My experience as well. In addition, while we were qualifying a WS paste we
encountered a shelf life issue that resulted in spattering and fines. It
seemed not to matter if the jar was opened. Two jars from the same lot. 

Opened one:
Used it for a week in trails.
Initially very good results. 
5 days in; solder fines after reflow that washed off.
7 days in; spattering, found solder on gold pads.

Opened second jar:
Day one; solder fines and spattering. 

We selected to another paste MFG. 

Guy

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, June 23, 2008 9:19 AM
To: [log in to unmask]
Subject: Re: [TN] Paste 'spattering' issue.

I can tell you from having performed dozens of paste qualifications that
there are a couple in the WS category that absorb moisture like a sponge.
They also tend to have many voiding issues. You may wish to try a different
paste. Contact me offline for recommendations. I have no interest in the
solder paste, I simply know which ones work best for different applications.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter Barton
Sent: Friday, June 20, 2008 2:05 AM
To: [log in to unmask]
Subject: [TN] Paste 'spattering' issue.

We are seeing an interesting phenomenon when using water soluble solder
paste on assemblies being reflowed in a vapour phase reflow process.

After reflow the assemblies show evidence of solder 'spattering' on the
assembly, not good news when there are large areas of exposed Gold contact
areas and components near to the soldered joints that must remain free of
ANY solder contamination.

The only way to prevent this spattering from happening at all is to bake the
assemblies after population and before reflow at 70 deg. C for around 15
minutes.

We do not see this phenomenon if the assemblies are reflowed in a convection
oven. Unfortunately this process is not viable for the assembly in question.

It is a large thermal mass and has heavy fixturing.

My assumption is that in the VP reflow process any water that could have
been absorbed from the atmosphere into the paste is effectively sealed in
when the hot vapour condenses on to the assembly. It then superheats as the
latent heat is given up and then effectively 'explodes' when the vapour
pressure overcomes the surface tension of the condensed vapour phase fluid,
thus spattering the solder. I do not believe that it is due to ramp rates as
the process ramp rate is less than 2 deg. C per second
- well within the recommended processing parameters.

The problem is not dependent on the age of the paste, it can happen from a
properly conditioned freshly opened pot onwards.

I do not have the option of sealing the placement process from the factory
atmosphere as there is manual population of odd form parts. That has a 15
minute cycle time. The factory atmosphere itself is within the normal realms
of temperature and humidity.

Also I cannot change to a different type of paste. It must be water soluble.

I have discussed this the paste manufacturer and they have not come across
this phenomenon before. The only advice so far is to lengthen the preheat
time but this gives no measurable improvement.

Has anybody else encountered this issue, and if so, have you found any other
solutions?





-----------------------------------------------------------------------
Peter Barton
Senior Process Engineer
ACW Technology Ltd
Dinas Isaf West
Tonypandy
Mid Glamorgan. CF40 1XX  Wales

Tel: 01443 425275 (direct)
Fax:  023 8048 4882
International Tel : +44 1443 425200
International Fax : +44 23 8048 4882
Website/URL:  www.acw.co.uk

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