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FCCSP IMC Growth under Reliability Stress follow Automotive Criteria
Keywords: Automotive, IMC, Flip Chip
The automotive integrated circuit (IC) market will outgrow by two even three times of the existing IC market. Market researchers predict that automotive semiconductors will occupy more than 15% of the total semiconductor market by 2025, especially for those analog IC of intelligence vehicle. It is noteworthy that flip chip package becomes the automotive devices solution gradually due to the higher efficiency & complexities of the pin design. More and more design houses are moving toward to flip chip or wafer level fan-out package design for automotive infotainment, radar & GPS application. These changes will enable automobiles become reliable and intelligent, so as let the packaging industry prioritize the development of advanced package for next generation of automotive market requirements. Focusing on the semiconductor industry, more and more devices turn their assembly form from legacy wire bonding to flip chip owing to the higher performance with shorter electrical signal transition path. Also, flip chip package can do more complex design since the transition outset will not be limited at outer ring but full die area. Unlike wire bonding, the flip chip assembly requires the media like pillar or solder bump to link signal between the chips & substrate, hence the interconnect of the microstructural evolution do associated to the reliability effectiveness. The Kirkendall Void issue had been raised up, and lots of experts bring forward the view of why KVs were generated and how to inhibit the voids existing to extend the life time for electronic devices. The Kirkendall effect is the atomic movement of a diffusion system with the result of mass flow accompany by a vacancy flow in the opposite direction. Although the voids exist at the intermetallic layer, the electrical signal may still be able to transit means functional test pass. Adding Ni layer is the most common solution to extend the lifetime by blocking the Cu/Sn IMC diffusion since it is more resistant to dissolution into solder joints, yet higher resistance will be due to the natural characteristic of Ni layer, even some of the magnetic sensitive devices may have concern for adding these material, hence the lifetime and void formation rate is extremely important for device when do the design from initial stage. However, the kirkendall void had been a well-known issue for long term reliability of semiconductor interconnects, while even the KVs existing at the interfaces of Cu & Sn, it may still be able to pass the condition of un-bias long term reliability testing, especially for 2,000 cycles of temperature cycling test and 2,000hrs of high temperature storage. Three kinds of bump structure LF only, Cu/Ni/SnAg, and Cu/SnAg Cu pillar were selected and allocated with two passivation type which are well known as polyimide(PI) and polybenzoxazole(PBO). Those DOE legs were built up with same mask, except for 1st layer of passivation, but just revised the mask tone for light transferred. The following daisy chain RDL & UBM process is the same. Hence, the three kinds of electrical connection point & passivation type were comparable for the automotive criteria reliability performance. A large numbers of KVs was observed after 200cycles of temperature cycling test at the intermetallic Cu3Sn layer which locate between the intermetallic Cu6Sn5 & Cu layers. Cu6Sn5 IMC will convert to Cu3Sn IMC at initial stage, then Kirkendall void will be found at the interface of Cu & Cu3Sn IMC, which has quality concerning issue if the voids density grows up. These kinds of voids will growth proportional with the aging time at initial stage, but slowing down attribute to the barrier layer of Cu3Sn & Cu interfaces. Besides, the Cu3Sn & Cu6Sn5 do affected seriously by heat, but Ni3Sn4 is not affected by heat or moisture. The first phase to form and grow into observable thickness for Ni and lead- free interface is Ni3Sn4 IMC, and the thickness has little relationship to the environmental stress since no IMC thickness variation between TCT, uHAST and HTSL stress test.
Xenia Liu, Section Manager
ASE Group
Taoyuan, R.O.C.

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