In a message dated 02/16/2000 10:23:20 AM Central Standard Time,
[log in to unmask] writes:

<< Hello Technet,

 I was wondering, is anybody out there doing 0.8 mm pitch BGA's on boards
that have to meet the IPC class 3 assembly requirements ?  My concern is that
with the thinner stencil that's needed to print the solder paste for these
FBGA's, there will be not enough solder for some of the other components.
 I think that a step-etched stencil is no option, since there is not enough
free area around the FBGA.

 By the way, does anyone have experience with the behaviour of fine-pitch
BGA's during shock and vibration ?

 Hope to hear from some of you !

 Daan Terstegge
 Unclassified mail
 visit my website: http://surf.to/smtinfo >>

Hi Daan!

I've built a few assemblies with micro-BGA devices on them and I know what
you're talking about.

Typically I use a 4-mil thick stencil, and what I ask to be done with the
stencils I get made with a micro-BGA footprint, is to do what my stencil
vendor calls a "solder paste compensation". Which in essence is to allow the
apertures to be just ever so slightly larger than the pads for all the 50-mil
stuff.

During reflow the paste will wick back to the pads giving you volume back
that you've lost reducing the thickness of the foil.

Also, in case you haven't tried it yet, I always have the micro-BGA apetures
in the stencil turned into squares instead of round, that really works well
for good paste release from the stencil.

That last board I built with a micro-BGA is a hi-rel board (but it was just a
prototype) and I did have one part that was on the verge of being
insufficient. It was those military style tantalum capacitors, you may have
seen them before...you know those orange ones with the little tit sticking
out on the polarized end. Those things need a lot of solder to make a good
fillet. Might try a 5-mil stencil on my next go...

Haven't "shaked and baked" any assemblies with micro-BGA on them yet, so I
don't have any direct knowledge. But as you may have read at the Tessera
page, there's some good thermal cycling performance that's talked about.

-Steve Gregory-

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