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May 2010

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From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 20 May 2010 16:52:30 -0500
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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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"Thayer, Wayne - IIW" <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
05/20/2010 03:17 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Thayer, Wayne - IIW" <[log in to unmask]>


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Subject
[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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