Hakan,

Regarding solderability of 4596 without flux - Hmmm!  Certainly works better
WITH flux.

Depending on such things as whether the conductive is on multilayer
dielectric (such as 5704) or directly on alumina, burnishing may, or may
not, help.  Check out both - as I am sure you already have.

Have not found plasma cleaning to be that beneficial.  

Increasing reflow temperature above your "normal" range will help facilitate
wetting. 

Can you pre-wet with solder and flux, then clean, and perform your flux-less
assembly (assuming you still need to do it flux-less)?

I believe that I have attempted to use my SST (Scientific Sealing
Technology) vacuum reflow equipment in an attept to solder to this
conductive as well, with mediocre success.  This was even after using the
purge/backfill capability o the equipment with high purity nitrogen (found
forming gas to be of negligible benefit <300-400°C).  If memory serves me
correct, I believe that I resorted to the pre-wetting technique - this way I
was assured of good wetting to the substrate before device attachment even
began.

Hope this helps somewhat.

PS - You did not indicate the solder alloy, nor the approximate area to be
wetted. 

Good Luck!

Steven Creswick - CTS RF Integrated Modules

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