| Summary: | 碩士 === 國立交通大學 === 材料科學與工程系所 === 94 === Due to the trend of miniaturization and the high performance, the dimension of solder bumps keeps decreasing and the current that each bump needs to carry keeps increasing, causing the current density in the solder bump to increase dramatically. As a result of the high current density, minimization of electromigration effect becomes the key to achieve high reliability. In flip chip technology, UBM (Under Bump Metallurgy) is often used to connect chip, solder and board. UBM is mostly composed of Ni or Cu which reacts with Sn to form IMC (Intermetallic compound). IMC is usually composed of Ni3Sn4 or Cu6Sn5 and is used to joint chip and solder. We conjecture IMC can reduce current crowding effect during electromigration test because of its higher resistivity than solder. For the reasons above, our goal is to make the IMC layer as smoothly as possible in order to achieve a longer life time for solder. In our experiment, two types of samples are used; both are aged at 90% of its melting point. One of the samples is thin-film UBM bump. Its structure is Ti 0.1 micro-m /Cr- Cu 0.3 micro-m/Cu 0.7 micro-m/Sn63Pb37/Au0.025micro-m / Ni(P) 5 micro-m/ Cu20 micro-m. Because solder ball is eutectic SnPb, its melting point is 183℃. We used 150℃ to age it. The other was thick -film bump, its structure was Ti 0.5 micro-m /Cu 0.5 micro-m/Cu5�慆icro-m /Ni3 micro-m/Sn80Pb20 /Au 1 micro-m/ electroless Ni 5 micro-m. Sn80Ni20 its melting point is 208℃. We used 170℃ to age it. At the end of the experiment, we found aging effect reduces solder ball life time under 0.28A and 150℃ during electromigration test in thin film structure. On the other hand, we observed an opposite result for thick-film bump structure. We speculated that voids form between UBM and solder in thin-film under thermal aging. However, complete and successive IMC formed in thick-film UBM. We used infrared ray to exam the sample failure in the Al trace or solder bump after electromigration test. Because thick-film UBM has Kelvin probes structure, we also used 4-point probe to measure bump resistance in thick-film bump. Before bump open, we found different failure mechanisms between aging-free and aging solder joint. Sample which age may form IMC bridge between chip and board
during electromigration test. It may be an important reason to prolong the sample life
time.
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