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June 2008

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Subject:
From:
Peter Barton <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Peter Barton <[log in to unmask]>
Date:
Fri, 20 Jun 2008 08:05:00 +0100
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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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