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July 2001

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Thu, 5 Jul 2001 12:23:35 -0500
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In our operation, we currently clean all bare boards before running them
through the assembly process.  After automated assembly (using RMA based
solder paste), the boards are cleaned again in our in-line cleaner or vapor
degreaser.  The bare boards are also cleaned in either the in-line cleaner
or vapor degreaser, both of which are probably over kill.  These boards go
into military radar systems.

My questions are:
1.      Most of our boards come in visually clean.  Occasionally, we get
some boards that look oxidized.  What is the risk of not cleaning?

*I guess I will dissent from the other responders.  Visual cleanliness is
seldom useful when you are trying to determine the true cleanliness of the
boards.  Relying on the IPC-6012 limit of 10 micrograms NaCl/in2 is poor,
at best.  If you have a board that is consistently 9.5-9.9, you meet the
spec, but you still have a contaminated board, in my opinion.  When I was
at CSL, we did a high volume of determining cleanliness of bare boards.  I
saw far too many times where the boards met IPC specs, but still had many
corrosion and metal migration issues.  This was more the case for no-clean
assemblers, but occurred occasionally with OA and RMA flux users.  This is
the risk of not cleaning.

*You are basically asking the question, do I need to clean my bare boards?
The answer is, It Depends.  Ask yourself two questions:  (1) how many board
suppliers do I have (each of which uses different materials and processes);
and (2) how confident am I that ALL my suppliers understand bare board
cleanliness and can consistently deliver clean product to me?  I did a
paper at Nepcon 95 looking at the bare board cleanliness of 6-8 board
suppliers to a major automotive assembler in Kokomo, IN (which shall remain
nameless, chuckle chuckle).  Some suppliers were rock steady all year.
Some had occasional blips.  Others wouldn't know a clean board if it ran up
and bit them.  If you are certain that your bare board suppliers understand
board cleanliness, and consistently provide you with acceptably clean
product for your hardware, then you don't need to clean.  But until you can
answer those questions, I suggest you keep cleaning.

*I will illustrate with two examples (it's my lunch hour and I can type as
much as I want):  One client had the job of watchdogging bare board
cleanliness.  He had a horrible time with electrochemical reliability on
their hardware.  He had 20 board suppliers from all around the globe.
Board cleanliness levels all over the map (pun intended).  His headaches
did not go away until he wittled the list down to about 4-5 and monitored
them closely.  The second client had a similar situation, but in his low
volumes, never had the clout to effect change at his board sources.  His
company had converted to no-clean assembly, but still had a big aqueous
cleaner in the warehouse.  In his case, that big cleaner went to the front
of his assembly line and all boards entering into the no-clean assembly
were cleaned to a known starting point, and his problems (at least the
board cleanliness ones) went away.

*You have the advantage of still cleaning and using RMA fluxes (or more
likely a ROL0 flux, since RMAs don't exist anymore), which are very
forgiving of board contamination.  The rosin tends to encapsulate board
surfaces and your cleaning may address the more aggregious residues on the
bare boards.  I have found, however, that fabrication residues become
progressively harder to remove with successive exposures to reflow
temperatures.

2.      We're considering a small batch cleaner - something on the order of
a Maytag dishwasher with dishwashing detergent.  I've seen this used in
some
small commercial operations.  How wide spread is this practice and does it
make sense?

*I cring just thinking about it.  Brian Ellis had some excellent comments.
If you go with a commercial dishwasher, they have seals and pumps made for
mild detergent solutions intended for dishes (go figure).  Those solutions
are not what you want on your electronics.  Lots of nasty surfactants and
most customers don't like their electronics with a fresh pine scent or the
smell of lemons.  On the other hand, if you put a good saponifier in the
unit, one that is good for cleaning electronics and RMA residues, the seals
and pumps will only last a few days.  I know of one fabricator that
regularly rinses their are boards in Cascade detergent as a final clean.
Nice shiny solder, nice clean board, loaded with ionic and organic crud.
If you are going to clean your bare boards, do it right and not on the
cheap.

If you would like me to go into more depth, then .......... (here it
comes)...... contact me off-line............. (couldn't resist)........

Doug Pauls
Rockwell Collins

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