In a message dated 97-07-25 00:06:55 EDT, you write:

>        Can some one out their throw some light on below mentioned problem
>  ,probable causes and solutions.
>         1. this defect noticed on 1.6mm double sided board, smobc,hal
finish.

My guess is that the problem lies with the HAL finish.  The thickness of the
board is most likely irrelevant.  The solder mask may or may not be absorbing
the HAL flux.  If it is a cheap mask or a mask not fully cured, then it will
absorb the halogenated fluxes when the laminate goes above the glass
transition temperature.  Post-HASL residues are often invisible to the naked
eye, and will often defy classical ionic cleanliness measurement methods.

>         2. after assembly and while testing at elevated temperature noticed

>   shorts during test cycle on the solder side between connector pads and 
> trace running thro.

If you are doing cyclical testing, you have a point at which you are going
from cold to hot.  You only need a few degrees difference between the
surrounding air and the energized product to condense a liquid water film
over the entire board.  When this happens, you have the three elements needed
for an electrochemical failure:  electricity, water, and an ionic contaminant
(from the HAL).  Your shorts begin at this point.  Polyglycol residues from
HAL also make it much easier for a water film to form on a product surface.
 Since an unpowered product is not very useful in the real world (slide rules
excepted), many manufacturers try to eliminate the water from the equation by
conformal coating.  This is a band-aid approach.  Water will go through an
organic coating, it just takes longer.  The only reliable solution is to
address the ionic residue at the source.

>         3.unable to figure out ,this shorts appear of very little dimension
and
>  are very small compared to pad /trace.

Because HAL residues affect the entire board surface, you only need a very
small amount to start the electrochemical faults.  The smaller the spaces,
the easier it is for problems to occur.  One thing to check is the coverage
of your traces by solder mask.  If the mask was too thin during application,
you may not have adequate coverage on the edge of your traces.  We had a
project recently where this was the case.  A high energy power supply had
inadequate solder mask coverage.  During thermal cycling tests, when the
water film formed, the water bridged from a high energy point to an
intervening component, causing an arc.  HAL residues and inadequate solder
mask was the cause.

If you have adequate solder mask in the areas of the failures, then the
problem is your HAL residues.  Check the post-HAL cleaning process.  We
recommend 140F-150F deionized water, preferably with a low level of
saponifier.  Unfortunately, most fabricators use either tap water or softened
water, which is not particularly effective against HASL residues.  

You can contact me off line if you are interested in some of our remedially
cleaning recommendations.  I'd offer them here, but it involves some
commercial products and I don't want to appear like a commercial.

Doug Pauls
Technical Director, CSL
[log in to unmask]

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