Development and evaluation of lead free reflow soldering techniques for the flip chip bonding of large GaAs pixel detectors on Si readout chip

Lead free reflow soldering techniques applying AuSn as well as SnAg electroplated bumps were chosen for the evaluation of the flip chip bonding process for a x-ray pixel detector. Both can be used in pick & place processes with a subsequent batch reflow suitable for high volume production. AuSn solder was selected due to its fluxless bondability, the good wettability and the self-alignment process capability and SnAg solder due to its more ductile behaviour and lower yield stress compared to AuSn. GaAs test chips with daisy chain and four point Kelvin probe structures together with appropriate Si test substrates were designed, manufactured and bumped. Test chips with 55 and 170 μm pitch and different chip sizes (maximum 16.3 down to 4 mm square) were used. AuSn bumps were deposited by electroplating Au first and Sn on top. Au bumps were also formed on substrate side. Two under bump metallizations (UBM) were used for the SnAg samples: Cu and Ni.