Whisker-free lead frames

A method of reducing the formation of tin whiskers in an electronic interconnection, the method comprising the steps of:

• providing an electrical contact comprising copper;
• forming a first lead-free layer on the electrical contact, the first lead-free layer comprising nickel having a thickness in a range of about 0.05 μm to 1.0 μm;
• forming a second lead-free layer adjoining the first lead-free layer, the second lead-free layer comprising tin, a weight percentage of nickel relative to tin being in a range of about 1 to 12;
• heating the contact and first and second lead-free layers to a temperature of between 232° C. and 270° C. for a time period between 5 seconds and 120 seconds to cause melting of the tin and formation of a substantially planar intermetallic material comprising copper, tin and nickel at a copper interface with the contact and having essentially no elemental non-bound tin.

An electronic assembly, comprising:

• a lead frame including a plurality of leads comprising copper;
• a chip bonded to the lead frame and encapsulated in a polymer composition;
• a first layer comprising nickel, the first layer formed on and interfacing with the leads and having a thickness between about 0.05 μm to 1.0 μm;
• and
• a lead-free second layer comprising tin, the second layer overlying the first layer and having a melting point between about 232° C. to 270° C., a weight ratio of nickel in the first layer to tin in the second layer being in a range of about 1% to 12%;
• wherein the plurality of leads and the first and second layers are positioned with respect to each other such that, when the second layer is melted, an intermetallic layer comprising copper, nickel and tin is formed at an interface with the leads after a sufficient time has elapsed, the intermetallic layer having substantially no elemental non-bound tin.