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July 2008

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 30 Jul 2008 08:53:42 -0500
Content-Type:
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Hi, Leif
Send your pictures to [log in to unmask]
 
Can you tell me what solder paste it is you are using? How was it
qualified for use?

Here are just some of the questions you need to ask yourself in order to
determine why you have an issue with solder fines:
Do you have a good documented solder paste handling procedure? How is
the paste handled, from vendor or distributor to your factory? When was
the last time you stopped in at the distributor to verify their stock is
being rotated and is kept refrigerated immediately upon receipt from the
factory? How often do they turn the packages upside down to prevent flux
separation? Are they a certified distributor who will pass on to you a
lot recall notice from your solder paste vendor if there is a known bad
lot?
How long is the paste allowed to sit out on the stencil and how many
times can a line of paste on the stencil be sheared (printed back and
forth) before it is removed and replenished with fresh paste? Are the
operators allowed to scrape up the unused paste on the stencil and
re-deposit it into a jar for re-use later? Is the jar or tube of solder
paste, once removed from refrigeration, allowed to set for two to four
hours (depending on paste vendor and paste type) to reach room
temperature prior to printing? Once removed from the refrigerator, is
unused solder paste allowed to be put back in the refrigerator? Are you
using Type 4 paste or Type 3? How good is the printer setup, ie, the
repeatability of the registration of the stencil to the PWB? Are you
performing some type of aperture reduction on all pads in general and at
least a 50% reduction on large belly pads to prevent solder fines from
being printed onto the board? If a board is misprinted, does the
operator know better than to simply wipe off the board (embedding the
paste into every space between the edges of the pads and the soldermask,
into every small via, into every through hole, etc.) but is there a
documented procedure detailing how the misprinted board is to be cleaned
to prevent this? How is the solder paste packaged, jar or tube? (Tubes
prevent a much larger volume of paste from being exposed to air and
humidity, and also help prevent re-use of solder paste that has been out
for awhile). Do you perform a good solder paste print inspection using a
3d AOI or some other type of automated inspection, and do you use the
data from this inspection process to detect (real-time) paste defect
trends and react to them with corrective actions?

Sorry to ask so many questions, but all of these can contribute to
fines, and there are many more factors that can cause them.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
Sent: Wednesday, July 30, 2008 8:29 AM
To: [log in to unmask]
Subject: [TN] Solder Paste with craters.

Technetters,

We are having an issue with solder fines on our lead free boards. We are
using SAC305 WS and no nitrogen. I noticed that our boards have craters
in the paste on the pads after being printed. Not all pads are deposited
this way, but maybe 25%. These craters have an air bubble in them. The
bubbles usually burst before the boards goes into the P&P, but the
crater stays. The screening process is exactly the same for leaded and
unleaded paste. The leaded paste does not behave this way. Could these
craters be a symptom of the cause of the solder fines? Anyone have any
experience with this?

How do I go about posting pictures to stevezeva.homestead.com?.


--
Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com

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