Micro-solder/adhesive hybrid joints for high-density, high-power, high-reliability, and reworkable module interconnection in mobile phones

In this study, in order to obtain high power handling capability and high reliability of solder joints as well as fine-pitch capability and reworkability of conventional adhesive joints, thermo-setting adhesive films containing 20 μm diameter Sn-58Bi and Sn-3.0Ag-0.5Cu solder particles were used as interconnection materials for module interconnection in mobile phones. For fluxless solder joining with simultaneous adhesive curing, ultrasonic bonding was performed to take advantages of mechanical vibration which breaks native solder oxides and rapid ultrasonic heating which enables fast solder joining and adhesive curing within 5 seconds. According to the experimental results, ultrasonic bonded micro-solder/adhesive hybrid joints showed excellent solder alloy joining with more than 10 at.% Au diffused from Ni/Au finish into solder bulk. At the same time, the micro-solder/adhesive hybrid joints showed higher than 90% curing of the adhesive matrix within 5 seconds. The micro-solder/adhesive hybrid joints showed no short-circuit with 200 μm pattern pitch, 360% improved power handling capability, and excellent reliability in an unbiased autoclave test (121°C, 2 atm, 100% relative humidity for 24 hours). In addition to this, micro-solder/adhesive hybrid joints could be easily reworked by peeling FPCs, cleaning adhesive residues with Acetone, and bonding a fresh FPC onto the cleaned electrodes. The reworked micro-solder/adhesive joints also showed exactly the same electrical properties and reliability as the as-bonded joints.