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Blisters on Aluminum

After silver plating machined aluminum we test the parts by heating to 175oC for one hour. During the soldering step a good percentage of the parts blister. Do you have any suggestions for eliminating this problem?
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Q: After silver plating machined aluminum we test the parts by heating to 175oC for one hour. If the parts pass this test our assembly department solders components to the part by heating to approximately 250oC using a hot plate. During the soldering step a good percentage of the parts blister. Do you have any suggestions for eliminating this problem? P. B.

 

A: There a number of things you should investigate. To plate aluminum you must first apply a zincate layer. The zincate layer can diffuse into the aluminum base material. An alloy will form that will blister. The same thing can happen with the copper strike applied over the zincate. The copper/zinc alloy formed may blister. You do not mention in your e-mail what type of zincate you are using. A proprietary ternary zincate will reduce the tendency to diffuse into the base aluminum and copper strike.

The amount of diffusion usually depends on the temperature and time. Heat testing of the parts prior to soldering may be “pushing” the parts close to the blistering point. The soldering step may then push the parts over the blistering point. Unless the specifications require the heat test, I would suggest reducing the length of time for the heat test or eliminating it completely. But, before eliminating the temperature testing completely run a simple test: Solder the parts to an aluminum part that has not been temperature tested and see if blisters are formed.

Also, there is information that suggests increasing the thickness of the copper layer will reduce the amount of blistering.
 

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