Bi based alloy for high temperature leadfree die attach

ELV and RoHS continue to require lead-free solders replaced to Pb-rich solders. Bi based alloys are good candidates because the solidus lines are almost more than 250 centigrade degrees. However Bi based alloys have poor wettability on Cu and Ni electrodes and Bi alloys are very brittle in comparison with Pb rich alloys. In this study, it is found out that a small amount of Sn addition in Bi drastically improve the wettability on Cu and Ni electrodes as good as in the addition in pure Pb. On the other hands, the Sn addition in Bi also decreases in the solidus line to 139C. However the temperature to start to melt at the solder joints can be increased to more than 250C after die bonding because Sn is easily reacted with the electrodes on Si dies and Cu-leadframes. Void ratio of Bi-2wt.%Sn alloy is as good as Pb-3Sn-1Ag alloy, when the Si dies of 5.5×5.5mm in area with Ni/Au flush back metals attach on the leadframes with Cu, Ni and Ag plating. Any delamination and crack don´t occur at the solder joints molded by epoxy resin after the joints dipped for 120sec at 250C, because the solder joints don´t start to melt at less than 250C and relax the stress in the solder joints at high temperature. Strain-stress curves at Bi alloys very resemble to Pb-3Sn-1Ag alloy at the low strain range nevertheless Bi alloys are very brittle.