Hi Ladies and Gentlemen,
Thanks for your responses - they ARE helpful. In answer to some of your questions,
for those who like to leave no stone un-turned, I have the following answers:
- we are not sure if HASL is the tinning process used by the PCB manufacturer.
- we use acrylic conformal coat sprayed on by hand at ambient temperature in
extracted environment.
- we allow the boards to air dry for 12 hours before we remove the masking tape.
- I am from the UK, and the term I use (tinning) is meant to suggest placement of
the tin-lead layer upon the bare copper of a PCB. Of the details of this process I
know nothing.
- oxidation of the copper was suggested as a problem; what are the likely causes of
this?
- intermetallics were also suggested as a problem; is this intermetallics prior to
tinning, or a lack of them forming after it?
For those of you who may not have seen the first mail, I have incorporated it below:
"My problem is on a subject about which I know little, and I write to you on behalf
of a friend upon whose project the following has manifested itself.
We have a line upon which we manufacture simple through-hole PCBs. We are using
externally supplied multi-layer boards with nothing high-density or high tech about
them. They come in looking nice and well tinned. For about six months they have
been flowing OK, with only one problem - a large ground plane caused a few problems
in one area for a little while and we had to re-adjust the profile on the flow
machine (this is, I am sure, irrelevent to the problem). We like to manufacture to
a very high standard, so it was concerning when some PCBs recently exhibited bare
copper in various places. We use a low tack professional masking tape to mask two
planes on the board during conformal coat - on removal, the tinning came away,
exposing bare copper! Even more seriously, at many places solder had not fully
wicked to the top of the through-hole, meaning very suspect joints. In various
places, solder had wicked away from pads, again exposing copper.
Is it possible that we are doing something wrong without realising, or shoud we be
chatting to our board suppliers?
What do you folks feel is the problem, and what do you recommend we look at?..."
Thankyou very much for your help - past and future,
Be well,
Richard Tilbrook
Junior Engineer.
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