Steve,
In another life I worked for Bendix Engine Controls, using polyimide 12 layer boards
between .060 and .080 thick, depending upon the module. These had what we
called "heat rails" bonded to them, which were copper stiffners .050 thick from what
I recall. All of our DIP and axial components were placed over the copper rails
for conductive heat dissipation purposes. These boards were roughly 7" x 8".
The reason we used copper rails with polyimide boards was for matching thermal
coefficients as closely as we could. Of course, these were units that were fuel
cooled, and the copper was clamped to a "cold wall" that had fuel flowing through
it. So we had a pretty extreme temperature range to deal with. We could not use
either aluminum for the stiffner or FR4 material for the circuit board because of
the potentially severe consequences of thermal mismatch.
We also pre-tinned all gold plated side brazed DIP's in a solder pot that we could
afford to get "gold polluted" before inserting in to the PWB. This was to prevent
the type of solder cracking that you described.
In summary, you may have two problems: An overly severe thermal mismatch
between the aluminum stiffner and the PWB/DIP lead combination, and what we
called "gold pollution" of the solder joints.
Hope this helps...
Arthur Kahlich
Solliday Engineering Corp.
Phone: (415)621-0616
Email: [log in to unmask]
-----Original Message-----
From: [log in to unmask] [SMTP:[log in to unmask]]
Sent: Wednesday, May 06, 1998 10:29 AM
To: [log in to unmask]
Subject: [TN] Solder Fracture Problems
All,
Stick with me on this...it gets kind of lengthy....
About a year ago, I posted a problem to the Tech-net regarding
solder fractures. At the time I got a lot of information on the subject.
Enough to help guide me in the right direction.
The PWB is a Polyimide material, .070" thick with 12 layers. All layers
are 1oz copper. The PWB has a bonded stiffener, 6061-T6 aluminum @.040"
thick. Board dimensions are 6" x 3.875".
Some components are placed over the stiffener, some are not. The
assemblies are prepared with straight through lead termination's and are
wave soldered, some post assembly performed, cleaned, and inspected. All
the assemblies exhibit good wetting and generally good solder
characteristics at this point.
The fractures all appear on gold plated, side brazed lead DIPs that are
sitting directly on the stiffener. The fractures occur mostly at the lead
fillet, but occasionally happen near the pad fillet Also worth noting is
that the fractures occur on the side of the leads (the thin portion of the
lead) with the more severe ones occurring near the ends of the DIPs and
lessening as you move toward the middle.
It turns out that because of the differences in Cte between the PWB and the
bonded stiffener, the board is warping during wave solder. This warp is
basically soldered in place which places the solder under constant loading
as the board tries to return to it's room temperature dimensions. The side
brazed DIPs do not provide sufficient stress relief and eventually the
solder fatigues and fractures at the weakest point.
Get to the point...what is your question you ask. Is there a way to
eliminate this warp during wave solder or will attaching another material
to the PWB only make things worse?? No we can't get rid of the side brazed
DIPs because they are a military part only manufactured in that
configuration.
Any help you can provide will be greatly appreciated.
Steve McBride
Frontier Electronic Systems
P.O. Box 1023 Stillwater Ok.74076
[log in to unmask]
(405) 624-5281
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