Krishna,

KC Teo whilst at Aztech (now at Philips CFT) observed similar defects
with fine pitch QFP's. The shorts drew solder away from the foot of
the lead leaving a lean joint, so rework had to do more than just
remove the short. Lean joints were also sometimes observed on adjacent
leads, indicating that the short must have a some point in the reflow
cycle have spanned more than just the two leads shorted. These were
often missed by rework operators. The occurance of these shorts shows
that the solder paste must have bridged due to e.g. printing defects/
paste displacement during placement or slumping. These observations
led to a detailed study of the effects of various process variables
(e.g. stencil thickness, aperture shape, flux type) on the occurance of
opens and shorts which are reported in Journal of Electronic Manuf.
They also led to our collaborating in modelling of short stability
using the Evolver, the results of which have also been reported in JEM.

David Whalley

>I hope all of you had a good weekend and are back for an exciting week.
>I had a strange problem of solder short a few days ago. The shorts were
>between 2 pins of an SOIC towards the plastic body, and not at the board
>level. Can the soldering experts of TechNet help me understand this
>strange phenomenon ?

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