Phil:

A white residue is a sign of a contaminated surface.  Although, I 
do not know if the presence of lead carbonate will cause 
dewetting, I can explain a possible source of the lead carbonate.

A chlorinated hot air leveling flux may leave an HCl residue 
which,if not thoroughly cleaned off the board, 
will attack the lead (Pb) in the solder producing lead 
chloride:

     Pb + 2HCl --> PbCl2 + 2H+

The lead chloride will adsorb water and carbon dioxide from the 
atmosphere and form lead carbonate, releasing hydrochloric acid, 
which will attack additional lead in the solder producing lead 
chloride:

     PbCl2 + H2O + CO2 --> PbCl2 + H2CO3 --> PbCO3 + 2HCl

     Pb + 2HCl --> PbCl2 + 2H+


This chain reaction can continue until all lead on the surface is 
consumed.  Lead carbonate is a white material and may be the 
white residue you are observing.

If you have any questions, please give me a call.

Harry

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______________________________ Reply Separator _________________________________
Subject: Dewetting & Pumice Scrub
Author:  [log in to unmask] at SMTPLINK
Date:    9/18/96 10:02 AM


Technetters 

In the mid 1970's Paul Davis of Tin Research wrote an article on dewetting as
result of pumice scrub.  Does anyone have a reference to where this was
published.  Also does anyone have any input as to dewetting as result of
pumice scrub in relation to a sequence of: (1) pumice scrub, (2)apply solder
mask, (3) hot air level, (4)solderability test and dewet..

Another question on dewetting.  Has anybody seen dewetting as a result of a
white residue on the lands and board which was analyzed as lead carbonate?
 And another:where does the lead carbonate come ?

Phil Hinton 
Hinton PWB Engineering 
[log in to unmask] 

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