Paresh Patel wrote:
"We have one SM chip on our boards which is put on by a contractor.
 Lately,
we are seeing excessive flux residues on and around the pads.  The
residues
have  look of frosted crystals.  The flux used is a no-clean flux.

Is it acceptable for these residues to be there?  Will it create any
problem
now or later?  What is acceptability standard?"


You should determine the nature of the residue before making any
conclusions but this information may be of some help.  Alliant
Techsystems participated in a CRADA, the final report is called
"Evaluation of Low-Residue Soldering for Military and Commercial
Applications:A Report from the Low-Residue Soldering Task Force."  In the   
    

study, comparisons were made between 'low residue' assemblies that were   
    

cleaned prior to conformal coat and those that were processed without a   
    

cleaning step.  One of the sub-groups processed at Alliant Techsystems
was soldered using low residue flux and then cleaned using a solution of   
    

75%IPA:25%water in an Ionograph.  I did not work on the project, but, if   
    

I remember correctly, this sub-group was 'ugly' ( the residue from the no   
    

clean flux was only partially removed and had turned white) but performed   
    

well in the testing.

At one time, copies of this report were being distributed by a member of   
    

the task force from Sandia National Labs but I think he has move on to a   
    

different organization.   The report has been summarized in several trade   
    

magazines.  If you need help locating a copy, you might ask the technical   
    

librarian at the EMPF ( 317-226-5623). Good luck.

Mary Davis
Sr. Materials & Process Engineer
Alliant Techsystems
206-356-3311
e-mail: [log in to unmask]




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