Flip-Chip Interconnect for Millimeter-Wave Packaging Applications
Doctoral thesis, 2012
In recent years, with the demands for wireless communication systems increas rapidly, the operating frequency for the portable wireless is moving toward millimeter-waves. Millimeter-wave wireless communication systems require not only suitable functional IC components but also competent package with low cost and good interconnect performance. To meet the demands for commercial applications, package with low power consumption, low cost, small size, and light weight becomes indispensable. However, unlike low frequency applications, millimeter-wave frequencies introduce significant parasitics and therefore the interconnect between IC chips and packaging carriers must be carefully managed in order to maintain good electrical performance. Conventional bond-wire induces significant parasitic inductance and thus results in unwanted effects, which could deviate the IC performance after assembly, especially at millimeter-wave frequencies.
Flip-chip interconnect has drawn lots of attentions for chip-level packaging at millimeter-wave frequencies due to several advantages over bond-wire, e.g., shorter interconnect length, smaller package size and higher throughput. However, at MMW frequency range, the proximity effect, or detuning effect, is a crucial issue for flip-chip due to the proximity of chip to substrate. The proximity effect may cause the flipped-chips to deviate from its original performance. Approaches like increasing the bump height, reducing the metal overlap and employing compensation design at the transition region have been proposed to improve flip-chip performance. In addition, flip- chip reliability is very crucial for industrial applications since it relies only on several metallic connections. Using underfill as a buffer layer between chips and carriers can significantly improve flip-chip reliability, but unfortunately, the trade-off is underfill-induced performance decay and deviation. Furthermore, cost-reduction is also very important for commercialization. Conventional ceramic-based carrier offers excellent chemical and physical properties but with higher cost. Using low-cost organic board might be a good solution to get lower cost with fair performance. However, the investigation for flip-chip on organic board is generally insufficient.
This dissertation covers an overall study for flip-chip interconnect for millimeter-wave frequencies. It can be divided into two parts. The first part is about active device packaging. Single MMIC chips and mm-wave modules were flip-chip assembled for demonstration. A V-band SPDT switch for half-duplex RF front-end switching was flip-chip assembled and RF characterized to 67 GHz. By adopting hi-compensation design, the packaged switch showed excellent frequency response and very low additional loss.
Moreover, a V-band frequency source with a 7 GHz oscillator and a x8 multiplier was flip-chip assembled onto a multi-chip carrier. For comparison, both the oscillator and x8 multiplier were also bonded as individual chips. From the measurement results, the flip-chip technique did not have any detrimental effects and the assembled module showed excellent phase noise of -112 dBc/Hz @ 1 MHz offset with high output power of 11 dBm, demonstrating outstanding performance for millimeter-wave frequency generation.
The second part is about material investigation in a flip-chip system. Underfill is generally required for improving flip-chip reliability. However, underfill in a flip-chip interconnect might introduce negative effects i.e., chip impedance mismatch and dielectric loss at millimeter-wave frequencies. To investigate and solve this issue, an epoxy-based was applied to a flip-chip structure and measured up to 67 GHz. By using pre-matching design and low-loss underfill, the flip-chip assembly exhibited excellent performances with return loss below -20 dB and insertion loss less than 0.6 dB. In addition, the reliability test revealed that the flip-chip assembly also performed excellent reliability. The other material investigation is about flip-chip carrier material. Low-cost Rogers RO3210TM organic laminate was employed to replace ceramic-based carrier for cost reduction and performance improvement. Both passive transmission lines and active discrete mHEMTs were flip-chip bonded onto RO3210TM. The test results showed that RO3210TM is a promising packaging carrier for commercial applications up to 50 GHz.