Undercooling of Sn-Ag-Cu alloys: solder balls and solder joints solidification

The degree of undercooling of three Pb-free solders Sn-4.5 %Ag-0.5 %Cu, Sn-4.0 %Ag-0.5 %Cu and Sn-2.5 %Ag-0.5 %Cu (wt.%) are determined using differential scanning calorimetry in two configurations (i) solder balls and (ii) solder joint samples prepared by reflowing solder balls on copper substrates (Ball Grid Arrays package). A large degree of undercooling was observed with solder balls, while the corresponding undercooling was significantly reduced in the case of solder joints. Samples were characterized using optical microscopy and scanning electron microscopy. The microstructure of the solder alloy is greatly affected by the reaction at the solder/copper interface. The role of the interfacial reaction between liquid solder alloy and copper substrate on the undercooling degree of the liquid alloy as well as on the microstructure of the solder joint is discussed.