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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