Matthew:
Although every board out there is different and it is best to view the
particular part to be able to comment on what is happening, here is what I think
you are dealing with.
I would agree with Kelly Shriver that your acrylic adhesive is most likely
giving you the trouble and it does sound like delamination. I do not know if
you are dealing with the adhesiveless type product Kelly mentioned or not. Even
with this adhesiveless substrate material many times there will be an acrylic
based coverlay on the circuit and it can still cause some of these issues.
One thing I have seen in the past that helps is to try to eliminate all of the
adhesive that you can. In other words will 1 mil adhesive work in your
application instead of 2 etc?
I have seen some cross hatching performed in the past on large ground plane
area's that are coverlayed to help prevent demlamination, however I would not
think this should be an issue on a .250" trace.
It is possible that there may be moisture or a contaminate under the coverlay.
You should try to verify or rule this out first.
One of the best things you can probably do is to try to get together with your
board fabricator and material suppliers to work on this together. I have
participated in this type of joint effort on flex/rigid boards in the past at a
fabricator and I have found it to be extremely helpful. Good luck.
The opinions and views expressed here are my own and do not necessary reflect
that of my employer.
____________________Reply Separator____________________
Subject: [TN] Flex circuit - discolored copper, delam?
Author: [log in to unmask]> Matthew Sanders <[log in to unmask]
Date: 5/10/99 6:54 PM
Hello all,
I have some 6 layer rigid flex boards with discolored copper in the flex
area. If I look at the flex (after 2.5 hour bake and 2 reflow cycles), I see
a uniform amber area for most of the circuit along a .250" trace; however,
there are some areas that are significantly lighter and some that are
significantly darker than the majority of the area. I'm not sure if I should
be concerned with this state of being, so I guess I have a few questions.
- What is this? One of the darker ones appeared to have the coverlay
delaminated (2 mil acrylic adhesive over 1 oz copper), but the lighter ones
seem to have no problems.
- How can I tell if it's a problem or if it'll cause any end-use
difficulties (or delaminate during use)? I've taken some samples and run
them through our reflow oven 24 times, soaked them in water for 3 hours, and
run them through reflow 4 more times. So far, the dark ones look like
there's delam (as shown by cross section), and the light ones look ok.
- What can my fabricator do to avoid it (assuming it is a problem)? Are
there better adhesives to use (epoxies, etc.)?
- Would cross-hatching the .250" trace reduce the liklihood of occurance?
Should I need to do this, or should a fully in control fab process not cause
this?
Ok, thank you all for your kind assistance. I'm never sure if I'm alone out
there in my rigid-flex woes, so I'm interested to see responses.
Thanks,
Matt Sanders
Matthew Sanders
PCB/Metal Procurement Engineer, Trimble Navigation Ltd.
[log in to unmask]
Phone: (408) 481-7817
Fax: (408) 481-8590
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