The best welding method for printed circuit boards

The best welding method for printed circuit boards

1, dip tin effect

When the hot liquid solder dissolves and penetrates into the surface of the metal to be welded, it is called metal dip tin or metal dip tin. The mixture of solder and copper forms a new part of copper, partly a solder alloy, which is called dip tin, which forms intermolecular bonds between the various parts to form a metal alloy copolymer. The formation of good intermolecular bonds is the core of the welding process, which determines the strength and quality of the solder joints. Only the surface of the copper is not contaminated, not exposed to the oxide film formed in the air to tin, and the solder and the working surface need to reach the appropriate temperature.

2 Surface tension

Everyone is familiar with the surface tension of water, which forces the cold water droplets on the coated metal plate to remain spherical, since in this case the adhesion of the liquid on the solid surface tends to diffuse less than its cohesion. Wash with warm water and detergent to reduce its surface tension, the water will be coated with grease coated metal plate and the outflow to form a thin layer, if the adhesion is greater than the cohesion will happen this situation.

Tin-lead solder has a greater cohesive force than water, making the solder spherical to minimize its surface area (the same volume case has the smallest surface area compared to other geometrical profiles to meet the minimum energy state requirements) The The effect of the flux is similar to that of the cleaning agent on the grease-coated metal plate. In addition, the surface tension is highly dependent on the degree of cleaning and the temperature of the surface. Only when the adhesion energy is much larger than the surface energy (cohesion) tin.

3 metal alloy co-generation

Copper and tin intermetallic bonds form grains, and the shape and size of the grains depend on the duration and strength of the temperature at the time of soldering. Welding less heat can form a fine crystal structure, the formation of the best strength of the excellent welding point. The reaction time is too long, whether it is due to the welding time is too long or because the temperature is too high or both, will lead to rough crystalline structure, the structure is gravel and crisp, shear strength is small.

Using copper as a metal substrate, tin-lead as a solder alloy, lead and copper do not form any metal alloy copolymer, but tin can penetrate into copper, tin and copper intermolecular bond in the solder and metal connection surface to form metal Alloy copolymer Cu3Sn and Cu6Sn5.

Metal alloy layer (n phase + ε phase) must be very thin, laser welding, the thickness of the metal alloy layer on the order of 0.1mm, wave soldering and hand soldering, the excellent welding point of the intermetallic bond thickness of more than 0.5μm. Since the shear strength of the solder joint 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 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 at which the solder joint is formed. Ideally, the welding should be completed in about 220 s for about 2 seconds under which the chemical diffusion reaction of copper and tin will produce an appropriate amount of metal The alloying materials Cu3Sn and Cu6Sn5 have a thickness of about 0.5μm. Inadequate intermetallic keys are common in cold soldering points or welds that do not rise to the appropriate temperature, which may cause the weld surface to be cut off. On the contrary, too thick metal alloy layer, common in excessive heating or welding too long for the welding point, it will lead to welding point tensile strength is very weak.

4 dip tin angle

When the temperature of the eutectic point of the solder is higher than about 35 ° C, when a drop of solder is placed on the surface of the hot coated flux, a meniscus is formed. To a certain extent, the ability of the metal surface to dip tin The The shape of the meniscus can be evaluated by the shape of the meniscus if the solder crescent surface has a clear undercut, shaped like a grease on a metal plate coated with grease, or even tends to be spherical, the metal is not weldable. Only the meniscus stretched into a small angle of less than 30 to have good weldability.