intermediate process should not use no-clean flux... it should use  
low residue or no residue flux... if they used no clean flux, you  
need to clean it before put on assembly (your part of parts qual  
should including amount  of flux residue allowable on the part).  On  
the other hand, what you can do on the assembly level, when you got  
such a part...you need to use strong (corrosive  possibly) flux/paste  
and clean the assembly after if you have to (stuck with the parts or  
vendor).  IMHO.
good luck.
jk
On Aug 24, 2017, at 2:11 PM, Steven Kelly wrote:

> Hi All,
> I have a new hi-density BGA connector I am trying to assemble. When  
> the supplier makes the connector they "solder balls onto the  
> pin" (lead free). We see dried flux on the balls and when we solder  
> the connector we are getting a few joints with head in pillow. The  
> connector manufacturer says the dried flux is not an issue - is  
> this true? Thanks. Steve Kelly