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June 2008

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Fri, 20 Jun 2008 11:40:48 +0300
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Many years ago I saw this happening, in the days when CFC-113 was used 
as a secondary blanket. The preheating by the CFC-113 was at too low a 
temperature. If your machine has a secondary vapour zone, I suggest you 
find a product with a BP of ~110°C and keep the assembly in that zone 
until it reaches equilibrium temp, before it goes into the reflow zone. 
If the machine is single zone, then you will just have to preheat the 
assemblies a bit more. There are no miracles.

Brian

Peter Barton wrote:
> 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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