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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