Fine Pitch and Low Temperature Bonding Using Solder Bumps and the Assessment of Solder Joint Reliability

We developed a bonding at low temperature using fine pitch Sn and In bumps at 120°C, and studied the reliability of the fine pitch In-Sn solder joints. The 30 μm pitch Sn and In bumps were joined together without flux at 120°C for 2 minutes under 70.2 MPa. A non conductive adhesive (NCA) was applied during solder joining. Thermal cycling test (0°C–100°C, 2 cycles/h) of up to 2000 cycles and 1000-h temperature and humidity test (40°C/95%) were carried out to evaluate the reliability of the solder joints. The bondability was evaluated by measuring the contact resistance (Rc) of the joints through the four point probe method. The solder joints were characterized by scanning electron microscopy with energy dispersive X-ray spectroscopy. Joints between the In and Sn bumps were successfully made at 120°C. The average contact resistance of the In/Sn solder joints was 10–14 mΩ before thermal cycling. Without the NCA, the solder joints between the Si chip and glass substrate failed electrically in the early stages of the thermal cycling test. The failure in the solder joint during thermal cycling was attributed to the thermal expansion mismatch between the Si chip and glass substrate. In contrast, no electrical failures were observed in the joints with applied NCA even after 2000 cycles. The flip chip joints with applied NCA were stable up to 1000 hours in the temperature and humidity test.