Hi Peter,

When vapour phase soldering was "normal" through about 1991 it was always
the practice to pre-heat assemblies to (at that time) drive off solvents.

It will always be the case that if you introduce a product with a component
(solvent - water etc) boiling point way below the soldering temperature into
a soldering temperature environment with sufficiently fast transfer
capability as is the case with your vapour bath, that you will get an
explosive out gassing as the low boiling point component turns to vapor.

In your case you are probably introducing a component with a boiling point
of 100C or lower into vapor with a temperature of 230C.

This is the same effect you will see if you drop one drop of water into a
pan of hot oil.

My advice with vapor phase is and always was to pre-heat before soldering to
drive off the volatiles.

John

 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter Barton
Sent: Friday, June 20, 2008 12:05 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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