Mr. Holton's reply (see the following) is right on target. Voids can be
caused by...
1. Curing the SMA too hot, too fast (particularly if trying to
simultaneously cure the adhesive and reflow the solder.)
2. Allowing the SMA to absorb moisture prior to cure.
3. Gross over-application of SMA such that it gets on the pads and mixes
with the solder paste.
Regarding moisture absorption, Loctite also has "less moisture sensitive"
SMA's (Chipbonders 3615, 3616, 3617, and 3618.)
Regards.
[log in to unmask] on 02/06/98 08:00:12 AM
Please respond to [log in to unmask]; Please respond to [log in to unmask]
To: [log in to unmask]
cc: (bcc: Darryl Small/LRH/Loctite Americas)
Subject: Re: [TN] Most weird problem: Solder between epoxy & fab or epoxy
& comp
I too have seen this problem in the past, the voids are caused by
outgassing of the material during cure or moisture within the adhesive or
in the PCB. If you cure the glue with a fast high temperature profile, the
problem is more likely to occur. For example, the adhesive can be cured
for 1.5 minutes at 150?C or 2.5 minutes at 125?C. The lower temperature
cure is better. Obviously you need to work with the thru-put of your
process, but slowing down the cure time worked for us and did not affect
thru-put.
The voids in the adhesive create a path for the flux and solder (via
capillary action), and i have seen shorts under components from one
termination to the other of pure solder, or worse yet, solder under the
component that did not create a short, but was close enough to the
termination that eventually a dendritic growth occured and completed the
short. HOWEVER, the main cause of the problem was that the adhesive dot
under the component was TOO BIG!! After the component was placed, the
adhesive dot covered the entire area under the component and onto the pads.
When the component was wave soldered, the pad became liquid, and due to
capillary action, solder was sucked into the adhesive voids, thus creating
the short. If you make your adhesive dot such that it does not touch the
pads or the end terminations then you have eliminated the path for the
solder, and thus no more shorts. With this design, even if you have voids
in the adhesive and solder is caught in the voids, it does not stretch from
pad to pad. Obviously it is best to eliminate the voids altogether.
Consider adhesive dot design:
With a single dot, make sure that it does not touch the pads or
terminations.
With a dual dot design: make sure that the dots do not touch the pads or
terminations. We also had another problem with dual dots, the gap between
the dots also created a pathway for flux or solder. You will find in many
books a suggestion to put a dummy trace between the pads. This acts as a
dam.
The adhesive could be a contributing factor. Hereaus had similiar problems
(as was mentioned with Loctite), but they have new formulations(PD944) to
help reduce the moisture and outgassing issues.
Ed Holton
Hella Electronics
[log in to unmask] on 02/05/98 09:02:04 PM
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To: [log in to unmask]
cc: (bcc: Ed Holton/Hella North America Inc.)
Subject: [TN] Most weird problem: Solder between epoxy & fab or epoxy &
comp
Dear Technetters,
I have seen the most weird problem in my 7 years in PCBA manufacturing
environment. Solder is getting between the epoxy and component and/or
between fab and epoxy thus shorting it out across terminations of SMT caps.
Does any one has seen similar problem? Any resolution? Our regular process
is epoxy application thr' stencil, component loading, curing and then hand
load & wave.
Any help or leads will be highly appreciated.
re,
ken patel
______________________________________________________
Ken Patel Phone: (408) 490-6804
1708 McCarthy Blvd. Fax: (408) 490-6859
Milpitas, CA 95035 Beeper: (888) 769-1808
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