Ed,
In many aspects, PTFE doesn't process the same as standard FR-4 and desmear & electroless are among these.
Due to its chemical properties PTFE has low tendency to react with most chemicals, that is why it cannot be electroless plated just like FR-4. The molecules need to go through some kind of chemical/mechanical attack, in order to allow them to react.
One method is the plasma etching - it could be of a chemical or mechanical nature, depends on the gas being used. In my opinion, the downside of plasma etching is the very smooth surface it creates.
The other possibility is sodium etching and there are several commercial products available (if you are interested, I can name a few). They are supposed to create a better surface topography for plating but they can involve some potential health hazards. Most of these chemicals are quite expensive, too.
In the beginning we avoided both of these surface treatments when producing double sided boards. We simply passed the board several times through our electroless line, until achieving complete coverage (which was rather easy to determine for double sided boards). I think that when you are producing more complicated products, this method's reliabilty is questionable.
DESMEAR - althogh theoretically possible, I haven't seen "resin smear" problems with PTFE. However, if you are concerned about it, you can use plasma etch to clean the hole walls. The chemical etching won't do the job. In such a case, you can consider using the sodium etch AFTER the plasma, in order to improve hole-wall surface topography and lower the surface tension.
Also, you may be aware of the fact that PTFE is processed differently from FR-4 because of mechanical aspects too: machinabilty, creep etc.
If you wish to discuss the matter further, I will be happy to assist you - through the forum/ direct E-mail/ offline/.......
Regards,
Yehuda
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* Yehuda E. Weisz
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* Tel: 972-3-5240362
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