Juan,

My guess is that the white stuff inside the contacts is what is typically referred to as "White Residue".  The best way I know to describe "White Residue" is that it is "decharacterized rosin".  During the soldering process the flux is heated to an elevated temperature and the outside surface or film of the flux residue that is exposed to the environment polymerizes or changes its physical properties and becomes "solvent insoluble".  When you try to clean white residue with a solvent like IPA that isn't very aggressive the rosin flux beneath the polymerized rosin dissolves and allows the "decharacterized" rosin to float downward and once it touches the surface of the contacts it sticks there and can't dissolved away.  You can try more aggressive solvents but they won't work very well.  The best way we've found to remove "White Residue" is to apply, by spraying or brushing, rosin flux to the "White Residue".  Yes, you did hear me correctly; put more flux on the contacts.  The trick is that the "decharacterized" rosin residue doesn't dissolve in the flux but it will float on the rosin.  Do not reheat the flux because you'll only form more white residue.  Wait several minutes to give the decharacterized residue to float on the rosin and then wash off the flux that has the white residue floating on it by aggressively spraying with a solvent. Chlorinated solvents work best because they aggressively remove the flux but IPA also works.

Regards,
George
George M. Wenger, Andrew Corporation
Reliability / FMA Engineer
Base Station & Subsystems Group
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Juan T. Marugán
Sent: Tuesday, September 06, 2005 6:33 PM
To: [log in to unmask]
Subject: [TN] Cleaning rosin flux residues

Hello teachers,

we have the following problem in some class 3 units: we have detected a
white stuff inside the contacts of female connectors. It could be the main
cause of the failures found during testing. Our thought is this substance
could be due to a reaction between the rosin flux used when soldered the
connectors whit the cleaning solvent (IPA)

Two questions:

1) Could you recommend a solvent to remove these residues. We have tried
with 141-b, but it could damage some parts of the unit.

2) We would like to send a contaminated connector to a lab for analisys.
Could you recommend a technique to determine these substances?

Thank you for your help.

Juan T. Marugán
Indra Sistemas SA
España

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