Una placa de circuito impreso ( PCB ) soporta mecánicamente y conecta eléctricamente componentes eléctricos o electrónicos utilizando pistas conductoras , almohadillas y otras características grabadas de una o más capas de láminas de cobre laminadas sobre y / o entre capas de láminas de un sustrato no conductor . Los componentes generalmente se sueldan a la PCB para conectarlos eléctricamente y sujetarlos mecánicamente.
Las placas de circuito impreso se utilizan en todos los productos electrónicos, excepto en los más simples. También se utilizan en algunos productos eléctricos, como las cajas de interruptores pasivos.
Las alternativas a los PCB incluyen la envoltura de alambre y la construcción punto a punto , ambas una vez populares pero que ahora rara vez se usan. Los PCB requieren un esfuerzo de diseño adicional para diseñar el circuito, pero la fabricación y el ensamblaje se pueden automatizar. El software de diseño asistido por computadora está disponible para hacer gran parte del trabajo de diseño. Los circuitos de producción masiva con PCB son más baratos y rápidos que con otros métodos de cableado, ya que los componentes se montan y conectan en una sola operación. Se pueden fabricar grandes cantidades de PCB al mismo tiempo, y el diseño solo debe realizarse una vez. Los PCB también se pueden fabricar manualmente en pequeñas cantidades, con beneficios reducidos.
Los PCB pueden ser de una cara (una capa de cobre), de doble cara (dos capas de cobre en ambos lados de una capa de sustrato) o de varias capas (capas exterior e interior de cobre, alternando con capas de sustrato). Las placas de circuito impreso multicapa permiten una densidad de componentes mucho mayor, porque las trazas de circuito en las capas internas ocuparían espacio de superficie entre los componentes. El aumento de la popularidad de las placas de circuito impreso multicapa con más de dos, y especialmente con más de cuatro, planos de cobre coincidió con la adopción de la tecnología de montaje en superficie . Sin embargo, los PCB multicapa hacen que la reparación, el análisis y la modificación en campo de los circuitos sean mucho más difíciles y, por lo general, poco prácticos.
El mercado mundial de PCB desnudos superó los 60.200 millones de dólares en 2014 [1] y se estima que alcanzará los 79.000 millones de dólares en 2024. [2] [3]
Descripción general
Un PCB básico consta de una hoja plana de material aislante y una capa de lámina de cobre , laminada al sustrato. El grabado químico divide el cobre en líneas conductoras separadas llamadas pistas o trazos de circuito , almohadillas para conexiones, vías para pasar conexiones entre capas de cobre y características como áreas conductoras sólidas para blindaje electromagnético u otros fines. Las pistas funcionan como cables fijados en su lugar y están aislados entre sí por el aire y el material del sustrato de la placa. La superficie de una placa de circuito impreso puede tener un revestimiento que protege al cobre de la corrosión y reduce las posibilidades de que se produzcan cortocircuitos en la soldadura entre las trazas o el contacto eléctrico no deseado con los cables desnudos sueltos. Por su función de ayudar a prevenir cortocircuitos de soldadura, el recubrimiento se denomina resistencia de soldadura o máscara de soldadura.
Una placa de circuito impreso puede tener varias capas de cobre. Un tablero de dos capas tiene cobre en ambos lados; Los tableros multicapa intercalan capas de cobre adicionales entre capas de material aislante. Los conductores de diferentes capas están conectados con vías , que son orificios enchapados en cobre que funcionan como túneles eléctricos a través del sustrato aislante. Los cables de componentes de orificios pasantes a veces también funcionan eficazmente como vías. Después de los PCB de dos capas, el siguiente paso suele ser de cuatro capas. A menudo, dos capas se dedican como fuente de alimentación y planos de tierra , y las otras dos se utilizan para el cableado de señales entre componentes.
Los componentes de "orificio pasante" se montan mediante sus cables conductores que pasan a través de la placa y se sueldan a trazas en el otro lado. Los componentes de "montaje en superficie" están unidos por sus cables a pistas de cobre en el mismo lado de la placa. Una placa puede utilizar ambos métodos para montar componentes. Las placas de circuito impreso con solo componentes montados en orificios pasantes ahora son poco comunes. El montaje en superficie se utiliza para transistores , diodos , chips IC , resistencias y condensadores. El montaje con orificio pasante se puede utilizar para algunos componentes grandes, como condensadores electrolíticos y conectores.
El patrón que se grabará en cada capa de cobre de una PCB se denomina "ilustración". El grabado se realiza generalmente con fotorresistencia que se recubre sobre la PCB, luego se expone a la luz proyectada en el patrón de la obra de arte. El material protector protege el cobre de la disolución en la solución de grabado. Luego se limpia el tablero grabado. Un diseño de PCB se puede reproducir en masa de una manera similar a la forma en que las fotografías se pueden duplicar en masa a partir de negativos de película utilizando una impresora fotográfica .
En tableros multicapa, las capas de material se laminan juntas en un sándwich alterno: cobre, sustrato, cobre, sustrato, cobre, etc .; Se graba cada plano de cobre, y las vías internas (que no se extenderán a ambas superficies externas del tablero multicapa terminado) se recubren, antes de que las capas se laminen juntas. Solo es necesario recubrir las capas exteriores; las capas internas de cobre están protegidas por las capas de sustrato adyacentes.
El epoxi de vidrio FR-4 es el sustrato aislante más común. Otro material de sustrato es el papel de algodón impregnado con resina fenólica , a menudo de color tostado o marrón.
Cuando una PCB no tiene componentes instalados, se le llama de forma menos ambigua placa de cableado impreso ( PWB ) o placa de cableado grabada . Sin embargo, el término "placa de circuito impreso" ha caído en desuso. Una PCB con componentes electrónicos se denomina conjunto de circuito impreso ( PCA ), conjunto de placa de circuito impreso o conjunto de PCB ( PCBA ). En el uso informal, el término "placa de circuito impreso" significa más comúnmente "conjunto de circuito impreso" (con componentes). El término preferido de IPC para placas ensambladas es conjunto de tarjeta de circuito ( CCA ), [4] y para placas posteriores ensambladas es ensamblajes de placa posterior . "Tarjeta" es otro término informal ampliamente utilizado para un "conjunto de circuito impreso". Por ejemplo, tarjeta de expansión .
Una PCB puede imprimirse con una leyenda que identifique los componentes, los puntos de prueba o el texto de identificación. Originalmente, la serigrafía se usaba para este propósito, pero hoy en día se usan generalmente otros métodos de impresión de mejor calidad. Normalmente, la leyenda no afecta la función del PCBA.
Una PCB mínima para un solo componente, que se utiliza para la creación de prototipos , se denomina placa de conexión . El propósito de una placa de conexiones es "romper" los cables de un componente en terminales separados para que las conexiones manuales a ellos se puedan realizar fácilmente. Las placas de conexión se utilizan especialmente para componentes de montaje en superficie o cualquier componente con paso de plomo fino.
Los PCB avanzados pueden contener componentes incrustados en el sustrato, como condensadores y circuitos integrados, para reducir la cantidad de espacio que ocupan los componentes en la superficie del PCB al tiempo que se mejoran las características eléctricas. [5]
Caracteristicas
Tecnología de orificio pasante
Los primeros PCB utilizaron tecnología de orificios pasantes, montando componentes electrónicos mediante cables insertados a través de orificios en un lado de la placa y soldados a trazas de cobre en el otro lado. Las placas pueden ser de una cara, con un lado de los componentes sin enchapar, o placas de doble cara más compactas, con componentes soldados en ambos lados. La instalación horizontal de piezas de orificio pasante con dos cables axiales (como resistencias, condensadores y diodos) se realiza doblando los cables 90 grados en la misma dirección, insertando la pieza en la placa (a menudo doblando los cables ubicados en la parte posterior de la placa). tablero en direcciones opuestas para mejorar la resistencia mecánica de la pieza), soldando los cables y recortando los extremos. Los cables se pueden soldar manualmente o con una máquina de soldadura por ola . [6]
La fabricación de orificios pasantes aumenta el costo de la placa al requerir que se taladren con precisión muchos orificios y limita el área de enrutamiento disponible para los rastros de señal en las capas inmediatamente debajo de la capa superior en las placas de múltiples capas, ya que los orificios deben pasar a través de todas las capas hasta el lado opuesto. Una vez que se puso en uso el montaje en superficie, se utilizaron componentes SMD de tamaño pequeño siempre que fue posible, con el montaje por orificio pasante solo de componentes que no eran adecuados para el montaje en superficie debido a requisitos de energía o limitaciones mecánicas, o sujetos a tensiones mecánicas que podrían dañar la PCB (por ejemplo, levantando el cobre de la superficie del tablero). [ cita requerida ]
Dispositivos de orificio pasante montados en la placa de circuito de una computadora doméstica Commodore 64 de mediados de la década de 1980
Caja de brocas que se utiliza para hacer agujeros en placas de circuito impreso. Si bien las brocas de carburo de tungsteno son muy duras, eventualmente se desgastan o se rompen. La perforación es una parte considerable del costo de una placa de circuito impreso de orificio pasante.
Tecnología de montaje superficial
La tecnología de montaje en superficie surgió en la década de 1960, ganó impulso a principios de la década de 1980 y se utilizó ampliamente a mediados de la década de 1990. Los componentes se rediseñaron mecánicamente para que tuvieran pequeñas lengüetas de metal o tapas de extremo que pudieran soldarse directamente sobre la superficie de la PCB, en lugar de cables para pasar a través de los orificios. Los componentes se volvieron mucho más pequeños y la colocación de los componentes en ambos lados de la placa se volvió más común que con el montaje de orificios pasantes, lo que permitió ensamblajes de PCB mucho más pequeños con densidades de circuito mucho más altas. El montaje en superficie se presta bien a un alto grado de automatización, lo que reduce los costos de mano de obra y aumenta considerablemente las tasas de producción en comparación con las placas de circuito de orificio pasante. Los componentes se pueden suministrar montados en cintas transportadoras. Los componentes de montaje en superficie pueden ser de un cuarto a un décimo del tamaño y peso de los componentes de orificio pasante, y los componentes pasivos son mucho más baratos. Sin embargo, los precios de los dispositivos semiconductores de montaje en superficie (SMD) están determinados más por el chip en sí que por el paquete, con una pequeña ventaja de precio sobre los paquetes más grandes, y algunos componentes con extremos de cable, como los diodos de conmutación de señal pequeña 1N4148 , son en realidad significativamente más baratos. que los equivalentes SMD.
Propiedades del circuito de la PCB
Cada trazo consiste en una parte plana y estrecha de la lámina de cobre que queda después del grabado. Su resistencia , determinada por su ancho, espesor y largo, debe ser lo suficientemente baja para la corriente que transportará el conductor. Es posible que las trazas de energía y tierra necesiten ser más anchas que las trazas de señal. En una placa de múltiples capas, una capa completa puede ser principalmente de cobre sólido para actuar como un plano de tierra para el blindaje y el retorno de energía. Para los circuitos de microondas , las líneas de transmisión se pueden colocar en forma plana , como una línea de banda o una microbanda, con dimensiones cuidadosamente controladas para asegurar una impedancia constante . En los circuitos de conmutación rápida y de radiofrecuencia, la inductancia y capacitancia de los conductores de la placa de circuito impreso se convierten en elementos de circuito importantes, normalmente no deseados; a la inversa, pueden usarse como una parte deliberada del diseño del circuito, como en filtros de elementos distribuidos , antenas y fusibles , obviando la necesidad de componentes discretos adicionales. Los PCB de interconexiones de alta densidad (HDI) tienen pistas y / o vías con un ancho o diámetro de menos de 152 micrómetros. [7]
Materiales
PCB compatible con RoHS
La Unión Europea prohíbe el uso de plomo (entre otros metales pesados) en artículos de consumo, una legislación llamada RoHS , directiva de Restricción de Sustancias Peligrosas. Los PCB que se venderán en la UE deben cumplir con RoHS, lo que significa que todos los procesos de fabricación no deben implicar el uso de plomo, todas las soldaduras utilizadas deben estar libres de plomo y todos los componentes montados en la placa deben estar libres de plomo, mercurio, cadmio y otros metales pesados. [8] [9]
Laminados
Los laminados se fabrican curando a presión y temperatura capas de tela o papel con resina termoendurecible para formar una pieza final integral de espesor uniforme. El tamaño puede ser de hasta 4 por 8 pies (1,2 por 2,4 m) de ancho y largo. Se utilizan diferentes tejidos de tela (hilos por pulgada o cm), el grosor de la tela y el porcentaje de resina para lograr el grosor final y las características dieléctricas deseadas . Los espesores de laminado estándar disponibles se enumeran en ANSI / IPC-D-275. [10]
La tela o material de fibra utilizado, el material de resina y la relación entre tela y resina determinan la designación del tipo de laminado (FR-4, CEM-1, G-10, etc.) y, por lo tanto, las características del laminado producido. Las características importantes son el nivel al que el laminado es ignífugo , la constante dieléctrica (e r ), el factor de pérdida (tδ), la resistencia a la tracción , la resistencia al corte , la temperatura de transición vítrea (T g ) y el eje Z coeficiente de expansión (cuánto cambia el espesor con la temperatura).
Hay bastantes dieléctricos diferentes que se pueden elegir para proporcionar diferentes valores de aislamiento según los requisitos del circuito. Algunos de estos dieléctricos son politetrafluoroetileno (teflón), FR-4, FR-1, CEM-1 o CEM-3. Los materiales preimpregnados bien conocidos utilizados en la industria de PCB son FR-2 (papel de algodón fenólico), FR-3 (papel de algodón y epoxi), FR-4 (vidrio tejido y epoxi), FR-5 (vidrio tejido y epoxi) , FR-6 (vidrio mate y poliéster), G-10 (vidrio tejido y epoxi), CEM-1 (papel de algodón y epoxi), CEM-2 (papel de algodón y epoxi), CEM-3 (vidrio no tejido y epoxi), CEM-4 (tejido de vidrio y epoxi), CEM-5 (tejido de vidrio y poliéster). La expansión térmica es una consideración importante, especialmente con las tecnologías de matriz de rejilla de bola (BGA) y troquel desnudo, y la fibra de vidrio ofrece la mejor estabilidad dimensional.
FR-4 es, con mucho, el material más utilizado en la actualidad. La cartulina con cobre sin grabar se llama "laminado revestido de cobre".
Con la disminución del tamaño de las características del tablero y el aumento de las frecuencias, las pequeñas no homogeneidades como la distribución desigual de la fibra de vidrio u otro relleno, las variaciones de espesor y las burbujas en la matriz de resina, y las variaciones locales asociadas en la constante dieléctrica, están ganando importancia.
Parámetros clave del sustrato
Los sustratos de la placa de circuito suelen ser materiales compuestos dieléctricos. Los compuestos contienen una matriz (generalmente una resina epoxi) y un refuerzo (generalmente una fibra de vidrio tejida, a veces no tejida, a veces incluso papel) y, en algunos casos, se agrega un relleno a la resina (por ejemplo, cerámica; se puede usar cerámica de titanato para aumentar la constante dieléctrica).
El tipo de refuerzo define dos clases principales de materiales: tejidos y no tejidos. Los refuerzos tejidos son más baratos, pero la alta constante dieléctrica del vidrio puede no ser favorable para muchas aplicaciones de alta frecuencia. La estructura espacialmente no homogénea también introduce variaciones locales en los parámetros eléctricos, debido a la diferente relación resina / vidrio en diferentes áreas del patrón de tejido. Los refuerzos no tejidos, o los materiales con poco o ningún refuerzo, son más caros pero más adecuados para algunas aplicaciones de RF / analógicas.
Los sustratos se caracterizan por varios parámetros clave, principalmente termomecánica ( temperatura de transición vítrea , resistencia a la tracción , resistencia a la cizalladura , de expansión térmica ), eléctrica ( constante dieléctrica , la tangente de pérdida , tensión de ruptura dieléctrica , la corriente de fuga , de seguimiento de resistencia ...), y otros (por ejemplo, absorción de humedad ).
A la temperatura de transición vítrea, la resina del compuesto se ablanda y aumenta significativamente la expansión térmica; exceder la T g ejerce una sobrecarga mecánica en los componentes de la placa, por ejemplo, las juntas y las vías. Por debajo de T g, la expansión térmica de la resina coincide aproximadamente con el cobre y el vidrio, por encima de ella aumenta significativamente. A medida que el refuerzo y el cobre confinan el tablero a lo largo del plano, prácticamente toda la expansión del volumen se proyecta al espesor y tensiona los orificios pasantes enchapados. La soldadura repetida u otra exposición a temperaturas más altas pueden causar fallas en el enchapado, especialmente con tableros más gruesos; por lo tanto, los tableros gruesos requieren una matriz con una T g alta .
Los materiales utilizados determinan la constante dieléctrica del sustrato. Esta constante también depende de la frecuencia, por lo general disminuye con la frecuencia. Como esta constante determina la velocidad de propagación de la señal , la dependencia de la frecuencia introduce una distorsión de fase en aplicaciones de banda ancha; Aquí es importante una constante dieléctrica tan plana en función de las características de frecuencia como sea posible. La impedancia de las líneas de transmisión disminuye con la frecuencia, por lo tanto, los bordes más rápidos de las señales reflejan más que los más lentos.
El voltaje de ruptura dieléctrica determina el gradiente de voltaje máximo al que puede estar sometido el material antes de sufrir una ruptura (conducción o arco a través del dieléctrico).
La resistencia de seguimiento determina cómo el material resiste las descargas eléctricas de alto voltaje que se arrastran sobre la superficie de la placa.
La tangente de pérdida determina la cantidad de energía electromagnética de las señales en los conductores que se absorbe en el material de la placa. Este factor es importante para altas frecuencias. Los materiales de baja pérdida son más caros. La elección de material innecesariamente de baja pérdida es un error de ingeniería común en el diseño digital de alta frecuencia; aumenta el costo de los tableros sin un beneficio correspondiente. La degradación de la señal por tangente de pérdida y constante dieléctrica se puede evaluar fácilmente mediante un patrón visual .
La absorción de humedad ocurre cuando el material se expone a mucha humedad o agua. Tanto la resina como el refuerzo pueden absorber agua; el agua también puede empaparse por fuerzas capilares a través de huecos en los materiales y a lo largo del refuerzo. Los epóxidos de los materiales FR-4 no son demasiado susceptibles, con una absorción de solo 0,15%. El teflón tiene una absorción muy baja del 0,01%. Las poliimidas y los ésteres de cianato, por otro lado, sufren una alta absorción de agua. El agua absorbida puede provocar una degradación significativa de parámetros clave; afecta la resistencia de seguimiento, el voltaje de ruptura y los parámetros dieléctricos. La constante dieléctrica relativa del agua es aproximadamente 73, en comparación con aproximadamente 4 para los materiales comunes de las placas de circuito. La humedad absorbida también puede vaporizarse al calentarse, como durante la soldadura, y causar agrietamiento y delaminación, [11] el mismo efecto responsable del daño por "popcorning" en el empaque húmedo de piezas electrónicas. Es posible que se requiera un horneado cuidadoso de los sustratos para secarlos antes de soldarlos. [12]
Sustratos comunes
Materiales que se encuentran a menudo:
- FR-2 , papel fenólico o papel de algodón fenólico, papel impregnado con resina de fenol formaldehído . Común en la electrónica de consumo con placas de una cara. Propiedades eléctricas inferiores a FR-4. Poca resistencia al arco. Generalmente clasificado a 105 ° C.
- FR-4 , un paño de fibra de vidrio impregnado con una resina epoxi . Baja absorción de agua (hasta aproximadamente 0,15%), buenas propiedades de aislamiento, buena resistencia al arco. Muy común. Se encuentran disponibles varios grados con propiedades algo diferentes. Típicamente clasificado a 130 ° C.
- Aluminio , placa de núcleo de metal o sustrato de metal aislado (IMS), revestido con un dieléctrico delgado termoconductor (utilizado para piezas que requieren una refrigeración significativa), interruptores de potencia, LED. Consiste en una placa de circuito delgada, generalmente de una sola capa, a veces de doble capa, basada, por ejemplo, en FR-4, laminada sobre chapa de aluminio, comúnmente de 0,8, 1, 1,5, 2 o 3 mm de espesor. Los laminados más gruesos a veces también vienen con una metalización de cobre más gruesa.
- Sustratos flexibles : puede ser una lámina revestida de cobre independiente o puede laminarse con un refuerzo delgado, por ejemplo, 50-130 µm
- Kapton o UPILEX , [13] una hoja de poliimida . Se utiliza para circuitos impresos flexibles , en esta forma común en la electrónica de consumo de factor de forma pequeño o para interconexiones flexibles. Resistente a altas temperaturas.
- Pyralux , una lámina compuesta de poliimida-fluoropolímero. [14] La capa de cobre puede deslaminarse durante la soldadura.
Materiales que se encuentran con menos frecuencia:
- FR-1, como FR-2, típicamente especificado a 105 ° C, algunos grados clasificados a 130 ° C. Perforable a temperatura ambiente. Similar al cartón. Poca resistencia a la humedad. Baja resistencia al arco.
- FR-3, papel de algodón impregnado con epoxi. Típicamente clasificado a 105 ° C.
- FR-5, fibra de vidrio tejida y epoxi, de alta resistencia a temperaturas más altas, normalmente especificado hasta 170 ° C.
- FR-6, vidrio mate y poliéster
- G-10, vidrio tejido y epoxi: alta resistencia de aislamiento, baja absorción de humedad, muy alta fuerza de unión. Típicamente clasificado a 130 ° C.
- G-11, vidrio tejido y epoxi: alta resistencia a los disolventes, alta retención de la resistencia a la flexión a altas temperaturas. [15] Típicamente clasificado a 170 ° C.
- CEM-1, papel de algodón y epoxi
- CEM-2, papel de algodón y epoxi
- CEM-3, vidrio no tejido y epoxi
- CEM-4, vidrio tejido y epoxi
- CEM-5, tejido de vidrio y poliéster
- PTFE , ("Teflon") - costoso, baja pérdida dieléctrica, para aplicaciones de alta frecuencia, muy baja absorción de humedad (0.01%), mecánicamente suave. Difícil de laminar, rara vez se utiliza en aplicaciones multicapa.
- PTFE, relleno de cerámica: costoso, baja pérdida dieléctrica, para aplicaciones de alta frecuencia. La variación de la relación cerámica / PTFE permite ajustar la constante dieléctrica y la expansión térmica.
- RF-35, PTFE relleno de cerámica reforzada con fibra de vidrio. Relativamente menos costoso, buenas propiedades mecánicas, buenas propiedades de alta frecuencia. [16] [17]
- Alúmina , una cerámica. Duro, quebradizo, muy caro, de muy alto rendimiento, buena conductividad térmica.
- Poliamida , un polímero de alta temperatura. Caro, de alto rendimiento. Mayor absorción de agua (0,4%). Puede utilizarse desde temperaturas criogénicas hasta más de 260 ° C.
Espesor de cobre
El espesor de cobre de los PCB se puede especificar directamente o como el peso de cobre por área (en onzas por pie cuadrado), que es más fácil de medir. Una onza por pie cuadrado es 1.344 milésimas de pulgada o 34 micrómetros de espesor. El cobre pesado es una capa que excede las tres onzas de cobre por pie 2 , o aproximadamente 0.0042 pulgadas (4.2 milésimas de pulgada, 105 μm) de espesor. Se utilizan capas de cobre pesadas para alta corriente o para ayudar a disipar el calor.
En los sustratos FR-4 comunes, el espesor más común es de 1 oz de cobre por pie 2 (35 µm); El grosor de 2 oz (70 µm) y 0,5 oz (17,5 µm) suele ser una opción. Menos comunes son 12 y 105 µm, a veces hay 9 µm disponibles en algunos sustratos. Los sustratos flexibles suelen tener una metalización más fina. Las placas con núcleo de metal para dispositivos de alta potencia suelen utilizar cobre más grueso; Lo habitual es 35 µm, pero también se pueden encontrar 140 y 400 µm.
En los EE. UU., El espesor de la hoja de cobre se especifica en unidades de onzas por pie cuadrado (oz / pie 2 ), comúnmente denominado simplemente onza . Los espesores comunes son 1/2 oz / ft 2 (150 g / m 2 ), 1 oz / ft 2 (300 g / m 2 ), 2 oz / ft 2 (600 g / m 2 ) y 3 oz / ft 2 (900 g / m 2 ). Estos funcionan con espesores de 17.05 μm (0.67 miles ), 34.1 μm (1.34 miles ), 68.2 μm (2.68 miles) y 102.3 μm (4.02 miles), respectivamente. La lámina de 1/2 oz / pie 2 no se usa ampliamente como un peso de cobre terminado, pero se usa para capas externas cuando el enchapado para orificios pasantes aumentará el peso del cobre terminado.Algunos fabricantes de PCB se refieren a la lámina de cobre de 1 oz / pie 2 como que tiene un espesor de 35 μm (también puede denominarse 35 μ, 35 micrones o 35 micrones ).
- 1/0: denota 1 oz / pie 2 de cobre en un lado, sin cobre en el otro lado.
- 1/1: indica 1 oz / pie 2 de cobre en ambos lados.
- H / 0 o H / H: indica 0,5 oz / pie 2 de cobre en uno o ambos lados, respectivamente.
- 2/0 o 2/2: indica 2 oz / pie 2 de cobre en uno o ambos lados, respectivamente.
Certificación de seguridad (EE. UU.)
El estándar de seguridad UL 796 cubre los requisitos de seguridad de los componentes para las placas de cableado impresas para su uso como componentes en dispositivos o electrodomésticos. Las pruebas analizan características como la inflamabilidad, la temperatura máxima de funcionamiento , el seguimiento eléctrico, la deflexión del calor y el soporte directo de piezas eléctricas vivas.
Diseño
Initially PCBs were designed manually by creating a photomask on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. The finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards.
Modern PCBs are designed with dedicated layout software, generally in the following steps:[18]
- Schematic capture through an electronic design automation (EDA) tool.
- Card dimensions and template are decided based on required circuitry and case of the PCB.
- The positions of the components and heat sinks are determined.
- Layer stack of the PCB is decided, with one to tens of layers depending on complexity. Ground and power planes are decided. A power plane is the counterpart to a ground plane and behaves as an AC signal ground while providing DC power to the circuits mounted on the PCB. Signal interconnections are traced on signal planes. Signal planes can be on the outer as well as inner layers. For optimal EMI performance high frequency signals are routed in internal layers between power or ground planes.[19]
- Line impedance is determined using dielectric layer thickness, routing copper thickness and trace-width. Trace separation is also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals.
- Components are placed. Thermal considerations and geometry are taken into account. Vias and lands are marked.
- Signal traces are routed. Electronic design automation tools usually create clearances and connections in power and ground planes automatically.
- Gerber files are generated for manufacturing.
Fabricación
PCB manufacturing consists of many steps.
PCB CAM
Manufacturing starts from the fabrication data generated by computer aided design, and component information. The fabrication data is read into the CAM (Computer Aided Manufacturing) software. CAM performs the following functions:
- Input of the fabrication data.
- Verification of the data
- Compensation for deviations in the manufacturing processes (e.g. scaling to compensate for distortions during lamination)
- Panelization
- Output of the digital tools (copper patterns, drill files, inspection, and others)
Panelization
Several small printed circuit boards can be grouped together for processing as a panel. A panel consisting of a design duplicated n-times is also called an n-panel, whereas a multi-panel combines several different designs onto a single panel. The outer tooling strip often includes tooling holes, a set of panel fiducials, a test coupon, and may include hatched copper pour or similar patterns for even copper distribution over the whole panel in order to avoid bending. The assemblers often mount components on panels rather than single PCBs because this is efficient. Panelization may also be necessary for boards with components placed near an edge of the board because otherwise the board could not be mounted during assembly. Most assembly shops require a free area of at least 10 mm around the board.
The panel is eventually broken into individual PCBs along perforations or grooves in the panel[20] through milling or cutting. For milled panels a common distance between the individual boards is 2 to 3 mm. Today depaneling is often done by lasers which cut the board with no contact. Laser depaneling reduces stress on the fragile circuits, improving the yield of defect-free units.
Copper patterning
The first step is to replicate the pattern in the fabricator's CAM system on a protective mask on the copper foil PCB layers. Subsequent etching removes the unwanted copper unprotected by the mask. (Alternatively, a conductive ink can be ink-jetted on a blank (non-conductive) board. This technique is also used in the manufacture of hybrid circuits.)
- Silk screen printing uses etch-resistant inks to create the protective mask.
- Photoengraving uses a photomask and developer to selectively remove a UV-sensitive photoresist coating and thus create a photoresist mask that will protect the copper below it. Direct imaging techniques are sometimes used for high-resolution requirements. Experiments have been made with thermal resist.[21] A laser may be used instead of a photomask. This is known as maskless lithography or direct imaging.
- PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis.
- Laser resist ablation Spray black paint onto copper clad laminate, place into CNC laser plotter. The laser raster-scans the PCB and ablates (vaporizes) the paint where no resist is wanted. (Note: laser copper ablation is rarely used and is considered experimental.[clarification needed])
- Laser etching The copper may be removed directly by a CNC laser. Like PCB milling above this is used mainly for prototyping.
The method chosen depends on the number of boards to be produced and the required resolution.
Large volume
- Silk screen printing – Used for PCBs with bigger features
- Photoengraving – Used when finer features are required
Small volume
- Print onto transparent film and use as photo mask along with photo-sensitized boards, then etch. (Alternatively, use a film photoplotter)
- Laser resist ablation
- PCB milling
- Laser etching
Hobbyist
- Laser-printed resist: Laser-print onto toner transfer paper, heat-transfer with an iron or modified laminator onto bare laminate, soak in water bath, touch up with a marker, then etch.
- Vinyl film and resist, non-washable marker, some other methods. Labor-intensive, only suitable for single boards.
Subtractive, additive and semi-additive processes
Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern. In additive methods the pattern is electroplated onto a bare substrate using a complex process. The advantage of the additive method is that less material is needed and less waste is produced. In the full additive process the bare laminate is covered with a photosensitive film which is imaged (exposed to light through a mask and then developed which removes the unexposed film). The exposed areas are sensitized in a chemical bath, usually containing palladium and similar to that used for through hole plating which makes the exposed area capable of bonding metal ions. The laminate is then plated with copper in the sensitized areas. When the mask is stripped, the PCB is finished.
Semi-additive is the most common process: The unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed bare original copper laminate from the board, isolating the individual traces. Some single-sided boards which have plated-through holes are made in this way. General Electric made consumer radio sets in the late 1960s using additive boards.
The (semi-)additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes to produce conductive vias in the circuit board.
Chemical etching
Chemical etching is usually done with ammonium persulfate or ferric chloride. For PTH (plated-through holes), additional steps of electroless deposition are done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.[22]
The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates.[23]
As more copper is consumed from the boards, the etchant becomes saturated and less effective; different etchants have different capacities for copper, with some as high as 150 grams of copper per litre of solution. In commercial use, etchants can be regenerated to restore their activity, and the dissolved copper recovered and sold. Small-scale etching requires attention to disposal of used etchant, which is corrosive and toxic due to its metal content.[24]
The etchant removes copper on all surfaces not protected by the resist. "Undercut" occurs when etchant attacks the thin edge of copper under the resist; this can reduce conductor widths and cause open-circuits. Careful control of etch time is required to prevent undercut. Where metallic plating is used as a resist, it can "overhang" which can cause short-circuits between adjacent traces when closely spaced. Overhang can be removed by wire-brushing the board after etching.[23]
Lamination
Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.[25]
The inner layers are given a complete machine inspection before lamination because mistakes cannot be corrected afterwards. Automatic optical inspection (AOI) machines compare an image of the board with the digital image generated from the original design data. Automated Optical Shaping (AOS) machines can then add missing copper or remove excess copper using a laser, reducing the number of PCBs that have to be discarded.[26] PCB tracks can have a width of just 10 micrometers.
Drilling
Holes through a PCB are typically drilled with drill bits made of solid coated tungsten carbide. Coated tungsten carbide is used because board materials are abrasive. High-speed-steel bits would dull quickly, tearing the copper and ruining the board. Drilling is done by computer-controlled drilling machines, using a drill file or Excellon file that describes the location and size of each drilled hole.
Holes may be made conductive, by electroplating or inserting hollow metal eyelets, to connect board layers. Some conductive holes are intended for the insertion of through-hole-component leads. Others used to connect board layers, are called vias.
When vias with a diameter smaller than 76.2 micrometers are required, drilling with mechanical bits is impossible because of high rates of wear and breakage. In this case, the vias may be laser drilled—evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias and can have diameters as small as 10 micrometers.[27][28] It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers and no outer layers. Laser drilling machines can drill thousands of holes per second and can use either UV or CO
2 lasers.[29][30]
The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes. These holes electrically connect the conducting layers of the PCB. For multi-layer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma etching. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper.
Plating and coating
Proper plating or surface finish selection can be critical to process yield, the amount of rework, field failure rate, and reliability.[31]
PCBs may be plated with solder, tin, or gold over nickel.[32][33]
After PCBs are etched and then rinsed with water, the solder mask is applied, and then any exposed copper is coated with solder, nickel/gold, or some other anti-corrosion coating.[34]
Matte solder is usually fused to provide a better bonding surface for bare copper. Treatments, such as benzimidazolethiol, prevent surface oxidation of bare copper. The places to which components will be mounted are typically plated, because untreated bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was coated with solder by hot air (solder) levelling (HASL aka HAL). The HASL finish prevents oxidation from the underlying copper, thereby guaranteeing a solderable surface. This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU, which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin, 0.7% copper, 0.05% nickel, and a nominal of 60 ppm germanium.[citation needed]
It is important to use solder compatible with both the PCB and the parts used. An example is ball grid array (BGA) using tin-lead solder balls for connections losing their balls on bare copper traces or using lead-free solder paste.
Other platings used are organic solderability preservative (OSP), immersion silver (IAg), immersion tin (ISn), electroless nickel immersion gold (ENIG) coating, electroless nickel electroless palladium immersion gold (ENEPIG), and direct gold plating (over nickel). Edge connectors, placed along one edge of some boards, are often nickel-plated then gold-plated using ENIG. Another coating consideration is rapid diffusion of coating metal into tin solder. Tin forms intermetallics such as Cu6Sn5 and Ag3Cu that dissolve into the Tin liquidus or solidus (at 50 °C), stripping surface coating or leaving voids.
Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias.[35][36] Silver, zinc, and aluminum are known to grow whiskers under the influence of an electric field. Silver also grows conducting surface paths in the presence of halide and other ions, making it a poor choice for electronics use. Tin will grow "whiskers" due to tension in the plated surface. Tin-lead or solder plating also grows whiskers, only reduced by reducing the percentage of tin. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Another coating issue is tin pest, the transformation of tin to a powdery allotrope at low temperature.[37]
Solder resist application
Areas that should not be soldered may be covered with solder resist (solder mask). The solder mask is what gives PCBs their characteristic green color, although it is also available in several other colors, such as red, blue, purple, yellow, black and white. One of the most common solder resists used today is called "LPI" (liquid photoimageable solder mask).[38] A photo-sensitive coating is applied to the surface of the PWB, then exposed to light through the solder mask image film, and finally developed where the unexposed areas are washed away. Dry film solder mask is similar to the dry film used to image the PWB for plating or etching. After being laminated to the PWB surface it is imaged and developed as LPI. Once but no longer commonly used, because of its low accuracy and resolution, is to screen print epoxy ink. In addition to repelling solder, solder resist also provides protection from the environment to the copper that would otherwise be exposed.
Legend printing
A legend is often printed on one or both sides of the PCB. It contains the component designators, switch settings, test points and other indications helpful in assembling, testing, servicing, and sometimes using the circuit board.
There are three methods to print the legend.
- Silk screen printing epoxy ink was the established method. It was so common that legend is often misnamed silk or silkscreen.
- Liquid photo imaging is a more accurate method than screen printing.
- Ink jet printing is increasingly used. Ink jet can print variable data, unique to each PWB unit, such as text or a bar code with a serial number.
Bare-board test
Boards with no components installed are usually bare-board tested for "shorts" and "opens". This is called electrical test or PCB e-test. A short is a connection between two points that should not be connected. An open is a missing connection between points that should be connected. For high-volume production, a fixture such as a "bed of nails" in a rigid needle adapter makes contact with copper lands on the board. The fixture or adapter is a significant fixed cost and this method is only economical for high-volume or high-value production. For small or medium volume production flying probe testers are used where test probes are moved over the board by an XY drive to make contact with the copper lands. There is no need for a fixture and hence the fixed costs are much lower. The CAM system instructs the electrical tester to apply a voltage to each contact point as required and to check that this voltage appears on the appropriate contact points and only on these.
Assembly
In assembly the bare board is populated (or "stuffed") with electronic components to form a functional printed circuit assembly (PCA), sometimes called a "printed circuit board assembly" (PCBA).[39][40] In through-hole technology, the component leads are inserted in holes surrounded by conductive pads; the holes keep the components in place. In surface-mount technology (SMT), the component is placed on the PCB so that the pins line up with the conductive pads or lands on the surfaces of the PCB; solder paste, which was previously applied to the pads, holds the components in place temporarily; if surface-mount components are applied to both sides of the board, the bottom-side components are glued to the board. In both through hole and surface mount, the components are then soldered; once cooled and solidified, the solder holds the components in place permanently and electrically connects them to the board.
There are a variety of soldering techniques used to attach components to a PCB. High volume production is usually done with a pick-and-place machine and bulk wave soldering for through-hole parts or reflow ovens for SMT components and/or through-hole parts, but skilled technicians are able to hand-solder very tiny parts (for instance 0201 packages which are 0.02 in. by 0.01 in.)[41] under a microscope, using tweezers and a fine-tip soldering iron, for small volume prototypes. Selective soldering may be used for delicate parts. Some SMT parts cannot be soldered by hand, such as BGA packages. All through-hole components can be hand soldered, making them favored for prototyping where size, weight, and the use of the exact components that would be used in high volume production are not concerns.
Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Or, even if all components are available in through-hole packages, it might be desired to take advantage of the size, weight, and cost reductions obtainable by using some available surface-mount devices. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress (such as connectors that are frequently mated and demated or that connect to cables expected to impart substantial stress to the PCB-and-connector interface), while components that are expected to go untouched will take up less space using surface-mount techniques. For further comparison, see the SMT page.
After the board has been populated it may be tested in a variety of ways:
- While the power is off, visual inspection, automated optical inspection. JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.
- While the power is off, analog signature analysis, power-off testing.
- While the power is on, in-circuit test, where physical measurements (for example, voltage) can be done.
- While the power is on, functional test, just checking if the PCB does what it had been designed to do.
To facilitate these tests, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise boundary scan test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.
In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard. The JTAG test architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes, by using circuitry in the ICs to employ the IC pins themselves as test probes. JTAG tool vendors provide various types of stimuli and sophisticated algorithms, not only to detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.
When boards fail the test, technicians may desolder and replace failed components, a task known as rework.
Protection and packaging
PCBs intended for extreme environments often have a conformal coating, which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax; modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another technique for applying a conformal coating is for plastic to be sputtered onto the PCB in a vacuum chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered extremely difficult.[42]
Many assembled PCBs are static sensitive, and therefore they must be placed in antistatic bags during transport. When handling these boards, the user must be grounded (earthed). Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components. The damage might not immediately affect function but might lead to early failure later on, cause intermittent operating faults, or cause a narrowing of the range of environmental and electrical conditions under which the board functions properly. Even bare boards are sometimes static sensitive: traces have become so fine that it is possible to blow a trace (or change its characteristics) with a static discharge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.
Construcción de leña
Cordwood construction can save significant space and was often used with wire-ended components in applications where space was at a premium (such as fuzes, missile guidance, and telemetry systems) and in high-speed computers, where short traces were important. In cordwood construction, axial-leaded components were mounted between two parallel planes. The components were either soldered together with jumper wire or they were connected to other components by thin nickel ribbon welded at right angles onto the component leads.[43] To avoid shorting together different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next interconnection layer. One disadvantage of this system was that special nickel-leaded components had to be used to allow reliable interconnecting welds to be made. Differential thermal expansion of the component could put pressure on the leads of the components and the PCB traces and cause mechanical damage (as was seen in several modules on the Apollo program). Additionally, components located in the interior are difficult to replace. Some versions of cordwood construction used soldered single-sided PCBs as the interconnection method (as pictured), allowing the use of normal-leaded components at the cost of being difficult to remove the boards or replace any component that is not at the edge.
Before the advent of integrated circuits, this method allowed the highest possible component packing density; because of this, it was used by a number of computer vendors including Control Data Corporation. The cordwood method of construction was used only rarely once PCBs became widespread, mainly in aerospace or other extremely high-density electronics.
Tableros de cables múltiples
Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting matrix (often plastic resin). It was used during the 1980s and 1990s. (Kollmorgen Technologies Corp, U.S. Patent 4,175,816 filed 1978) As of 2010, Multiwire was still available through Hitachi.
Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach allowed designers to forget completely about the routing of wires (usually a time-consuming operation of PCB design): Anywhere the designer needs a connection, the machine will draw a wire in a straight line from one location/pin to another. This led to very short design times (no complex algorithms to use even for high density designs) as well as reduced crosstalk (which is worse when wires run parallel to each other—which almost never happens in Multiwire), though the cost is too high to compete with cheaper PCB technologies when large quantities are needed.
Corrections can be made to a Multiwire board layout more easily than to a PCB layout.[44]
There are other competitive discrete wiring technologies that have been developed.
Historia
Before the development of printed circuit boards, electrical and electronic circuits were wired point-to-point on a chassis. Typically, the chassis was a sheet metal frame or pan, sometimes with a wooden bottom. Components were attached to the chassis, usually by insulators when the connecting point on the chassis was metal, and then their leads were connected directly or with jumper wires by soldering, or sometimes using crimp connectors, wire connector lugs on screw terminals, or other methods. Circuits were large, bulky, heavy, and relatively fragile (even discounting the breakable glass envelopes of the vacuum tubes that were often included in the circuits), and production was labor-intensive, so the products were expensive.
Development of the methods used in modern printed circuit boards started early in the 20th century. In 1903, a German inventor, Albert Hanson, described flat foil conductors laminated to an insulating board, in multiple layers. Thomas Edison experimented with chemical methods of plating conductors onto linen paper in 1904. Arthur Berry in 1913 patented a print-and-etch method in the UK, and in the United States Max Schoop obtained a patent[45] to flame-spray metal onto a board through a patterned mask. Charles Ducas in 1927 patented a method of electroplating circuit patterns.[46]
The Austrian engineer Paul Eisler invented the printed circuit as part of a radio set while working in the UK around 1936. In 1941 a multi-layer printed circuit was used in German magnetic influence naval mines. Around 1943 the USA began to use the technology on a large scale to make proximity fuzes for use in World War II.[46]
After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army. At around the same time in the UK work along similar lines was carried out by Geoffrey Dummer, then at the RRDE.
Motorola was an early leader in bringing the process into consumer electronics, announcing in August 1952 the adoption of "plated circuits" in home radios after six years of research and a $1M investment.[47] Motorola soon began using its trademarked term for the process, PLAcir, in its consumer radio advertisements.[48]
Even as circuit boards became available, the point-to-point chassis construction method remained in common use in industry (such as TV and hi-fi sets) into at least the late 1960s. Printed circuit boards were introduced to reduce the size, weight, and cost of parts of the circuitry. In 1960, a small consumer radio receiver might be built with all its circuitry on one circuit board, but a TV set would probably contain one or more circuit boards.
Predating the printed circuit invention, and similar in spirit, was John Sargrove's 1936–1947 Electronic Circuit Making Equipment (ECME) which sprayed metal onto a Bakelite plastic board. The ECME could produce three radio boards per minute.
During World War II, the development of the anti-aircraft proximity fuse required an electronic circuit that could withstand being fired from a gun, and could be produced in quantity. The Centralab Division of Globe Union submitted a proposal which met the requirements: a ceramic plate would be screenprinted with metallic paint for conductors and carbon material for resistors, with ceramic disc capacitors and subminiature vacuum tubes soldered in place.[49] The technique proved viable, and the resulting patent on the process, which was classified by the U.S. Army, was assigned to Globe Union. It was not until 1984 that the Institute of Electrical and Electronics Engineers (IEEE) awarded Harry W. Rubinstein the Cledo Brunetti Award for early key contributions to the development of printed components and conductors on a common insulating substrate. Rubinstein was honored in 1984 by his alma mater, the University of Wisconsin-Madison, for his innovations in the technology of printed electronic circuits and the fabrication of capacitors.[50][51] This invention also represents a step in the development of integrated circuit technology, as not only wiring but also passive components were fabricated on the ceramic substrate.
Originally, every electronic component had wire leads, and a PCB had holes drilled for each wire of each component. The component leads were then inserted through the holes and soldered to the copper PCB traces. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. The patent they obtained in 1956 was assigned to the U.S. Army.[52] With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are inefficient since drilling holes is expensive and consumes drill bits and the protruding wires are cut off and discarded.
From the 1980s onward, small surface mount parts have been used increasingly instead of through-hole components; this has led to smaller boards for a given functionality and lower production costs, but with some additional difficulty in servicing faulty boards.
In the 1990s the use of multilayer surface boards became more frequent. As a result, size was further minimized and both flexible and rigid PCBs were incorporated in different devices. In 1995 PCB manufacturers began using microvia technology to produce High-Density Interconnect (HDI) PCBs.[53]
HDI technology allows for a denser design on the PCB and significantly smaller components. As a result, components can be closer and the paths between them shorter. HDIs use blind/buried vias, or a combination that includes microvias. With multi-layer HDI PCBs the interconnection of stacked vias is even stronger, thus enhancing reliability in all conditions. The most common applications for HDI technology are computer and mobile phone components as well as medical equipment and military communication equipment. A 4-layer HDI microvia PCB Cost is equivalent in quality to an 8-layer through-hole PCB. However, the cost is much lower.
Recent advances in 3D printing have meant that there are several new techniques in PCB creation. 3D printed electronics (PEs) can be utilized to print items layer by layer and subsequently the item can be printed with a liquid ink that contains electronic functionalities.
Manufacturers may not support component-level repair of printed circuit boards because of the relatively low cost to replace compared with the time and cost of troubleshooting to a component level. In board-level repair, the technician identifies the board (PCA) on which the fault resides and replaces it. This shift is economically efficient from a manufacturer's point of view but is also materially wasteful, as a circuit board with hundreds of functional components may be discarded and replaced due to the failure of one minor and inexpensive part, such as a resistor or capacitor. This practice is a significant contributor to the problem of e-waste.[54]
Ver también
- Breadboard
- C.I.D.+
- Design for manufacturability (PCB)
- Electronic packaging
- Electronic waste
- Microphonics
- Multi-chip module
- Occam process – another process for the manufacturing of PCBs
- Point-to-point construction
- Printed electronics – creation of components by printing
- Printed circuit board milling
- Printed electronic circuit – similar name, different part
- Stamped circuit board
- Stripboard
- Veroboard
- Wire wrap
PCB materials
- Conductive ink
- Laminate materials:
- BT-Epoxy
- Composite epoxy material, CEM-1,5
- Cyanate Ester
- FR-2
- FR-4, the most common PCB material
- Polyimide
- PTFE, polytetrafluoroethylene (Teflon)
PCB layout software
- List of EDA companies
- Comparison of EDA software
Referencias
- ^ "iconnect007 :: Article". www.iconnect007.com. Retrieved 2016-04-12.
- ^ Research, Energias Market. "Global Printed Circuit Board (PCB) Market to Witness a CAGR of 3.1% during 2018-2024". GlobeNewswire News Room. Retrieved 2018-08-26.
- ^ "Global Single Sided Printed Circuit Board Market - Growth, Future Prospects and Competitive Analysis and Forecast 2018 - 2023 - The Industry Herald". The Industry Herald. 2018-08-21. Retrieved 2018-08-26.
- ^ IPC-14.38
- ^ https://www.electronicdesign.com/technologies/embedded-revolution/article/21799095/use-embedded-components-to-improve-pcb-performance-and-reduce-size
- ^ Electronic Packaging:Solder Mounting Technologies in K.H. Buschow et al (ed), Encyclopedia of Materials:Science and Technology, Elsevier, 2001 ISBN 0-08-043152-6, pages 2708–2709
- ^ https://www.freedomcad.com/2018/08/21/why-use-high-density-interconnect/
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enlaces externos
- PCB Fabrication Data - A Guide
- The Gerber Format Specification