DEPRESSING THE MELTING-POINT OF SOLDERS AND BRAZES BY EUTECTIC ALLOYING

Alloying as a means of depressing the melting point of metals is exami ned. Eutectic reactions in binary alloys can bring about substantial r eductions in melting point and this phenomenon is put to good we in so lders. Additional alloying can give rise to further reductions in melt ing point but, as demonstrated using selected examples drawn from sold ers and brazes and backed by simple theoretical arguments, this trend follows the law of diminishing returns. The pattern of alloying behavi our is then considered in relation to lead-free solders, which leads t o the conclusion that there are no 'drop-in' replacements for lead-tin eutectic solder, that melt at a closely similar temperature. There wo uld appear to be only two realistic options, one based on the bismuth -42wt% tin binary eutectic melting at 139 degrees C and the other a so lder based on the silver-copper-tin alloy system which has a ternary e utectic reaction ata temperature of 217 degrees C.