Patent ID: 7776651

Claim:
A method for joining first and second components having mismatched coefficients of thermal expansion (CTE), comprising: forming a plurality of solder bumps comprising a lead-free solder on at least one of the first and second components, wherein the lead-free solder is a solder alloy comprising 85-96% tin and 4-15% Indium by weight percentage (wt. %), wherein the lead-free solder alloy comprises at least one element from the following group: Ni and Ge, wherein the combined wt. % of said at least one element is less than 2 percent; disposing the first component opposite the second component with the plurality of solder bumps disposed therebetween; heating the solder to a reflow temperature; and cooling the first and second components to re-solidify the solder to form a plurality of solder joints between the first and second components, wherein the solder is caused to deform during cooling as a result of the CTE mismatch of the first and second components, and wherein the solder comprises a composition that undergoes a phase transformation when it is cooled from the reflow temperature that reduces residual stress in the solder that would normally appear under the same deformation, wherein the phase transformation comprises a martensitic phase transformation and wherein the rate of cooling is controlled to produce a needle-like Martensite microstructure.