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Question about solder masking BGA vias...

Steve Gregory

#30597

Question about solder masking BGA vias... | 18 September, 2004

I have a question about solder masking via's in a BGA pattern, and want to see if I'm thinking correctly about this. This problem is kind of along the same line as Carol's problem earlier.

I think it's a bad idea to leave the via's free from solder mask on the topside of the board beneath the BGA, and tent the bottomside via's. To me, you're just asking for trouble down the road because you'll trap flux residue and all kinds of stuff in the via's that won't be able to be cleaned out well. This is a 676 ball 1.0mm pitch BGA pattern. The board finish is ENIG not HASL. Am I wrong about this?

I am going to be using a no-clean paste, but I will be cleaning the boards using Kyzen Aquanox 4512P in a Aqueous Technologies SMT800 batch cleaner. I'm not expecting to be able to clean beneath the BGA at all with this pitch and number of balls.

Our customer seems to think that there shouldn't be a problem, but if I'm really concerned about this, they want me to show them some sort of documented proof...can anybody help me with this?

Thanks!

-Steve Gregory-

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#30605

Question about solder masking BGA vias... | 20 September, 2004

The Intel BGA Developer�s Guide [ http://developer.intel.com/design/packtech/ch_14.pdf ] says:

14.8.3.3 Plated Through Hole (PTH) Isolation Regardless of the technique used for the mounting pads shape or definition, isolation of the plated through hole (PTH) from the mounting pad is important. If the PTH is contained within the mounting pad, solder will wick down the PTH. The amount of solder that wicks depends on many factors, including PTH finish and coating variations. Because of this, the results are somewhat unpredictable. Some solder joints may be unaffected, while others will be starved to the point of creating opens. The worst result is a partially starved joint with severely reduced cross section. This joint can have significantly lower fatigue life and result in early system failure. Because the quality of the solder joint is guaranteed by control rather than inspection, designs/processes that result in random distributions are generally considered unacceptable, and the PTH-in-pad design is not suggested. All vias located between the BGA ball pads must be covered with solder mask. It is suggested that, at minimum, vias on the top side be covered with solder mask. The bottom side can also be covered.

Additionally, from our notes [unsure of the exact source], Intel's thoughts on vias: * All vias located between the BGA ball pads must be covered with solder mask. This will prevent solder from wicking over to the via pad. A via pad size of 24 thou is recommended. * Regardless of the technique used for the mounting pad shape or definition, isolation of the plated through hole [PTH] from the mounting pad is important. If the PTH is contained within the mounting pad, solder will wick down the PTH. The amount of solder that wicks depends on many factors, including PTH finish and coating variations. Because of this, the results are somewhat unpredictable. Some solder connections may be unaffected, while others will be starved to the point of creating opens. * The worst result is a partially starved connection with severely reduced cross section. This connection can have significantly lower fatigue life and result in early system failure. * Because the quality of the solder connection is guaranteed by control rather than inspection, designs / processes that result in random distributions are generally considered unacceptable, and the PTH-in-pad design is not recommended. All via located between PBGA pads must be covered with solder mask. It is suggested that as a minimum, vias on the top-side be covered with solder mask. The bottom side can also be covered, but left the customer's discretion.

Other BGA fabs probably give the same guidelines. Why wouldn't they?

Beyond that, as we posted on Carol's thread, 2221A, 4.5.1 requires top and bottom side via tenting.

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