A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive traces, pads and other features etched from conductive sheets, such as copper, laminated onto a non-conductive substrate, or PCB laminate. PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layered. Conductors on different layers are connected with plated through holes called vias. Advanced PCBs may contain components, such as capacitors, resistors or active devices, embedded in the PCB laminate. The through holes are formed after all layers of the PCB are laminated together. A drill or laser is commonly used to form each through hole. The through hole walls for multi-layers PCBs can be made conductive and then electroplated with copper to form the plated-through holes.
A conventional fabrication technique for making a multi-layer PCB is to start with a two-sided copper-clad laminate, etch the circuitry on both copper sides, then laminate to the top and bottom prepreg and copper foil. Lamination is done by placing the stack of materials in a press and applying pressure and heat for a period of time. This results in a one piece product. Additional layers can be similarly added to the top and/or bottom. Once all the layers are added, the PCB is drilled to form through holes, plated to form plated through holes, and the top and bottom copper foils are etched to form conductive traces on the top and bottom layers.
Typical PCB thickness is in the range of 1-2 mm. For applications such as mainframe computers, servers and routers, a backplane is used which is a thicker PCB, for example 8-10 mm or greater. As PCBs are designed with more and more layers, the larger PCB thicknesses result in larger aspect ratios. The aspect ratio is the ratio of the through hole length to the through hole diameter. The larger aspect ratios exceed the fabrication limit for plating the through holes, even with more advanced plating technologies such as reverse pulse plating.
A conventional fabrication technique for making thicker PCBs is to laminate together multiple thinner PCBs. Each of the thinner PCBs can be made in a manner similar to that described above, having fully formed plated through holes. The thinner PCBs are stacked on top of each other and properly aligned, the plated through holes in each thinner PCB having been positioned to aligned with corresponding plated through holes of the other thinner PCBs when stacked. As each plated through hole is intended to provide an electrical connection from the top to the bottom of the stack, the aligned plated through holes on adjacently stacked thinner PCBs need to be electrically connected. Conventional methods apply a conductive adhesive at the inter-thinner PCB electrical connection point between aligned plated through holes, sometimes referred to as a Z-interconnect. However, using a conductive adhesive to form the Z-interconnect results in reliability, due in part to the coefficient of thermal expansion (CTE) mismatch between the conductive adhesive, which is typically a polymer epoxy, and the metal plating of the plated through hole, which is typically copper. Also, during the lamination process bubbles, voids or trapped air may form in the conductive adhesive, further negatively effecting reliability of the Z-interconnect.
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