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SMT-July2016

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112 SMT Magazine • July 2016 than that of joints soldered by the first reflow process. The reason for the slight difference in shear strength can be assumed to be the grain structure of IMCs. Typically, the total time of re- flow process is around 8–10 mins for a commer- cial reflow oven with 12 zones, including initial ramp, soaking, spike in general. The heating time is longer enough to form a uniform flat IMC layer with larger grain size. On the other hands, the lower thermal stress was induced during laser soldering to enhance the strength of solder connection. Besides, the thickness of IMC layer is much lower in case of using laser soldering (<1 µm), because the very short sol- dering time. We also can deduce the energy de- livery was sufficient for the formation of strong solder connection with finer grain structure due to a rapid cooling rate and leading to solidifica- tion fast. As stated above, this is also a positive result, as it leads to better electric and thermal characteristic. Conclusions Laser soldering provides a clean non-con- tact process which involves transferring energy to the soldering location using a precisely con- trolled beam. Hence, laser soldering can not only used in assembly of electronic components as a candidate to other soldering technologies, but also illustrated a good prospect for particu- lar applications. In this research, we bring theory to practice that a novel solder paste of alloy composition of SAC305 in a dispensing way is developed for laser soldering technology. The following con- clusions can be formulated: 1. According to the rheological properties of solder paste based on the flux formulas, SP1 is proved to be more applicable for dispensing process. 2. The relationship between fundamental design of flux and soldering performance was investigated by thermogravimetric analysis. It is clear that SP1 could be contributed to mitigate the issues of splash and solder balling when sol- dering using laser. 3. Reliability test of soldered joint with SP1 showed good results in voiding and mechani- cal properties. Since laser soldering is a very rapid process, each joint can be completed in the range of a second and the wasted energy is minimal. An additional advantage is that the joint cools very quickly that brings tougher joints with better mechanical properties owing to the finer grain structure and short inter-me- tallic zone. This leads to excellent electrical and thermal solder joint characteristics. Acknowledgement This work was supported by Likuan Opto Tech Inc. in laser soldering machine. SMT References [1] N. Suenaga, M. Nakazono, and H. Tsuchi- ya, "Laser soldering," Welding International, vol. 2, no. 3, pp. 269–276, Jan. 1988. [2] Y. H. Tian and C. Q. Wang, "12 Laser soldering A2 – Zhou, Y.," in Microjoining and Nanojoining, Woodhead Publishing, 2008, pp. 299–326. [3] P. M. Beckett, A. R. Fleming, R. J. Foster, J. M. Gilbert, and D. G. Whitehead, "The use of lasers for the soldering of high density inter- connects," in 1994/220), IEE Colloquium on Interconnection Technology (Digest No, 1994, pp. 4/1–4/4. [4] A. Flanagan, A. Conneely, T. J. Glynn, and G. Lowe, "Laser soldering and inspection of fine pitch electronic components," Journal of Materials Processing Technology, vol. 56, no. 1–4, pp. 531–541, Jan. 1996. [5] Z. Han, S. Xue, J. Wang, X. Zhang, L. Zhang, S. Yu, and H. Wang, "Mechanical proper- ties of QFP micro-joints soldered with lead-free solders using diode laser soldering technology," Transactions of Nonferrous Metals Society of China, vol. 18, no. 4, pp. 814–818, Aug. 2008. [6] T. Oppert, G. Azdasht, E. Zakel, and T. Teutsch, "Laser assisted soldering and Flip-Chip attach for 3-D packaging," in 31st International Conference on Electronics Manufacturing and Technology, 2006, pp. 86–86. [7] C.F. Bohman, "The Laser and Microsol- dering", Society of Manufacturing Engineering, Tech. paper N0 AD74-810 Mich 48128, 1974, pp.19–20. [8] R. Batorfi, Z. Illyefalvi-Vitez, O. Krammer, R. Bunea, and P. Svasta, "Optimizing techno- logical process for laser soldering of fine-pitch A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY

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