Matthew,
Yes, other adhesive systems (epoxy and some of the FR formulations)
could help.
Coverfilm bond strength tests could help you to discriminate between the
relative rankings of performance, but the absolute values measured may
not have any direct meaning to your application. (The standard tests
often fail to discriminate for the particular failure mode you're trying
to deal with.)
There are a good many things that could cause uneven discoloration, and
others have suggested the key bad ones. However, some sources of
discoloration are quite meaningless in terms of physical performance, so
unless they're falling apart don't give up on your circuits too easily.
You may be exceeding the rated capabilities of the coverfilm adhesive.
What temperature was your 2.5 hr bake? What temperatures were the reflow
cycles?
Acrylics in particular are prone to oxidation and embrittlement leading
to loss of bond strength with exposure to time/temperature. In the
process of making the 6 layer rigid flex your supplier may have used up
most of the adhesive's useful temperature resistance. Acrylics also
provide some of the poorest barriers to moisture transmission.
My only addition to Dr. Sallo's review of Sputtered vs. Cast would be
the DuPont AP products, and some very thin cast products that Rogers was
doing for one special customer.
Andy Magee
Flex Guru - Consulting
[log in to unmask]
(937) 435-3629
Matthew wrote,
We're currently using adhesiveless base material on the parts due to
some
low yield problems we'd seen in the past. Unfortunately, the
delamination
we're seeing is coverlay to copper where we're using the acrylic
adhesive.
Does anyone have experience with epoxys or other adhesives for that
component?
As for the tweezer method, that's similar to what we've been doing as a
check; I feel pretty good about the method, but I'm worried about the
possibility of this discoloration, even if it doesn't give me the
tactile
delam feedback, having some problems later in life. I worried that even
the
C-SAM, while it can verify the parts as-is, won't necessarily tell me if
I'll see any future problems (or if it's ok now, will it be ok in the
future?). Does testing the bond strength between the cover and the
copper
help?
This is something of a separate thread, but for adhesiveless flex
materials,
is there any practical functional difference between cast polyimide and
sputtered copper (/11 and /12 I think). Is it a difference I should
worry
about when specifying materials?
Thanks for the information so far!
Matt
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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