Hi Werner,
I'm a little confused on the solder wetting. Can solder become liquidous
and not form a metallurgical bond because either not enough time while
liquidous or insufficient peak temperature? Or is a metallurgical bond
formed as soon as the solder becomes liquidous unless some other controlling
factor prevents the bond?
Kind Regards
Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]
> -----Original Message-----
> From: Werner Engelmaier [SMTP:[log in to unmask]]
> Sent: Thursday, May 31, 2001 4:05 PM
> To: [log in to unmask]
> Subject: Re: [TN] Package reliabilty...
>
> 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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