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April 1998

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From:
Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 21 Apr 1998 13:33:14 EDT
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Dear "straight and narrow path',
Sorry, but I have to come down on the side of your friend. The pad sizes for
chip components are derived primarily from wave soldering needs where pad
areas need to be large enough to be touched by the wave. If you do reflow,
that is totally unnecessary. In fact, studies in Germany have shown
reliability improvements with both narrower pads as well as placements that
are off-center. Both of these conditions result in smaller solder fillets. It
is large solder fillets that frequently cause the cracking of capacitior
chips, particularly in cold temperature operation or testing. Narrower pads
also reduce the likelihood of dombstoning—again smaller fillets=lower surface
tensions.  In fact, for these components there is not technically justifiable
need for fillets. For larger chip components on FR-4, you would get more
solder joint reliability by producing solder joints with larger solder joint
heights between PCB and component (remember solder columns for CBGAs).

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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