Reflow soldering process is widely implemented in the electronics industry. This method allows the attachment of electronic components to a printed circuit board (PCB) through the melting of solder paste, which makes the interconnection between them. The reflow soldering process must ensures the correctly melting of the solder paste and heating of the adjoining surfaces, without the electronic components suffer overheating or any other type of damage. Solder paste is the most widespread material in the SMT (Surface Mount Technology) process using reflow soldering. An ideal solder paste will increase production efficiency, decreasing the amount of defects associated with the reflow soldering process. However, several factors affects the performance of the solder paste, from rheology, printability, and reliability to the adhesion strength of components and the ability to avoid defects related to reflow. Therefore, all these factors need to be considered during the selection of a solder paste for a specific application. The rheological properties were determined using both a double cylinder (PHYSICA-RHEOLAB MC1) and a double plate (Malvern) rheometers. The later enable the determination of viscoelastic properties. The present paper analyses the rheological behavior of a SAC405 solder paste, a mixture containing a metal alloy powder (25–45 μm) and a flux which at its base is a resin. The tests were carried out at conditions (temperature and shear rate) of relevance to the printing process. The results obtained show that the paste viscosity closely follows the Herschel-Bulkley model and shows a thixotropic behavior without fully recovery between applications. In addition, the viscosity decreases with the increase of shear rate confirming that the solder paste is a non-Newtonian fluid, shear thinning in behavior. The oscillatory tests have shown that the transition from elastic to viscous behavior occurs at a shear stress above 35 Pa. On the other hand, the creep/recovery test confirms that the level of solicitation influences the capacity of recovery of the solder paste.

Air jet impingement technology receives considerable attention due to its high performance for heat transfer enhancement in thermal equipment, providing high heat transfer rates. Due to its inherent characteristics of high average heat transfer coefficients and uniformity of the heat transfer over the impinging surface, this technology is implemented in a variety of engineering applications and industrial processes, such as reflow soldering, drying of textile, cooling of turbojet engine blades and fusion reactors. Multiple jet impingement involves several variables such as: jets arrangement, jet diameter, nozzle-to-surface distance, nozzle shape, jet-to-jet spacing, jet velocity and Reynolds number, among others. However, the total control of all these parameters is still one of the remarkable issues of the thermal design of jet impingement systems. In some industries that have implemented this technology in their processes, such as reflow soldering, the range of values of these variables are established through empiricism and “trial and error” techniques. To improve the process and to reduce time and costs, it is fundamental to define accurately all the process parameters in order to obtain an optimized design with a high degree of control of the heat transfer over the target surface. To perform an accurate and complete study of the multiple jet impingement variables for a specific application, the development of both experimental and numerical studies is fundamental in order to obtain reliable results. In that sense, this work reports the project and construction of a purpose-built test facility which has been commissioned, using a PIV system. This experimental setup is based on the oven used in the reflow soldering process. The optimization of the multiple jets geometry which is integrated in the experimental setup is herein described and discussed both experimentally and numerically. The numerical simulation of the jet impingement inside the oven was conducted using the ANSYS software, specially designed to predict the fluid behavior. Regarding the relevance of the multiple jet impingement, this work intends to improve the knowledge in this field and to give reliable and scientifically proved answers to the industries that apply this technology in their processes.

No abstract

A common failure mode of electronic PCB’s is the appearance of cold solder joints between the component and PCB, during product life. This phenomenon is related to solder joint fatigue and is attributed mainly to the mismatch of the coefficients of thermal expansion (CTE) of component-solder-PCB assembly. Although some experiments show that newer lead-free tin-silver-copper (Sn-Ag-Cu, or SAC) solders perform better than the older SnPb ones, with today’s solder joint thickness decreasing and increasing working temperatures, among others, the stresses and strains due to temperature changes are growing, leading to limited fatigue life of the products. As fatigue life decreases with increasing plastic strain, creep occurrence should have significant impact, especially during thermal cycles. In order to improve mechanical properties, but also as an attempt to reduce maximum reflow cycle temperatures due to component damage and production costs, various SAC solder alloying additives are being considered to use in industrial production facilities. Solder paste producers are proposing new products based on new solder paste formulations, but the real life effects on thermo-mechanical performance aren’t well known at the moment. In this paper a dynamic mechanical analyser (DMA) is used to study the influence of Bismuth (Bi) addition, up to 5 wt %, on SAC405 solder paste, in terms of creep behaviour. Creep tests were made on three-point-bending configuration, isothermally at 30 °C, 50 °C and 75 °C, and three different stresses of 3, 5 and 9 MPa. The results shown not only a significant Bi concentration influence on creep behaviour but also a noticeable temperature dependence.

No abstract