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Hi Wayne - well, "it depends" ($$$$ for Doug's Diet Mt. Dew fund) on the
component weight versus solder buoyancy. We measured a whole bunch of
CLCCs is the late 80's as part of a process investigation and found out
the typical solder joint standoff was 3-4 mils for devices with 20-44 I/O.
I would suspect that if you measured a statistically valid number of
samples of a specific component type, you would find that there is a
typical standoff that is consistently achieved for a given solderpaste
stencil and pad pattern set. This is a very interesting question as the
IPC specifications always list "evidence of wetting" and not a specific
value.
Dave Hillman
Rockwell Collins
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[TN] Solder joint height
Should there be a spec for minimum solder joint height for leadless
components? An old paper from Werner (mid 90's, I believe) popped up on
the Internet when I queried this, and Werner has a statement in it that
joints less than about 50 microns thick are basically all intermetallics,
but that is not necessarily a bad thing since the intermetallics are
stronger than the solder (but more brittle).
I figured since we often try to get additional height for BGA joints to
improve reliability, we should be concerned about QFNs and LCCs.
I grabbed a 1.5mil (37 micron) tungsten rod out of our cleanroom (used for
clearing clogged wirebond capillaries)and started seeing if I could get
that under my leadless components. Couldn't get it under any of them,
likewise with the QFNs.
Checked Technet archives and didn't see an obvious thread, but maybe I
didn't have the correct search parameters--if you just enter "height" you
get a couple of thousand hits.
Wayne Thayer
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