Ultrathin soldered flip chip interconnections on flexible substrates

Flip chip assembly of silicon ICs on flexible substrates has gained more interest in the last years. On the one hand there are numerous low cost applications for this technology such as smart cards and smart labels, on the other hand flexible substrates offer a wide potential for highly complex folded packages and 3D modules. Using conventional flip chip assembly processes the stand-off is too high compared to the dimensions of chip and substrate. Until now solder application by stencil printing is only used successfully down to 150 μm pitch with bump heights larger than 70 μm due to the solder paste and stencil features. Deposition technologies for smaller solder volumes together with new assembly processes have to be developed and qualified to realize suitable contact heights. With the immersion soldering process a cost effective maskless bumping process for thin solder layers has been developed. Thermode bonding has been investigated as a promising fast flip chip technology for thin soldered contacts on flexible substrates. In this paper the process development for two different solder materials in combination with noflow underfiller materials will be presented for flip chip contacts of less than 10 μm height. The reliability of thin solder joints is a key issue. Therefore, the failure mechanisms and the ageing behaviour were studied. The intermetallic phase formation has a larger influence because the intermetallics consume the majority of the solder alloy. The effect of intermetallic growth for the different solder materials as well as the impact of the small stand-off during ageing are investigated in this paper. Special emphasis is put on failures resulting from the joint geometry of ultrathin contacts. The promising results of a reliability test program consisting of thermal cycling, temperature/humidity testing and multiple reflow tests are discussed.