1 tin contamination
When a hot liquid solder dissolves and permeates the surface of the metal to which it is welded, it is said to be metallic or metallic.The molecules of the mixture of solder and copper form a new kind of alloy that is partly copper and partly solder.The formation of good intermolecular bonds is the core of welding technology, which determines the strength and quality of welding points.Only the surface of the copper is free from contamination, no oxide film formed by exposure to the air can touch the tin, and the solder and working surface need to be at an appropriate temperature.
2 surface tension
We are all familiar with the surface tension of water, which keeps the droplets of cold water on the greasy metal plates globular because, in this case, the adhesion that causes the liquid on the solid surface to diffuse is less than the cohesive force.Wash with warm water and detergent to reduce the surface tension. The water drains the greased metal sheet and forms a thin layer.
Tin - lead solder is even more cohesive than water, making the solder a sphere to minimize its surface area (in the same volume, the sphere has the minimum surface area compared to other geometric shapes to meet the minimum energy state requirements).The effect of flux is similar to that of cleaner on the coated metal sheet. In addition, surface tension is also highly dependent on surface cleanliness and temperature.
Production of metal alloy copolymerization
The metal bond between copper and tin forms grains whose shape and size depend on the duration and strength of the welding temperature.Less heat during welding can form fine crystalline structure and form excellent welding spot with best strength.The reaction time is too long, whether due to excessive welding time or high temperature or both, resulting in a coarse crystalline structure that is sandy, brittle and of low shear strength.
Using copper as the metal base material and tin-lead as the solder alloy, lead and copper will not form any metal alloy copolymerization. However, tin can permeate into copper, and metal alloy copolymer Cu3Sn and Cu6Sn5 can be formed by the intermolecular bonds between tin and copper on the joint surface of solder and metal, as shown in the figure.
For laser welding, the thickness of the metal alloy layer is 0.1mm. In wave soldering and manual soldering iron welding, the thickness of the intermetal bond of the superior welding point is mostly over 0.5mm.Since the shear strength of the welding spot decreases as the thickness of the metal alloy layer increases, it is often tried to keep the thickness of the metal alloy layer below 1 valve m, which can be achieved by making the welding time as short as possible.
The thickness of the metal alloy copolymer layer depends on the temperature and time of forming the welding point. Ideally, the welding should be completed within about 220t 2s. Under this condition, the chemical diffusion reaction of copper and tin will produce a suitable metal alloy binding material Cu3Sn and Cu6Sn5 with a thickness of about 0.5m.Inadequate intermetallic bonding is commonly seen at cold welding contacts or welding points that are not raised to the appropriate temperature, which can lead to disconnection of welding surfaces.In contrast, too thick metal alloy layers, commonly seen in over-heating or welding of welding points for too long, will result in very weak tensile strength of the welding points, as shown in the figure.
4 wetting Angle
Higher than the eutectic point of solder temperature around 35 ℃, when a drop of solder placed in hot coating helps the flux on the surface, has formed a meniscus, to some extent, the metal surface wetting ability through the crescent shape to evaluate below.If the solder has a visible undercutting edge on the curved surface, which is shaped like water beads on a greased metal plate, or even tends to be spherical, the metal is not weldable.Only the meniscus is stretched to less than 30.The small Angle has good weldability.