Not being an expert on assembly... let alone BGA assembly, I would like to
ask the help of my fellow technetter's on the following issue :

In a recent conversation with a customer regarding BGA assembly, the
following observations/concerns were raised :

1) Does the presence of a region of high density plated through via holes
under the BGA device, conduct sufficient heat during bottom side wave
solder, to jeapordize the BGA solder attachement? This customer is designing
a BGA with bottom side glue and wave, and the concern is that a second
reflow of the ball joints could corrupt the joint integrity of the top-side BGA.

2) In such a scenario, the need of a turbulent wave for bottom side wave
solder is believed to cause solder to flow up the vias causing solder shorts
or potential solder bridges on the top side, under the BGA. For this reason
the customer is considering solder mask plugging on the bottom side of the
boards.

3) Solder mask plugging is also considered to prevent flux entrapment under
the BGA, due to potentially corrosive flux contamination flowing through the
via and under the BGA device during wave solder.


Can anyone help me on what is the current "state of the union" on BGA
assembly? Is plugging of vias considered mandatory or is it dependent on the
assembly process (ie flux type)? Is the top side, bottom side or both sides
plugged with solder mask? Do people design boards with BGA AND bottom side
wave solder? How about double sided paste and reflow with BGA?

All suggestions, comments and advice would be greatly appreciated. 

Dave Rooke
Circo Craft - Montreal Canada
D. Rooke
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