Electronic Packaging Research

Bonding principle of Bi/Sn/Au solder joints

Achieving high quality fluxless solder joint in air ambient
Developing fluxless lead free FCOF (flip chip on flex)
Understanding microstructure and phase formation of joints
 Flip chip packaging for high temperature and power devices



Solder joints and Microstructure of Bi/Sn/Au

(a) SAM acoustic image of a Bi/Sn/Au solder joint
(b) SEM image of Bi/Sn/Au solder joint
Our lab has transmission mode Scanning Acoustic Microscopy. Resolution: up to 10micrometer

Fluxless tin rich Au-Sn flip chip bonding process

1. Formation of Cr/Au pad

2. Formation of solder bumps through stencil mask on Si wafer

3. Chip mounting and joint formation in H2 ambient

Solder bump formation



(a) Multilayer tin rich eutectic Au-Sn bump after deposition
(b) The tin rich eutectic Au-Sn solder bump after re-flow in hydrogen