PCB Capabilities
Via-in-Pad PCB
Filled and plated vias under component pads for maximum routing density.
Via-in-pad places plated vias directly beneath surface-mount component pads instead of routing them to adjacent via locations through short traces (dog-bone fan-out). This technique is essential for fine-pitch BGAs, QFNs and LGAs where the pad pitch is too tight to route vias between pads, and increasingly common in high-density designs where the routing space saved by eliminating fan-out traces enables a lower layer count or a smaller board outline. AstroPCB fabricates via-in-pad boards with epoxy-filled, copper-capped vias that provide flat, solderable pad surfaces for reliable assembly.
Why via-in-pad is necessary
A BGA with 0.5 mm ball pitch has pads spaced at 0.5 mm center-to-center. After accounting for pad diameter and solder mask opening, the gap between adjacent pads is approximately 0.15-0.2 mm — not enough space to route a via and a trace between pads. The only escape routing option for inner-row balls is to place the via directly in the pad. Without via-in-pad, fine-pitch BGAs with more than two rows of balls cannot be routed on a reasonable number of layers.
QFN and LGA packages with exposed thermal pads also require via-in-pad. The exposed pad must be soldered to a copper area on the PCB for both electrical grounding and thermal conduction. Thermal vias within this pad area conduct heat from the component through the board to internal copper planes or a bottom-side heatsink. These vias must be filled and capped to prevent solder from wicking down through the vias during reflow, which would starve the thermal pad joint and create voids that increase thermal resistance.
Filled and capped via process
The standard via-in-pad manufacturing process follows IPC-4761 Type VII: the via is drilled, plated, filled with non-conductive epoxy, and then plated over with copper to create a flat surface. The copper cap is planarized (polished flat) to ensure uniform solder paste deposition during stencil printing. The result is a pad that behaves identically to a solid copper pad from the assembly perspective — solder paste prints evenly, the component sits flat, and the solder joint forms without wicking or voiding artifacts.
For thermal applications, conductive fill (IPC-4761 Type VI) replaces the non-conductive epoxy with silver-filled or copper-filled epoxy. Conductive fill provides a thermal path through the via while still preventing solder wicking. The thermal conductivity of conductive fill is lower than solid copper but significantly better than air or non-conductive epoxy, making it a practical compromise for thermal via arrays.
Design rules for via-in-pad
Via diameter is typically 0.2-0.3 mm (8-12 mil) for mechanically drilled vias placed in BGA pads. Smaller vias are easier to fill reliably but provide less plated-barrel cross-section for current carrying. For laser-drilled microvias (0.1-0.15 mm), filling happens naturally during the copper plating process and does not require a separate epoxy fill step. Pad size must accommodate both the component solder joint requirements and the via drill plus annular ring. Work with the fabricator to confirm that the pad geometry supports reliable filling and capping for the specified via size.
Not all vias on a board need to be filled and capped. Specify via-in-pad only where required (under component pads) and leave other vias as standard open vias. Selective filling reduces fabrication cost compared to filling every via on the board.
Via-in-pad vs dog-bone routing
Dog-bone routing places the via adjacent to the pad, connected by a short trace. This avoids the cost of via filling but consumes routing space and increases the minimum board area. For BGAs with 0.8 mm pitch and above, dog-bone routing is practical and avoids the via-fill cost premium. Below 0.65 mm pitch, via-in-pad becomes necessary for inner-row escape. The choice between the two approaches affects both fabrication cost and routing density — discuss with the manufacturing engineer during the design phase to find the optimal balance.
Request a via-in-pad quote
Upload your design files and specify which vias require filling and capping. The engineering team will confirm the fill process and include it in the fabrication quotation.