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By Hsiang-Chuan Chen, Ya-Ching Chuang, Jen-Yio Shiu, and Chang-Meng Wang, Shenmao Technology Inc. and Watson Tseng, Shenmao America Inc.
Laser soldering method becomes attractive in the packaging and assembly of surface-mounted devices and microelectronics. The method transfers laser energy for reflow process with a non-contact procedure and determines the soldering location precisely. It is critical to consider formulated solder paste due to the amount of heat absorbed by flux and alloys when applying the method to reliable joints. A new solder paste of alloy composition of Sn-3.0Ag-0.5Cu in a dispensing way is developed for the automatic laser soldering processes. This paper presents the results of soldering BGA spheres on circuit boards with minimal voiding and no splash or solder balling issues. Cross-sectional studies showed the reduction of intermetallic formation in well-formed solder joints. In addition, the joints have high shear strength due to lower thermal stress during the process. This new solder paste was proved to be suitable for repair of electronic components and manufacturing electronic parts that cannot be soldered in a conventional reflow oven.
With the miniaturization of modern electronics with selective soldering and the uses of heat-sensitive electronic components, conventional reflow soldering process can no longer satisfy the requirements in advanced packaging. In recent decades, several alternative reflow soldering processes, such as infrared radiation reflow, vapor phase reflow, and laser soldering, were proposed. The inferior uniformity of temperature is mostly concerned as the use of infrared radiation reflow though the running cost is inexpensive. The characteristics of vapor phase reflow soldering is absolutely opposite to infrared radiation reflow. Compared with various soldering processes, laser soldering technology has distinct advantages over both former ones, in which the entire assembly is passed through an oven that can lead to problems concerning the heat-resistance properties of components or the printed circuit boards (PCBs). The key benefits of laser soldering are listed below:
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 leading 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 process parameter can be accurately controlled based on different components types to provide repeatable results. Furthermore, it enables soldering in narrow confines on high-density board assembly.
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 issue of package during reflow process can be mitigate 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 electronic 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, even for flip-chips bumping in 3D package.
In 1976, laser was first commercially applied in interconnections of microelectronics by C.F. Bohman. 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: deposition of solder paste, different alloy compositions, types of laser beam used, and both time and power of laser soldering, in order to optimize technological procedure of laser soldering. This paper presents a new specific formulated solder paste in a dispensing way for laser soldering 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 soldering 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), respectively. 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 voiding 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.
Preparation of Solder Paste and Test Vehicle
The new dispensing solder paste for laser soldering 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.
Table 1. Type 4 powder size—% of sample by weight.
Flux mainly comprises rosins, thixotropic agents, activators, solvents and other additives. In the first part of the research, two kinds of fluxes 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 soldering 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.
Table 2. The description of two lead-free solder pastes.
In the second part of the research, BGA test vehicle was simply carried out using BGA spheres (SAC305, 0.64 mm in diameter), soldered on the pads (0.50 mm in diameter, two kinds of surface finish, OSP and ENIG) of PCBs. Prior to soldering, the pads were covered with a thin layer of additional solder paste of SP1 to ensure good solder joint formation.
To read this entire article, which appeared in the July 2016 issue of SMT Magazine, click here.