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Electronic solder and its behavior in brazing process
Electronic brazing, also known as "micro-connection", is characterized by the small and fine size of the joined object, which is significantly different from ordinary welding. Some factors that can be ignored in traditional welding technology, such as solubility, diffusion layer thickness, surface tension, strain, etc., will have an important impact on the weldability and welding quality of micro-connection. With the rapid development of microelectronics technology, especially the breakthrough of chip and VLSI, micro-connection has changed from micro-feature to micro-feature, which makes the connection process diversified, the connection equipment tends to be precise, and the connection technology tends to be complicated.
Soft brazing is mainly used in electronic precision joining, including soldering, dipping, wave soldering, reflow soldering, B G A (ball grid array) soldering, hot pressing, ultrasonic soldering, hot pressing and diffusion brazing of bump joints, etc. The solders used are mainly tin-based alloys, including binary, ternary and quaternary alloys formed by Sn, Pb, Sb, Cu, B i, In, A g, A and so on. These alloys are mainly used in ingots, bars, rods, wires, plates, strips, foils, sheets, balls, powders, pastes and other products.
Whichever brazing method or solder product is adopted, the common point is that the solder is melted at a temperature lower than the solid phase line temperature of the base metal, so that the liquid solder can wet and capillary flow in the gap or surface of the base metal and fill the gap or spread. At the same time, the liquid solder can dissolve and diffuse between the interface of the liquid solder and the solid base metal to form intermediate metal compounds (IM C). When the heat source is removed, the liquid solder is then cooled, solidified and bonded joints of real cash.
When brazing, the solder is usually selected according to the melting temperature. In production practice, the seamless joint is often used as a good criterion for brazing according to the full infiltration gap of liquid solder. In the process of brazing thermal cycle, the behavior of solder mainly manifests as phase change, but a series of interfacial physical and chemical reactions occur during the phase transformation of solder. These reactions begin with the melting of solid solder and finally solidify of liquid solder. Therefore, solder behavior can be further divided into three stages: melting, interfacial reaction (including wetting, flow, filling, spreading, dissolution and diffusion), solidification and crystallization, and the most important of which is interfacial reaction. The wetting and diffusion of liquid solder are generally only within a few microns to tens of microns on the surface of base metal, which indicates that the physical and chemical properties of liquid solder are mainly referred to in the process of soldering besides melting and crystallization.
