Speaking entirely empirically, we've designed many PWB's w/BGA
components. The parts have ranged from ~80 to >500 pins. Most
of the applications have been in high-value/low-qty products, per-
haps 100's/yr.
As it has happened, 99% of those applications were in realms al-
ready reflow-over-wave with vias plugged-and-tented. I have had
some visibility into the post-assembly scenes and have not been
made aware of unintended reflowing or any reliability problems.
(Of course, some of these were designed with soldermask-defined
pads, and I haven't heard of problems associated with that, either ;)
There was one product destined for much higher production, with
the same setup, but the project got dropped at pre-production
stage. I don't think the costs associated with the plugging
were a factor.
In many of these cases there were in-circuit test requirements.
Faced with an already cleanly encapsulated PWB in the BGA areas,
we elected to add testpoints under the BGAs as non-drilled.
One recent product was set up for reflow/reflow, don't know the
results yet. Offhand I'm not sure why a BGA would present any
different accidental-reflow profile than any other part; large
QFP's have just as many vias packed almost as tightly together
and I think they have lower solder volumes as well.
Hope I've helped,
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
[log in to unmask] 508 649 9800
--------------------------------------------------------------------------
From: SMTP%"[log in to unmask]" "D. Rooke" 9-MAY-1996 22:14:47.84
To: [log in to unmask]
CC:
Subj: DES/ASSY : BGA design question
>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
([log in to unmask])
|
