The thermal expansion of Cu is 17 ppm/C and Al is 24 ppm/C. This amounts
to a 0.0007 inch / inch difference for every 100C temperature change.
"Relatively the same" may not be insignificant in some applications.

The actual circuit distortion would be a function of the force balance
in the composite structure:
Composite Strain (in/in) = Change in Temp. x Sum (Layer Modulus x Area
of Layer x CTE of Layer) / Sum (Layer Modulus x Area of Layer)

Andy Magee
Flex Guru - Consulting
[log in to unmask]
(937) 435-3629

Earl Moon wrote,

You could place the stiffener in the middle of the MLB no matter the
material selected. Aluminum and copper expand at relatively the same
rate in the X-Y so make your choice as copper because you can plate to
it (easily) or through it and it can serve as a thermal transfer
mechanism.

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