Wafer Level Solder Bumping and Flip Chip Assembly with Solder Balls Down to 30μm

摘要

The most important technology driver in the electronics industry is miniaturization mainly driven by sizereduction on wafer level and cost. One of the interconnection technologies for fine pitch applications with thepotential for highest integration and cost savings is Flip Chip technology. The commonly used method ofgenerating fine pitch solder bumps is by electroplating the solder. This process is difficult to control or evenimpossible if it comes to ternary or quaternary alloys. The work described in this study addresses the limitationsof existing bumping technologies by enabling low-cost, fine pitch bumping and the use of a very large variety ofsolder alloys. This flexibility in the selection of the solder materials and UBM stacks is a large advantage if it isessential to improve temperature cycling resistance, drop test resistance, or to increase electromigration lifetime.The technology allows rapid changeover between different low melting solder alloys. Tighter bump pitches anda better bump quality (no flux entrapment) are achievable than with screen printing of solder paste. Because nosolder material is wasted, the material costs for precious metal alloys like Au80Sn20 are much lower than withother bumping processes. Solder bumps with a diameter between to date 30 μm and 500 μm as well as smalland large batches can be manufactured with one cost efficient process.To explore this potential, cost-efficient solder bumping and automated assembly technologies for the processingof Flip Chips have been developed and qualified. Flip Chips used in this study are 10 mm by 10 mm in size,have a pitch of 100 μm and a solder ball diameter of 30 μm, 40 μm or 50μm, respectively.Wafer level solder application has been done using wafer level solder sphere transfer process or solder spherejetting technology, respectively. The latter tool has been used for many years in the wafer level packagingindustry for both Flip Chip and chip scale packaging applications. It is commonly known in the industry as asolder ball bumping equipment. For the described work the process was scaled down for processing solderspheres with a diameter of 30 μm what was never done before that way worldwide.The research has shown that the underfill process is one of the most crucial factors when it comes to Flip Chipminiaturization for high reliability applications. Therefore, high performance underfill material was qualifiedinitially [1].Final long term reliability testing has been done according to MIL-STD883G, method 1010.8, condition B up tothirteen thousand cycles with excellent performance of the highly miniaturized solder joints. SEM/EDX andother analysis techniques will be presented. Additionally, an analysis of the failure mechanism will be given andrecommendations for key applications and further miniaturization will be outlined.