Flip chip for high frequency applications : materials aspects

Sammanfattning: Flip chip has since decades been the primary choice for chip interconnect for high performance circuits. Over the last few years, interest from the microwave arena has increased, and at this point in time microwave flip chip is a possible option for volume production.This thesis is based on an extensive literature survey in combination with experiments done in collaboration with Ericsson, Flextronics in Linköping and Chalmers University of Technology in Gothenburg. The backbone of the literature survey was published as a chapter on high frequency interconnects in the reference book “Area Array Interconnection Handbook” (2002), edited by Karl Puttlitz and Paul A. Totta, who are known as the fathers of flip chip. This literature study has since then been expanded and the latest search wasdone in June 2007.Four groups of experiments are reported in this thesis. The fi rst experiments are on stud bumping thin film substrates for subsequent fl ip-chip bonding. The wire was 17-μm diameter Au1Pd and bumping was uniform and successful, after some initial equipment problems.The second is a set of tests on flip-chip bonding using gold stud bumps, gold tin solder-on-substrate, and plated gold pillars on alumina carriers. To evaluate the joining process using these different bumps, chips made of aluminawith coplanar waveguide transmission lines were thermocompression bonded to the bumped carriers. Bonder parameters were assessed related to the different bump types and materials. The bonding results were analyzed using shear tests, transmissive x-ray and scanning electron microscopy oncross-sections.The third experiment lot contains early results on reliability of stud-bump flip chip of gallium arsenide microstrip chips. Since the sample availability was very low and the joining process still under development, the results are vague, although it was fruitful to establish adequate methods of analysis and test.The fourth set of experiments involved the same alumina on alumina assembly, and flip-chip underfill and its impact measured up to 70 GHz. Three different underfill materials were applied to the test vehicles described above. Before and after underfilling, the test vehicles were measured for S parameters and compared.This study concludes that for flip chip on gold pads, the preferred joining process is soldering, just as for silicon chips with aluminum or copper pads. Since solder bumping on gold pads is hard to come by, the second best choice is plated pillars, on carrier or chip, and thirdly, gold wire stud bumping.Underfill for microwave applications should not necessarily be dreaded to the extent that it is today. This study has proven that the matching of lines isnot affected and the changes in dips can be considered in the early phases of the design process for chip and carrier.