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July 2008

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Subject:
From:
"Creswick, Steven" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Creswick, Steven
Date:
Thu, 31 Jul 2008 05:45:35 -0400
Content-Type:
text/plain
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text/plain (162 lines)
Leif,

You have received so many great suggestions thus far that I feel
somewhat silly asking this, but ...

   You are using a metal squeegee, correct ... !?

I will go back to my corner now...

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Wednesday, July 30, 2008 12:31 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste with craters.

I have not see that kind of print anomaly. Obviously this will change
the
solder volume and it is undesirable. While the craters are not causing
the
fines. They could be symptomatic of a failure in the material that
results
in fines. 

The flux should dry and then the solder coalesce onto the feature at
reflow.
Some pastes fail in so that the flux runs all over carrying the solder
spheres with it off the pads. Then at reflow the flux burns off leaving
fines stranded everywhere.

I have run into this problem and I have receive bad solder paste from
reputable suppliers that do this fresh from a sealed jar. 

You could observe this with a BGA rework station, hot air or IR pen.
Compare
the problem paste with fresh paste and paste that does not produce
fines.
I think you will see a difference. 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Wednesday, July 30, 2008 11:59 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste with craters.

Hi Leif!

I have your pictures posted. Go to:

http://stevezeva.homestead.com/files/air_bubbles_and_craters_1.jpg

http://stevezeva.homestead.com/files/air_bubbles_and_craters_2.jpg

Is the percentage of metals the same as your leaded paste? I also think
the
rheology the paste won't be the same with as it is with the leaded
paste, so
there will most likely have to be changes with the set-up of the print.
It
almost looks like the separation speed is too fast, and the paste isn't
given time to pull away from the inside of the apertures causing the
crater...

Steve

-----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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