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topic 661

Brittle Sn/Ni intermetallics


Please advise of any known options for avoiding a microcrack problem encountered when brazing a ceramic substrate into an Alloy 48 housing with Au/Sn. The substrate is plated with electroless Au (80 u" minimum) over electroless Nickel-Boron (100 u" minimum) over a refractory base metallization. Since this housing is supposed to be hermetic, the microcracks can be detected immediately after brazing via use of Mil-Std-883 [link by ed. to spec at TechStreet], method 1014 leak tests. Microsections indicate the cracks are present at the Au/Sn-to-Ni interface.

I suspect the root cause is the formation of a brittle Sn/Ni intermetallic, but I'm not sure how to correct the problem. Note that these same substrates can be successfully brazed using Au/Ge without microcrack formation. Unfortunately, process changes on the order of changing to Au/Ge braze or using electroless Nickel-Phosphorous on the ceramic substrate aren't allowable. Any suggestions?

Patrick Stanton

Why can't Au/Ge be used?temp?. I've brazed packages with Au/Ge in the past with no problems.If you suspect a Sn/Ni intermetallic being formed,try using more Au plating. Also look at your cooling profile. Could it be too fast. Good luck, Tom.

Tom Sivula
plating shop - Harbor City, California


Au/Sn forms a brittle intermetallic phase at low % Sn content. The Sn in Au/Sn braze alloys has a tendency to scavenge Au (either sputtered or plated).

80 u" Au should be sufficient as long as the braze soak time is not too long. You may be getting porosity at the Au to Au/Sn interface as the solid solution %Sn gets into the dangerous 1-5% range (if my memory serves me correctly).

Au/Ge is a better material for hermeticity although the braze temp. is higher than that for Au/Sn. I've regularly used Au/Ge on carriers that formed the bottom of a hermetic package. I'm not sure if the Ge will cause a problem in a vacuum oriented package requirement.

Another thing that may help you is the use of a forming gas environment for the Au/Sn. Typically, Au/Sn die attachments require a scrub in for good and uniform braze. The scrub requirement is less if forming gas is used. The scrub requirement is more if surface contamination is present. Another option (if you can stand the low temperatures) is an indium alloy with wetting elements (I believe antimony is used). Call Indium Corp of America for info on these types of alloys.

Jeff Albom
- El Granada, California

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