Physical characteristics of the hottest packaging

Physical characteristics of packaging and assembly

there are five physical characteristics that are particularly important for today's packaging and assembly: metallurgical phase transition temperature

, which has practical implications. Liquidus temperature can be regarded as equivalent to melting temperature, and solidus temperature is equivalent to softening temperature. For a given chemical composition, the range between the liquidus and the solidus is called the plastic or viscous stage. The solder alloy selected as the connecting material must be suitable for the final use temperature under the worst conditions. Therefore, the alloy is expected to have a liquidus at least twice as high as the desired maximum service temperature. When the service temperature is close to the liquidus, solder usually becomes mechanically and metallurgically "fragile"

the electrical conductivity of solder connections describes the transmission performance of their electrical signals. By definition, conductivity is the movement of charged ions (electrons) from one position to another under the action of an electric field. Electronic conductivity refers to metal, and ionic conductivity refers to oxide and nonmetal. The conductivity of solder is mainly produced by electron flow. Resistance - contrary to conductivity - increases with temperature. This is due to the weakening of electronic mobility, which is directly proportional to the average free path of electronic movement when the temperature rises. Anhui Yuantong water treatment equipment Co., Ltd. has won the Anhui famous brand title line (mean free path). The resistance of solder may also be affected (increased) by the degree of plastic deformation

the thermal conductivity of metals is usually directly related to conductivity, because electrons are mainly conductive and thermally conductive. (for insulators, however, phonon activity dominates.) The thermal conductivity of solder decreases with the increase of temperature

since the beginning of surface mount technology, the problem of coefficient of thermal expansion (CTE) has been often discussed, which occurs when the CTE of SMT connection material properties usually differ greatly. A typical assembly consists of FR-4 board, solder and components without pins or with pins. Their respective coefficient of temperature expansion (CTE) is 16.0 ×/°C(FR-4); twenty-three ×/°C(Sn63/Pb37); sixteen point five × Maybe this practice belongs to scratch test/° C (copper pin); And 6.4 ×/° C (alumina Al2O3 pinless element). Under the fluctuation of temperature and the switching of power supply, these CTE differences increase the stress and strain in the welding point, shorten the service life, and lead to early failure. Two main material properties determine the size, crystal structure and melting point of CTE. When the material has a similar lattice structure, it should focus on the above two types of construction machinery. Their CTE is opposite to the melting point

the surface tension of molten solder is a key parameter, which is related to the wettability and subsequent weldability. Due to the combination of fracture on the surface, the relative strength of the attraction between the molecules on the surface is weaker than the molecular force inside the solder. Therefore, the free surface of the material has higher energy than its interior. For the melted solder that melts the pad, the surface of the pad must have higher energy than the surface of the molten solder. In other words, the lower the surface energy of the molten metal (or the higher the surface energy of the "metal" pad), the easier it is to melt wet

information source: SMT information