Double-sided PCB Fabrication Process I

PCB fabrication process of double-sided PCB is more complicate than that of single-sided PCB. One of the major differences is that double-sided PCB board normally requires electroplating while single-sided PCB fabrication process does not. The design process using CAD is similar except that you have more layers for your circuit to route. In order to minimize cross talk and noise coupling, top and bottom traces should be routed at orthogonal to each other. Additional fabrication processes of double-sided PCB involving electroless copper plating, copper plating and metallic etch resist plating. There are more parameters to be considered; for example, the size of PTH, top and bottom layer isolation and current density of PTH comes into considerations.

PTH is a metallized hole that provides a conductive path between layers of PCB. It is also known as via hole since it provides connection between locations. PTH can be made by using chemical deposit or direct metallalization. As the demands for more compact circuit increase, direct metallization will become more popular. In addition, laser ablation with fine line technology is needed for interconnection. Pin Grid Array Package (PGA) and Ball Grid Array package (BGA) with 50 Mil pitch make micro-vias and fine line technology indispensable. State of the art BGA technology requires a design rule of 6 Mil spacing, 6 Mil trace, 24 Mil diameter pad, 24 Mil diameter mask and 12 Mil diameter hole.

Drilling
The raw laminate is first cut into the required size for drilling. All holes, including PTH holes are drilled using a high-speed numerically controlled drilling station. For manual drilling, a optical drilling is necessary for accurate hole positioning so as to ensure a correct front-back registration. Noted that in a PTH process, the hole size should be slightly larger than that of the single-sided PCB to accommodate the increase in PTH wall thickness due to electroplating.
After drilling, the PCB should be deburred and cleaned so as to remove the dirt and oxidation on copper and debris in the hole. Remove all sharp copper edges so that it cannot interfere with the electroplating process. The PCB should be cleaned using alkaline detergent and slightly etch in a 10% sulfuric acid bath.

Electroless Copper Plating
Metallization is required on the wall of the drill holes to make them conductive. It can be done by chemical deposition or direct metallization. For double layer boards, chemical deposition is common. It is an electroless process because the deposition of copper is done in the absence of electrical current. The objective of the electroless copper plating process is to deposit a very thin layer of copper over the entire surface of the copper cladding and on bare hole walls. This layer should be thick enough to allow the electrical current during the subsequent electrolytic plating. Normally, a thickness of 0.001 Mil to 0.0025 Mil is acceptable for further electroplating at low start current. The thickness of the PTH will be increased to the required level after the electrolytic copper plating.