The most usual cause of dewetting is myriad tiny unwettable areas in an 
otherwise wettable metal. A very common cause is abrasive cleaning, 
where abrasive particles are implanted into the copper surface, but 
there are other causes. What happens is that IMCs are formed where the 
e.g. copper is solderable and this bridges over the contaminants. When 
the solder flows away, the bridging breaks down, leaving areas with 
thickish solder and areas with just the residual IMC, which is very thin.

Brian

Louis Hart wrote:
> TechNetters, would someone fluent in soldering and terminology please advise or comment? In the IPC T-50H definition of dewetting, there is mention of a 'thin coating of solder' on the base metal. Pictures of dewetting in A-600 appear to show exposed base metal. A customer is complaining about dewetting on some PCBs that have all surfaces covered with solder, thin in some regions, thick in others. I can't see clearly the contact angle between the thick solder and thinner areas, but it may not be much different from 90 degrees. To me, it appears solderability should be OK, but I want to understand the dewetting phenomenon better before discussing with the customer.
> 
> By what physical mechanism does dewetting occur? How does this mechanism differ from nonwetting?
> 
> Thanks for any guidance...Louis
> 
> Louis Hart
> Compunetics
> Monroeville, PA
> USA
> [log in to unmask]<mailto:[log in to unmask]>
> 412-858-1272
> 
> 
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