Why is brazing called "seamless" welding!

【Summary Description】 When brazing, the solder is generally selected according to the melting temperature. In production practice, the liquid solder is often used to fully infiltrate the gap and make the joint "seamless" as a good criterion for brazing. The behavior of the solder during the brazing thermal cycle is mainly manifested by the phase change, but a series of interfacial physical and chemical reactions occur during the solder phase change, which begin with the melting of the solid solder and finally the solidification of the liquid solder. Therefore, the behavior of solder can be further divided into three stages: melting, interfacial reaction (including wetting, flow, filling, spreading, dissolution, diffusion, etc.), solidification and crystallization, and the most important of which is the interfacial reaction. The wetting and diffusion of liquid solder are generally only carried out within a few microns to tens of microns on the surface of the base material, indicating that the technological properties of the solder in the brazing process mainly refer to the physical and chemical properties of the liquid solder except for melting and crystallization. No matter which brazing method and solder product are adopted, the common point is that the solder is melted under the condition of lower than the solidus temperature of the base material, so that the liquid solder generates wetting and capillary flow in the gap or surface of the base material to fill the gap or spread, and at the same time, dissolution and diffusion occur between the interface of the liquid solder and the solid base material to generate intermediate metal compound (IM C). When the heat source leaves, the liquid solder then cools and solidifies to realize the bonding joint of the metal. Electronic brazing, also known as "micro-connection", is characterized by a significant difference between the size of the connected object and ordinary welding. Some factors that can be ignored in traditional welding technology, such as dissolved amount, diffusion layer thickness, surface tension, strain, etc., will have an important impact on the weldability and welding quality of micro-connections. With the rapid development of microelectronics technology, especially the breakthrough of chips and ultra-large-scale integrated circuits, micro-connections have entered micro-features from micro-features, which makes the connection process tends to be diversified, the connection equipment tends to be precise, and the connection technology tends to be complicated. Electronic precision connection mainly adopts soldering, including soldering iron welding, dip welding, wave soldering, reflow soldering, B G A (ball grid array) welding, hot pressure welding, ultrasonic welding, hot pressure ultrasonic welding and diffusion brazing of bump connection, etc. The solder used is mainly tin-based alloys, including binary, ternary and quaternary alloys formed by Sn, Pb, Sb, Cu, B I, In, A g, A u, etc. These alloys are mainly used as ingots, strips, rods, wires, plates, strips, foils, sheets, balls, powders, pastes and other products.

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