Hi Ed - in addition to Werner's comments, one other area that you might
investigate is your soldering profile. You stated " The unit thus far is a
ceramic substrate, silver palladium pads" - if your soldering process does
not have reasonable control of time/temperature then you could run the risk
of having the Ag/Pd metallization diffuse into you solder joints leaving
you with a solder connection sitting on top of a ceramic! A cross section
of a solder joint coupled with a SEM analysis would give you insight if
this is a possible cause. You could also conduct a Dissolution of
Metallization test on the substrate to see how if fairs. Good Luck.
Dave Hillman
Rockwell Collins
[log in to unmask]
[log in to unmask] on 04/24/98 07:23:39 AM
Please respond to [log in to unmask]; Please respond to [log in to unmask]
To: [log in to unmask]
cc: (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
Subject: Re: [TN] high temperature solders
Werner
You are right, the problem is the testing requirement, but guess what, the
test is real, and required by the Big Three for some electronic modules,
depending upon location in the car( the engine compartment)!!!! This
thermal shock condition is not to replicate actual conditions in the car,
you are right, a car will experience those temperatures, but would not see
that temperature transition, but this is an accelerated life testing. The
general public wants a car, and all its parts to last for many years. They
also don't like going to the dealership and being told that the problem is
a bad module or circuit board. Otherwise, cars that last a few years could
be easily built and sold (Yugo, Geo Metro, shall I go on!) Thus, to meet
the consumers needs, the push is to make electronic assemblies that will
last, 10 years is the targeted life. So. . .
Thanks for the suggestion about the leads. We are seriously considering
this idea. . .
I have received some excellent suggestions, thanks for everyone's help thus
far!!!
[log in to unmask] on 04/23/98 09:39:40 PM
To: [log in to unmask], Ed Holton/Hella North America Inc.
cc:
Subject: Re: [TN] high temperature solders
Hi Ed,
Your real problem is the testing requirement. While, for automotive
applications cycling of -40C to +140C with 30 minute dwell times makes
sense,
the10 sec transition most likely does not (where in an automobile to
you get a
temperature ramp of 18C/sec?even reflow soldering ramps are
limited to
2C/sec). It is this T-shock condition of 1080 C/minute that is
killing your
assemblies. You are setting up hugh transient warpages in both
components and
substrate putting the solder joints in tension. When I was
still at Bell Labs,
we once soldered two ceramic components together (to
assure that there was
absolutely no CTE-mismatch) and T-shocked them. Our
results were similar to
yours; both solder joints as well as metallizations
failed. Your problem is
neither solder related nor process related; it
comes from your sales people
accepting a job with a real crazy requirement.
On the other hand, I really would not want a car with some electronics
that
could withstand only 1000 cycles. From a design point-of-view, your
only possible solution is to put leads
onthese components that are very
compliant in the z-direction. Not a very
practical solution, not to speak
of the cost.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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