Earl Moon

Earl, I guess I am naive about long-term reliability issue of
placing 603's on 402 pads.  This is not the way to ensure fewer
cracked solder joints and to increase long term reliability.

Processbility and reliability of 603 chips on 603 pads are far
superior with few defects.  One issue I agree is that bulky
solder fillets do not mean a better reliability than smaller
fillets.  It could be, some cases would cause more cracked
joints.

The way I worked out the issue is to use 603 chips on 603 pads,
805's on 805 pads, and 1206's on 1206 pads, but reduce
solderpaste stencil apertures considerably (and/or a thinner
stencil than 8 mils).  This would permit smaller solder fillets by
depositing less paste.  Paste volume is about 60% to 70% than
one to one stencil opening.

This makes SMT inspectors and QA happy, and ensures enough
fillets for mechanical integrity and visual acceptance for long
term reliability.

Matthew
Norsat International Inc.

>>> <[log in to unmask]> March 2, 1999  1:33 pm >>>
We've talked about this before and other mechanisms effecting
defect in chip devices. The company
I'm now with goes to a little extreme - though having researched
this over the years. They even use
0402 pads and dimensions to mount, reflow, and operate
0603's.

There will be a change when moving to wave solderable, glue
mounted 0603 types to allow dispensing.
However, we use smaller pads to ensure smaller fillets to
ensure fewer cracked devices and longer
term reliability.

Earl Moon

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