SMT007 Magazine

SMT-July2016

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104 SMT Magazine • July 2016 the heat-resistance properties of components or the printed circuit boards (PCBs). The key ben- efits of laser soldering are listed below [2] : 1. Non-contact and controlled heating: In laser soldering, a precisely focused laser beam not only provides a strict control of quantity heating on the desired soldered location lead- ing to a fast and non-destructive of an electrical joint, but also minimizes energy consumption for the demand of low running cost. 2. Precise and controllable process: The pro- cess parameter can be accurately controlled based on different components types to pro- vide repeatable results. Furthermore, it enables soldering in narrow confines on high-density board assembly [3] . 3. No excessive thermal damage. This local confined energy transfer as heat input will not lead to damage to the surrounding materials, especially to the heat-sensitive components. 4. Reduce mechanical stress. The warpage is- sue of package during reflow process can be mit- igate due to the relatively small rise of package temperature with rapid ramping and cooling. 5. Easy to apply in a variety of demands for soldering, such as components that cannot be reflow in a conventional oven, using different substrates that require a soldering temperature different than the usual ones, repair of electron- ic components, and joining of fine wire bonds. Hence, laser soldering have attracted more attention as a new soldering method in the packaging and assembly of microelectronics, optoelectronics, and flat package IC [4, 5] , even for flip-chips bumping in 3D package [6] . In 1976, laser was first commercially applied in interconnections of microelectronics by C.F. Bohman [7] . During the last few years, soldering process using laser have been a well-established technology, mostly taking into consideration of various parameters of the soldering process: de- position of solder paste, different alloy compo- sitions, types of laser beam used, and both time and power of laser soldering [8-9] , in order to opti- mize technological procedure of laser soldering. This paper presents a new specific formulated solder paste in a dispensing way for laser sol- dering with high quality solder joints. The first part of the research revealed some important details how a solder paste has been developed to be applicable in dispensing and laser solder- ing in terms of flux formulas in the solder paste. The correlations were realized on the study of rheological characterization of solder paste and thermogravimetric analysis by rheometer and thermogravimetric analyzer (TGA), respective- ly. In the second part of the work, experiments on the ball-grid-array (BGA) solder spheres with solder paste on pads were carried out. The void- ing check was observed by X-ray machine. The cross-section inspection of IMC formation was investigated by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), as well as the quality of joints soldered. Experimental Procedure Preparation of Solder Paste and Test Vehicle The new dispensing solder paste for laser sol- dering was developed in mixing solder powder and flux. Alloy of Sn-3.0Ag-0.5Cu (SAC305) was used with the particle size of Type 4, which is followed by the Standard of IPC-TM-650 2.2.14 as Table 1. It is commonly used in assembly of PBGA components nowadays. Flux mainly comprises rosins, thixotropic agents, activators, solvents and other additives. In the first part of the research, two kinds of flux- es are formulated with different ingredients, in which rosins, thixotropic agents and additives are controlled the same, in order to realize the effects on dispensing characterization and sol- dering performance using laser. Table 2 present the brief summary of the flux compositions for both samples. These two solder paste samples with different flux formulas in this study are named SP1 and SP2. SP1 is a newly developed for laser soldering technology in a dispensing way, and SP2 is a conventional one. In the second part of the research, BGA test vehicle was simply carried out using BGA Table 1. Type 4 powder size—% of sample by weight. A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY

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