controlling solder on a gold metallized surface

[TurboLS]

what's happening, guys?

i have a rather annoying issue to deal with. i am trying to selectively solder onto a completely gold metallized surface with AgSn or SAC solder. I have tried all different kinds of solder masks and tapes, but the adhesives breakdown at these RoHS reflow temps. Aside from machining the gold after metallization to stop flow, what else could I try?

I am also trying to make this solder as flat as possible (thickness between 100um and 150um).

any input is appreciated.

 

[tomwalz]

You said any info so here goes. I know something about the process with tungsten carbide, cermets and ceramics but not with gold. However what you want is a highly wettable surface (to get 100 micron depth) bordered by a highly non-wettable surface. If you are doing a search you might consider that wettability is often defined by contact angles so add that term.

The first thing that comes to mind is carbon. A little bit of the right oil or grease can greatly hinder wetting. It is commonly supposed in industry that the heat of brazing burns these materials off to provide a clean surface. However there is almost always some sort of residue.

Put simply, I would suggest that you get your surface excruciatingly clean using a caustic based cleaner with a good rinsing agent then paint a ring of grease or oil around the area.

We also control the flow with the amount of braze alloy and the amount of flux.

There are also commercial “braze stop” agents but we have never found one that knocked us off our feet but we never looked all that much.

We can also control wetting by treating the surface with acids but I don’t know how well this will work with gold. You might check and see if you are using gold or a gold alloy.

We have more on this at cs4u.org under newsletters. I apologize but I don’t know how good that site is now. They are working on it.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[TurboLS]

thanks for the input. i may try the carbon idea. i'm also looking through the newsletters now.

 

[MikeHalloran]

Jewelers supply houses carry an assortment of "anti-fluxes", that are specifically intended to control solder and silver solder joints. You could probably screen some of the stuff around your joint and control its shape nicely.



Mike Halloran
Pembroke Pines, FL, USA