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Materials and Process Selection for High Frequency Flip Chip
Keywords: Flip chip, GHz, bumping
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. Four groups of experiments are reported. The first experiments are on stud bumping thin film substrates for subsequent flip chip bonding. The wire was 17-m 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 alumina with 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 on cross-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 indeterminate, 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 is not affected and the changes in dips can be considered in the early phases of the design process for chip and carrier.
Katarina Boustedt, Sr. Research Engineer
Ericsson AB
Molndal SE-431 84,

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