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July 2008

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Subject:
From:
Mike Fenner <[log in to unmask]>
Reply To:
Date:
Mon, 28 Jul 2008 10:59:51 +0100
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Unless you are manufacturing with WS fluxes, I have to ask "Why are you
doing this? What benefits do you think a water wash flux is bringing you?
Why not use a No clean specially designed for repair flux?"

I guess this is the scenario behind your question
Uncleaned residues, especially water wash, typically can manifest themselves
visually first on ceramic caps. 
What you are most likely seeing though is an indirect effect which is Ag
migration rather than direct corrosion from the flux residues.
YOU can get this from any improper cleaning process, not just water wash.

You meet all the criteria, nice bit of damp, held in place by coating, by
definition you have a potential difference across the device and a nice
supply of silver from the terminations, add in a nice conductive path from
part washed residues and bingo after a relatively short time you have pretty
patterns on device tops.

YOU usually see it first on the larger devices, which is slightly counter
intuitive if you are thinking in terms of corrosion from residues, but as Ag
migration caused by the residues, the biggest PD/opportunity is usually on
the larger devices. The smaller ones will follow though.

Brush cleaning and stuff like that is possibly OK for rosin/no cleans but
its definitely not for Water wash. One of the difficulties of water washable
repair is the reluctance to reprocess complete assemblies through a complete
wash and dry cycle. 
This is something you will have to address, possibly by getting an
additional different "short cycle/off line machine" or not water wash repair
or...

To directly answer your 3 questions.

1- How should a WS flux be cleaned after a rework?
A: Completely. Then dried.  You can't really clean topically with a brush,
and certainly not dry. Really it's a whole board job in a machine.

2- Can WS be cleaned with alcohol?  If not what would happen?
A: No,  You will leave flux residues on the work . These can give you low
SIR, electro migration and so on. The flux is water washable not alcohol
washable (assuming you are referring to IPA/similar). Alcohol can aid drying
though.


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.

A: Yes

Hope this helps.

Regards 

Mike Fenner 
Indium Corporation 

T: + 44 1908 580 400 
M: + 44 7810 526 317 
F: + 44 1908 580 411 
E: [log in to unmask] 
W: www.indium.com 
Pb-free: www.Pb-Free.com 

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Indium Corporation
Incorporated in New York, USA
Registered office: (Not for correspondence) 
77-97 Harpur Street, Bedford , UK
Registered number: fc16473


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jean-François
Bissonnette
Sent: Friday, July 25, 2008 2:59 PM
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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