Hi Pete and Steve,
Just to make sure that misinformation does not get disseminated.In a message
dated 05/30/2001 23:44:39, [log in to unmask] writes:
>BGA's, especially CBGA's have given us a real headache, mostly because the
>assembly process has to be spot on. We're making VME conduction-cooled
>cards with both PBGA's (35x35mm 1.27mm pitch) and CBGA's (21x21mm 1.27mm
>pitch), that are used in an avionics box. We had some boards made where the
>manufacturing process was OK for the other components but the BGA solder
>joints looked a little rough for avionics quality standards. They survived
>initial temperature cycling, but failed very quickly with only gentle,
>single axis vibration perpendicular to the board surface. The poor joints
>were blamed and we're working on improving the process with a different
>board stuffer.
Vibration failures do not occur in real solder joints, that is solder joints
that truly metallurgically bond. Your failures are, I bet, interfacial
failures of "SJs" that never properly wetted. Most likely bec ause the CBGAs
have a larger thermal mass and acted as heat sinks letting the solder balls
never reach proper reflow conditions.
>From more long-term experience in a previous prison camp, it was LCC's,
>particularly 20 pin LCC's that showed signs of solder joints cracking under
>qualification testing (15g accelerations and thermal cycling from -40 to
>+125 deg). The problem was improved by putting a pad underneath to lift
>them off the board surface and using a stepped solder stencil to deposit 10
>mils paste on the pads for these devices. The higher solder joints gave
>greater flexibility.
Nothing new about reducing the strains in solder joints from a given thermal
expansion mismatch by making them taller. It is NOT greater "flexibility,"
but reduced strain.
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], Website: www.engelmaier.com
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