Via in Pad (VIP) Plated Over (VIPPO)

Increasing signal speeds, PCB functional density and PCB layer, thickness are increasing the challenges associated with PCB design, especially with respect to the signal integrity. These constrains have forced chip manufacturers to produce fine pitch chips (BGA, QFN, flip chips…), and PCB designers to use Via-in-Pad (VIP) Plated Over (VIPPO) structures in conjunction with traditional designs, such as dog bone traces, microvias, skip vias and pad-with-trace along with incorporating VIPPO in order to achieve routability and signal integrity requirements.

What’s Via in Pad?

The final type of via filling in IPC-4761 is known as Via-in-Pad (VIP), and as it implies, VIP refers primarily to cases where the via is placed entirely within the pad of a surface mount device (SMD). Many PCB designers are finding taking advantage of this Via in Pad fabrication process more than ever before, as BGA packages get smaller and smaller, so that escaping traces becomes more and more difficult.

Not Filled or Filled Via in Pads?

If vias in pads do not be filled, your assembled board will likely be missing connection between some of the BGA balls and the board. If you placing the via in the BGA pad, your only option is to have the vias filled and plated over at the PCB manufacturing house. If you placing the vias between the pads, you have two options. You can put soldermask dams on the short trace between the pad and the open via. This will prevent solder paste from migrating. The other, and the better, option is to cap the vias with solder mask. This gives a bit of extra protection in case of any of the solder mask dams are too thin or chip off. Just make sure you cap these things on the solder side. If you cap them on the back, solder paste can still spread on the trace and partway into the via. That still puts the electrical and mechanical connection at risk.

Soldermask Filled or Epoxy Filled?

Solder mask cannot be used for filling the vias (microvias) in pad when forming Via in Pads since removing solder mask clearance from the SMT pad would make it un-solderable. In the solder mask filling, there was concern that nearby vias might wick away (straw and suck) some solder paste from a pad, so of course this is an even greater concern when the via is within the pad itself. In this case, MADPCB’s PCB production team will selectively fill the vias with an epoxy material but not solder mask before the surface finish is applied, which requires some additional cost and lead time compared to standard vias.

Conductive or Non-Conductive Epoxy Filled?

The epoxy is most often a non-conductive substance, since conductive substances expand much more quickly with heat, which can be troublesome during the reflow soldering process. Non-conductive filling does not impede the via’s ability to conduct electrical current, since the walls are still plated with copper just like any other via. In cases where the via must carry out a lot of current or heat through the PCB board, conductive epoxies are available as well. When the via in pad is plated through and then filled with a conductive or non-conductive epoxy plugging ink, the epoxy ink is then planarized to create a flat surface and plated over to get a fully filled and capped vias in pads.

When you send inquiry with PCB design files to MADPCB for quote, it would be helpful to note the presence of any vias within component pads. These will be taken into account for the pricing and lead time you see on your quote, and the CAM engineer can help you to determine if specific vias on your board will require filling of any kind.

Via-in-Pad Fabrication Process

The Via in Pad technology proves to be a good technique to save real estate while making the layout of the PCB thus allowing denser PCBs. Not all PCB manufacturers can conduct via-in-pad manufacturing process as excellent as MADPCB. Here we list the main Via-in-Pad fabrication process for thru-hole PCB boards as below.

Maybe you find this via-in-pad process is for thru-hole boards. What's the microvia in pad process? Yes, but similar. In Microvia in Pad Process, the required selective microvias need to be fully filled plugged (/filled) with epoxy ink, then planarized on top or bottom copper layer, plated-over to get invisible capped microvia surface.

Manufacturable Via-in-Pad Technical Parameters

Parameters

Standard

Special Fab

Special Fab

Filling

IPC 4761 Type VII

IPC 4761 Type VII

None

Min Drill Diameter

0.2mm

0.15mm

0.1mm

Min Pad Diameter

0.4mm

0.35mm

0.3mm

Max Drill Diameter

0.5mm

0.4mm

-

Max Pad Diameter

0.7mm

0.6mm

-

Min Feature Pitch (Fp)

0.6mm

0.55mm

0.5mm

Aspect Ratio of Via

1:12

1:12

1:10

Aspect Ratio of Blind Via

1:1

1:1

1:1