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April 2009

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Thu, 23 Apr 2009 13:35:02 -0500
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This is probably the result of multiple root causes.

First, the solder paste fines are trapped between the edge of the solder mask and the edge of the pad. Either the paste registration was off such that the squeegee pressed the paste in that space, and it just happens to be close to the size of the individual fine's average radius. This can also happen if the stencil printer pressure is too high.

Second, if the registration of the print was off, and the operator attempted to clean off the printed deposit in order to attempt a second print, but he/she simply wiped the paste off before cleaning the misprinted board, again the fines are wedged into the tight space between the pad and the soldermask. They will not wash away.

Third, I noticed that some of the fines did not reflow, but they are sitting up on top of the SJ, which did reflow. I suspect that these actually shifted when cleaning the PWB after reflow, not because the reflow temperature was insufficient. However, you should thermocouple the area and validate your reflow profile is hot enough to achieve full liquidus. Running the board too fast and trying to compensate by cranking up the temperature will sometimes lead to this condition, the PWB needs to achieve a good temperature soaking to ensure the paste all melts and agglomerates (and does not conglomerate).

If you are attempting to run solder paste that is Type 4 or higher, you need to utilize some type of aperture reduction in order to get all of the paste on top of the pad (5%-10% aperture reduction is typical), and not out on the PWB or in the space between the pad and the soldermask. In order to successfully run Type 4 or higher, your paste printing process needs to be repeatable to a sigma level of 5.3 or better, including the setups.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
Sent: Thursday, April 23, 2009 1:16 PM
To: [log in to unmask]
Subject: [TN] Solder Ball Problem

I am trying to get to the root of a little solder ball problem.  The problem
is specific to a single part and was evident on the first run of this board
a few months ago.  We are seeing it again on this run although the common
denominator here is that the PCB's are all from the same production batch.

It appears that the formation of solder balls can be reduced by pre-baking
the PCB's (100°C for 4-8 hrs) but not eliminated.  The best results are
obtained by pre-conditioning the PCB's by running them through the reflow
oven first,  letting them cool, then print, populate and re-flow.  The
majority of balls now formed are dealt with at wash.

The PCB's are simple double sided, HASL finish.  I suspect a problem with
the solder finish (HASL) but what exactly that "problem" is I am not sure.
FYI, paste is water soluble.

http://stevezeva.homestead.com/files/Solder_ball_1.jpg - this photo is of a
board that was pre-baked for about 4 hours @ 100°C, it was then printed and
re-flowed.  It does not matter if the 4 hour bake improved things you can
see that the results are pretty spectacular.

The following pictures are really more of the same except that these boards
have been populated and washed. 

http://stevezeva.homestead.com/files/Solder_ball_2.jpg
 
http://stevezeva.homestead.com/files/Solder_ball_3.jpg
 
http://stevezeva.homestead.com/files/Solder_ball_4.jpg

Any insight would be appreciated.

(thanks to Steve for hosting the photos)

Regards
John Parsons

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