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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Conformal Coating BGA's

#32005

Conformal Coating BGA's | 12 January, 2005

I was looking to see how others are handling conformal coating PCBs containing BGAs. We have looked at underfills, but none meet the reworkability we need.

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#32010

Conformal Coating BGA's | 12 January, 2005

We conformal coat our boards and get pretty good coverage along the edges of BGA. If you need coverage along the edges and are not getting it, your supplier may be able to recommend a different material or application method.

Most often, underfill is used to help balance differences in thermal expansion rates between: * Board substrate * Unpackaged components [ie, bare die, flip chips, etc]

We have talked about reworkable underfills previously. Search the fine SMTnet Archives for contacts of suppliers that may meet your requirements.

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#32015

Conformal Coating BGA's | 12 January, 2005

The two options I see are (not including underfill):

1) Have complete conformal coating coverage of all balls with no webbing in between balls because of CTE mismatches. If all the balls are not coated we see failure in humidity testing.

or

2) Seal the solder balls prior to coat. This works, but is labor intensive. Plus, we haven't found a material that can be applied, touching the solder balls, and not deform them in thermal cycling. I'm thinking it would have to be pretty soft in order to compensate for the higher CTE.

If anyone has a better solution or material recommendation please let me know.

Thanks!

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#32032

Conformal Coating BGA's | 13 January, 2005

We agree that coating BGA solder balls with conformal coat is a bad idea due to differences between the z-dimension thermal expansion between the conformal coat and solder.

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Bob R.

#32085

Conformal Coating BGA's | 14 January, 2005

Whether or not conformal coating creates a reliability problem if it bridges the gap between the board and bottom of the part depends on the properties of the coating. A low modulus coating is fine, but one thats modulus increases at colder temperatures will fatique the bumps in thermal cycling.

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