From a quality perspective, when choosing the process for high-frequency boards (PCBs), resin plug holes are the preferred method. However, in terms of the entire process flow and cost, resin plug holes for high-frequency boards may introduce an additional step and incur higher costs compared to using green solder mask plug holes. Green solder mask plug holes, on the other hand, offer a simpler process, as they can be performed in conjunction with surface ink application in a cleanroom, or holes can be plugged before printing. Nevertheless, green solder mask plug holes can experience post-plug shrinkage issues, which might not be suitable for cases with high requirements or specific designs.
The choice between high-frequency board (PCB) resin plug holes and green solder mask plug holes is often driven by the need to accommodate BGA components. Traditional BGAs might have VIAs connecting from pad to pad on the backside for routing. However, in cases where BGA routing is too dense for VIAs to pass through to the backside, VIAs can be directly drilled from the pad and routed to another layer, and then the holes are filled with copper plating to create a pad, a process known as VIP process. If VIAs are placed on the pad without using resin plug holes, it may result in solder leakage causing backside short circuits or front-side solder voids.
The process of high-frequency board (PCB) resin plug holes involves drilling, plating, plugging, baking, and grinding steps. After drilling, the holes are plated through, then resin is plugged into the holes and baked, followed by grinding to make the surface even. Since the ground resin after grinding doesn't contain copper, another layer of copper plating is applied to create a pad. These steps are performed before the original high-frequency board (PCB) drilling process, treating the holes that need to be plugged first, followed by drilling other holes according to the normal process flow.
Usually, green solder mask windowing is used for conductive holes, which are plug-in holes with diameters above 0.35mm. Since these holes are used for plug-in connections, there should be no insulating green solder mask inside the holes. Green solder mask plug holes are mainly applied in subsequent SMT production for mainboards of high-frequency boards (PCBs) with BGAs, typically for non-plated through holes (NPTH) with diameters below 0.35mm. The purpose of plugging these holes is to prevent these non-component holes from becoming short circuits due to acid-alkali oxidation and corrosion in long-term natural environments, which may impact electrical performance. The standard for plugging holes in BGAs is that the holes should be non-transparent, preferably filled to two-thirds of the hole's depth, and the plugged area should not be covered by solder mask. Many customers using BGAs often require hole plugging.
The methods of resin plug holes and green solder mask plug holes differ in terms of hole filling, and in other aspects such as acid-alkali resistance, resin plug holes generally hold an advantage over green solder mask plug holes
