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July 2008

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Subject:
From:
Kevin Glidden <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kevin Glidden <[log in to unmask]>
Date:
Fri, 25 Jul 2008 12:04:30 -0400
Content-Type:
text/plain
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Jean,

That is a very loaded question.  The issue at hand here is not so much that
the flux is water soluble or not, it is the activity level of the flux.
Typically, water soluble fluxes are an M or H activity level.  And, again,
typically, the more active the flux, the more harmful it can be if the
resides are left in place.  You will find that many of the experienced
experts and reliability guru's alike will respond that the #1 KEY to the
whole thing is that you can do whatever you like in your manufacturing
process, as long as you can PROVE it yields a robust and reliable product.
That boils down simply to ensuring a robust cleaning process to remove ALL
flux residues and to characterize any remaining residues (via Ion
Chromatography and / or SIR testing) as benign or harmful.  In fact, this is
a requirement of J-STD-001D section 3.1.  The testing protocols are in
Appendix C.   The last step is on-going process monitoring to ensure you
stay within your established limits. This can be costly and time involved
(consult a qualified laboratory for assistance), but the diligence pays in
dividends.  At the simplest level, we do not allow use of water soluble flux
(high activity level) for ANY rework.

Kevin Glidden
Manufacturing Engineer
Luminescent Systems Inc.

-----Original Message-----
From: Jean-François Bissonnette [mailto:[log in to unmask]] 
Sent: Friday, July 25, 2008 9:59 AM
To: [log in to unmask]
Subject: [TN] Cleaning WS fluxes and other evil

Hello Technetters,

 

We experienced some problems after reworking circuit boards, I would like to
have inputs on the following.

 

The problem found is capacitor leakage.  The capacitor was added manually
after automated production.  The boards were coated with acrylic conformal
coating.

 

1-       How should a WS flux be cleaned after a rework?

2-       Can WS be cleaned with alcohol?  If not what would happen?

3-       When reworking a SMT component, can WS flux be trapped under the
component and cause leakage.  The rework was cleaned with a brush or similar
method.

 

We think that the rework method was as follow (to fix a possible
contamination under the component)

a-       SMT component is removed (possibly directly through the conformal
coating)

b-       Using coating thinner, the area is cleaned and verified under UV to
ensure no coating is present.

c-       WS flux is added

d-       Component is soldered back in place

e-       Area is cleaned with isopropyl alcohol

f-         Coating is added

 

I think I've read that cleaning WS flux with alcohol is a NO NO!  Can anyone
confirm and comment? 

 

Jean-Francois 

 


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