Re: dirty boards

John Larkin wrote:
We sent out a batch of 35 VME module kits to be built by a contract
assembly house. There are lots of high-value resistors on this board

http://www.highlandtechnology.com/DSS/V360DS.html

in the filters and such. Boards started failing in test and it seems
to be caused by ionic contamination trapped under parts. The stuffers
used water-soluble flux (which is contrary to our rules) and obviously
didn't clean the boards enough. They claim to use a super
high-pressure conveyerized spray cleaner with super-clean water. I'm
skeptical about the cleanliness of their system, and they just told us
that the cleaning line "just broke" so now they can't rerun the
boards.

So, what's your experience? Can a water-soluble flux be reliably
cleaned off to decent leakage levels? Can they really clean under
surface-mount parts?

Last time this happened, some years ago with another assembler, we
nabbed a sample of their wash water, and it was 20x as conductive as
tap water.

I'm thinking in terms of slowly hand-scanning each board with a
water-pic sort of high-pressure wand, with single-use distilled water,
or something like that. It looks tricky to clean under surfmount
parts, especially with water. Our normal process is RMA flux followed
by solvent wash in a vapor degreaser.

Is there anything that can be added to the wash water to reduce its
surface tension, to get under parts better, but that isn't itself a
source of leakage? Maybe an alcohol/water mix?

Are there any lead-free implications as regards leakage? This board
uses regular pb/sn solder, but it may become a concerm some day.

In one case, I successfully improved the leakage resistance of a board with surface mount parts, but it didn't include a battery or any electrolytic capacitors. I first soaked the board in warm 99% pure isopropyl alcohol for an hour or so, to remove any covering of rosin flux (some parts had been hand soldered), and then cooked the boards for an hour or so in near boiling (perhaps 85 C) distilled water, followed by a couple rinses in cold, clean distilled water.

I then baked the boards in a vacuum oven at 50 C, overnight, to make sure all the surface bound water escaped.
.

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