24 years ago (yes 24!), I gave a paper at InterNepcon, in Brighton, UK, 
entitled "Contamination on Components  and Small Assemblies". I no 
longer have a copy, but I'm fairly sure I evoked the need to have all 
the components supplied clean to avoid just this kind of problem.

Brian

Steve Kelly wrote:
> Good Morning,
> 
> We are assembling a BGA connector onto a flex circuit. The BGA has leaded
> balls and the BGA manufacturer uses a no-clean process to attach the balls
> to the connector and does not clean afterwards. 
> 
> When we assemble the BGA connector to the flex we use water soluble paste
> and then we clean. After damp heat cycling we have seen some shorts. When we
> take off the connector we see flux residues on the underside of the
> connector but not on the flex.  So my question is - what are the concerns
> about mixing no-clean and water soluble? Is the failure likely due to
> inadequate cleaning or is there something else I should be concerned about.
> I realize this is probably not the way it should be done but some days you
> just do what the customer wants - good or bad. Thanks in advance. Steve
> Kelly
> 
>  
> 
> Steve Kelly
> 
> (416) 750-8433 (work)
> 
> (416) 750-0016 (fax)
> 
> (416) 577-8433 (cell)
> 
>  
> 
> 
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