Earl,
See my comments below.

>The guru is right as always - is that why he deserves the title?

Thanks for the complement. I can be wrong, I just try not to do it in
public too often.

>Pyralux is an acrylic film. Unfortunatley, it is the only practical
>adhesive system used to laminate flex and rigid/flex circuitry. It is
>not like a resin based system that is thermosetting and cross links to
>ensure innerlaminar integrity. Plus, the stuff has a Tg of "room"
>temperature and "cold" flows with a mind of its own.

There are several other very practical adhesive systems available for
flex and rigid-flex in volume use around the world.

Pyralux has been shown to have a roughly 50/50 split in thermoplastic
vs. thermosetting properties. The Tg of Pyralux is 40C. Fully cured the
adhesive has a modulus of 85,000 psi, an elongation of 270%, and a
tensile strength of 3,500 psi. During lamination (heating rate of
5C/min) the uncured Pyralux has a steadily decreasing viscosity until
about 140C, where there is an abrupt drop to its minimum. The apparent
viscosity eventually begins to increase again as curing advances. Cured
Pyralux adhesive has a tensile modulus of 590 MPa at 22C. This drops to
<1 MPa at 150C initially, but after holding at 150C for 250hr the
modulus increases to 50MPa due to the "oxidation" or over curing. (We've
all seen the darkening and embrittlement that results from advanced
thermal aging of acrylic)

>When a flex circuit is laid up and laminated, a certain amount of
>delamination usually is assured. You cannot get the Kapton and adhesive

>to conform to circuit trace edges, thus "soda strawing" is effected
>adjacent to them extending to the next dielectric thickness. This can
be
>the source of further, unacceptable delamination. The answer here is
>using lighter copper weights (thicknesses) as half ounce copper for
>example.

Admittedly, you have to know how to properly process flex, but
delamination is far from assured. There are several effective tools that
can be applied to preclude soda strawing and many other forms of
lamination defects. Poor manufacturing practices can indeed lead to
further damage in assembly and end use. While thinner copper can be
helpful, it may not be an option for all applications. Other ways to
control this issue include; modifying the lamination conformance
materials, adjusting the adhesive thickness, or changing the lamination
heating rates.

>No repairs are possible. The only way to ensure rework unnecessary, is
>to prevent it by assuring designs and suppliers are qualified.

Amen

>Earl Moon

Andy Magee
Flex Guru
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