plating microstrip circuits

[dowjones]

anybody has experience on finishing-plating microstrip circuits?. I have read that in electronics is usual the hasl finising and also an organic finishing called osp. other alternatives such as inmersion gold, inmersion silver and NiAu plating are commented.

can somebody in a simple way describe advantages and drawbacks?. soldering is a requirement for my finishings

thanks

 

[hobbymat]

Yes, I've got quite a lot of experience on this issue.
The chosen plating depends on the application, frequency range and also the components that are to be attached to the microstrip structures.

If you want a good, conductive finish that tolerates oxidation, choose NiAu plating. This can also used if you choose not to solder your transistors but want to use conductive epoxy instead for attaching the active components. NiAu can also be wirebonded, if you want to use a bare die (MMIC). I've used NiAu plating for 50 GHz and up, succesfully. Just take care that the plating thickness is thin enough not to cause inaccuracy to the line impedance.

There have been some reliability issues with NiAu plating, especially when very small-pitch BGA components are used, as the low solder volume in each ball gets mixed with an excessive amount of gold. Gold-tin alloy is a very brittle metal, and tolerates temperature cycling and other mechanical stresses rather poorly. In these cases it might be better to use immersion silver.

If you want to have something more inexpensive, you may be able to use OSP for less than 10 GHz. However, OSP is not very conductive so it's not suitable for other than solder joints. OSP is cheap, however.

The gold plating (with Ni underneath, usually) is almost always immersion gold. Electrolytic gold plating requires that all the pads are connected together with a thin conductor (that can be removed manually after plating). I've used this in some military applications, but not commercial ones.

Hobbie

 

[dowjones]

thanks a lot hobbymat.

in principle my case is under MIL conditions, and my feeling is that my circuits are going to be gold plated.

could you please indicate where can i learn more?.