All of this makes perfect sense if you consider the mechanisms.

ImSn has a shelf life, which is controlled by the spontaneous formation of the Tin/Copper intermetallic, which begins to form as the Tin is plated at the interface.   When the Tin has totally converted to the Tin/Copper intermetallic, it can no longer be soldered.

Heat accelerates the formation of the intermetallic.

Your first pass is accelerating the formation of the intermetallic dramatically.   You are likely simply converting the whole of the ImSn to intermetallic, and it is game over.

Rudy Sedlak
RD Chemical Company


--- On Fri, 7/4/08, Rex Waygood <[log in to unmask]> wrote:

> From: Rex Waygood <[log in to unmask]>
> Subject: [TN] Immersion Tin Problem
> To: [log in to unmask]
> Date: Friday, July 4, 2008, 2:35 AM
> We do not usually use Immersion Tin finish boards but we
> have some with
> a problem.
> The boards are about 10-12 weeks old, stored in original
> packing, double
> sided PTH and lead free processed.
> Side 1 processed OK
> Side 2 had some non-wet problems (Profiles and dwells etc
> are industry
> 'norm')
> Hand soldering has proved to be nigh on impossible.
> The immersion tin is within the thickness specified
> (typical values 1.1
> to 1.2 micron)
> 
> Should we have avoided ImSn (as we have done previously)?
> 
> Is the spec for the ImSn OK?
> 
> Has the PCB mfr done something wrong?
> 
> Have we? (apart from accepting ImSn)
> 
> Rex
> 
> 
> Rex Waygood
> Technical Manager
>  
> PartnerTech Poole Ltd 
> Benson Road
> Poole
> Dorset BH17 0RY
> United Kingdom
>  
> Tel: +44 (0)1202 674333
> Fax: +44 (0)1202 678028
> DDI: +44 (0)1202 338222
> Mob: +44 (0)7887 997403
>  
> [log in to unmask]
> www.PartnerTech.co.uk
> 
> 
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