8/13/2019 Through Hole via in Pad

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Design Note

Through Hole Via in Pad

The need for making the printed circuit board smaller, putting vias in the pads may seem like a viable optionto gain a lot of space. If these vias are filled and plated over then - yes - it is a good design solution. If the

vias are open, then it is not a good solution.

Open vias can lead to tomb stoning, poor and missing solder connections and solder blobs on the back side

of the board. Open vias on BGAs can also lead to the solder material being sucked off of the pad.

This design note discusses via structures in BGA, LGA and similar devices, but in general - make your

design without through hole via in pad.

The very best advice is:

If possible - avoid through hole via in pad. If you cannot avoid it, then via plugging is the next best

solution.

IPC-4761 Design Guide for Protection of Printed Board Via Structures, says in sect.: 1.1: This guideline

provides designers, fabricators and/or users with information on existing methods for the protection of via

structures on printed boards. In addition to detailing some of the advantages of via protection, production

and materials issues are given to aid the user in evaluating the benefits and concerns for each type of

protection.

In general holes can be filled and plugged with special filling pastes, resin fromthe prepreg and conventional soldermask (LPI) see NOTE 1. Pastes with good

thermal conductivity are available.

Via holes can also be filled and plated over with copper (capped).

For BGA devices it is not recommended to have through hole vias right under

the solder balls. If the design makes it necesary, then the via holdes must be

plugged and plated over as a IPC-4761 type VII, filled and capped via. See

NOTE 2.

For LGA and similar devices via in pad design is slightly better than the design

without vias in the solder pad. If heat dissipation is not a concern, avoid vias in pad, if it is strongly desired,

again use IPC-4761 type VII filled and capped vias.

NOTE 1: Using conventional solder mask to cover through hole vias is not recommended. The amount of

filling is not predictable and the variation from PCB batch to - batch is a serious concern.

NOTE 2: The additional cost for filled and capped vias is between 5% - 25% of the PCB cost, the more

complex design the lower is the additional cost percentage.

As a capped via means an extra copper plating process, etching may be an issue for line/space < 100

micron.

BB Electronics A/S

Via in Pad, September 2012

Peer Fisker