Solder preforms and solder alloy assembly methods

1. A method of assembly, comprising:

applying SAC solder paste to an electronic substrate to form a solder paste deposit;

placing a low temperature preform in the SAC solder paste deposit, wherein the low temperature preform has a melting temperature below the melting temperature of the SAC solder paste;

processing the electronic substrate at a reflow temperature of the SAC solder paste to create a low temperature solder joint; and

processing the low temperature solder joint at a reflow temperature that is lower than the reflow temperature of the SAC solder paste;

wherein the reflow temperature of the low temperature solder joint is between 138-170° C.

2. The method of claim 1 , wherein the low temperature preform is a composition consisting of tin and bismuth.

3. The method of claim 2 , wherein the composition consists of 42% by weight tin and 58% by weight bismuth.

4. The method of claim 2 , wherein the composition further consists of silver.

5. The method of claim 1 , wherein the solder joint consists of tin, silver, copper, and bismuth.

6. The method of claim 5 , wherein the solder joint consists of 49-53% by weight tin, 0-1.0% by weight silver, 0-0.1% by weight copper, and 46-50% by weight bismuth.

7. The method of claim 1 , further comprising a second solder process at a temperature between the lower reflow temperature and the reflow temperature.

8. The method of claim 1 , wherein the composition consists of 55% by weight tin and 45% by weight bismuth.