[Eliyaooo]

Hello to All,

I'm having difficulties with bonding epoxy resin coated polyimide to other polyimide substrate.
The defect is local separations between those two surfaces (after heat press c-staging).
I'd like to mention that those separations appears as small air bubbles.
Any thoughts/ideas??

Thanks!!

[Hunter2]

The problem is that Polyimide surfaces have not big roughness. Do you built up rigidflex PCB? You need a pretreatment of your Polyimide to get some roughness. Not easy,(NaOH or KMnO₄/NaOH treatments in advance.)

[Eliyaooo]

Thank you for the quick response..!

Unfortunately i can't treat the surface with NaOH solution because my polyimide surface contains metallic thin pattern, which will interact with the solution and etching process will begin. This will require complicate process development, like adding photolithography steps to protect the metallic foil.

[Hunter2]

Well that makes the things more complicated. I dont know what product it is, but tin makes for me normaly only sense at the end of process for some soldering purpose later on. To cover a thin pattern polyimide with epoxy???

[Eliyaooo]

Is there any good reference to vacuum b-staging?? (Degassing the epoxy resin while curing?!?)

[Hunter2]

Maybe there was a misunderstanding. You wrote

my polyimide surface contains metallic thin pattern

.

Do you mean tin Sn or do you mean there is some thin metal, which kind of metal copper,gold, nickel?

If it is not tin then NaOH will not harm copper or gold.

Your question: I am not so farmilar with the pressing purpose, even I work in the PCB industry. I am more educated on the wet processes.

[Eliyaooo]

It is a copper-iron.... alloy.
Heat pressing is for c-staging the epoxy.
NaOH under what condition? time,temp?

[Hunter2]

Its a kind like invar, isn't it?

Well the solution should be 40 -50 g/l NaOH and Temp. 60-80 ° C.

These solutions normaly used in combination with Permanganate oxidieser for hole cleaning (desmear) of printed circuitboards.

It could be also necessary to use a solvent (Sweller).

Other possibilities are Tetra etch^R normally used to treat fluorine polymers.

Also some plasma application with special gasmixture maybe can be used.

[Eliyaooo]

Thank you very much for the information.
Yes it's like Invar (proprietary alloy I'm trying to patent).
Is such a conditions you mentioned may change the physical properties? like thermal resistance, electrical resistance?

[Hunter2]

I dont think so, because only the surface will be changed.

[Enthalpy]

The superficial resistance (from track to track on one polyimide side) may change a lot through the soda action. Washing well with deionized water after the alkali can improve.

Na+ is one of the worst ions for semiconductor reliability - in case you have chips on the printed circuit, be they bare or plastic encapsulated. A heavier alkali, or if possible Ca++, would then be preferable from this aspect.

Could the polyimide degas during the hot pressing? PI is known to release >1% mass vapour under vacuum. Baking it before putting the epoxy, like 1h +125°C, would improve then.

[Eliyaooo]

Thanks for the answer Enthalpy.

It's interesting to mention that those separations appear only near the metallic thin foil.
It seems like the epoxy resin runs aways from those small regions and leaves uncoated zone.
What can cause such phenomena? and what treatment can be applied without changing the electrical resistance of the foil?

[Enthalpy]

Could it just be a matter of thickness? Some flexible circuits have 35µm metal and 25µm polyimide. Then, thicker epoxy might just do the trick.

Some metals influence the polymerization of epoxy.

Surface resistance: even if degraded, it will be in the giga-ohm range, not necessarily a worry for your circuit. Thorough washing (brush?) with deionized water improves a lot.

[Eliyaooo]

I agree with you regarding the adhesive thickness, but that would increase the resin squeeze out (Bleed out) from the covercoat, which will be a disaster for my micro-assembly.