Technetters,

All of our boards include BGAs. Many of these animals are rather big
(896 to 1410 balls) with full matrix balls, and running high frequencies
(up to 2 GHz). The designers insist on mounting plenty of bypass
capacitors under the BGA, on the opposite side, to shorten the filtering
trace length. This means that I find myself with through via in the
components (0402) pads. Workmanship problems are not their concern, they
tell me while walking to their morning coffee.

So, I am evaluating the technology of filled via. As a side effect, I
may find myself being praised by the designers - the first time since
ages - because of clearing some extra space used before for vias. This
will make my day.

What I had in mind is to define the following process:
1.      Produce the board until, inclusive, via holes plating.
2.      Fill the vias and polish the fillers.
3.      Coat externally with another copper layer
4.      Drill for the through holes components
5.      Coat with copper and etch the external layer again.

Did anyone experience with filled vias in PCBs of this thickness? Was it
a good experience, or would you prefer to forget this nightmare?

As of the foot print of the components: The board thickness is 3 mm,
hence (aspect ratio 1:10) the minimum drill is 0.3 mm (12 mil); the
resulted (annular ring of 0.125 mm) pads diameter shall be 0.55 mm (22
mil). There are two options:
1.      To use this pad for the 0402 components without modifications
2.      Square the inner part of the pad (the side under the component).

The pads design must conform also to ROHS processes. Which of the pad
options is better? Or may be - another alternative should be preferred?

Regards
Ofer Cohen
Manager
Quality Assurance, Reliability and Production Technologies
Seabridge Ltd. - A Siemens Company
Siemens COM FN A SB 

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