Precise flip chip assembly using electroplated AuSn20 and SnAg3.5 solder

In order to reduce costs in packaging especially for optoelectronic devices technologies are desirable that enable precise assembly at low cost. A flip chip assembly approach is presented using the self-alignment mechanism in combination with mechanical stops. Besides eutectic AuSn20 solder (80 wt.-% Au, Tm = 280 °C) which is widely used in optoelectronics the eutectic SnAg3.5 solder (96.5 wt.-% Sn, Tm = 221 °C) was tested as an alternative which is compatible to organic substrates. Both types of solder bumps were deposited using electroplating and were reflowed prior to flip chip assembly. The phase transformations taking place during the reflow of Au/Sn solder bumps are described. Furthermore, flip chip reflow soldering experiments of test vehicles using AuSn20 and SnAg3.5 solder proved that it is possible to achieve self-alignment in three directions. The final position of the chips in regard to the substrates was solely determined by the mechanical stops on both chip and substrate side being in direct contact to each other.