Why does solder crack in electronics PCB?

The question in the article's title most often arises when repairing electronics for various purposes. To begin with, I will explain the reasons for the mechanical destruction of solders, and then we will delve into the roots of this problem. Considering the fundamental principles helps design durable PCBs and select the right materials and technologies to prevent premature failure of solder joints and PCBs.

There is not a single molecule on the planet that is not in motion. Absolutely everything vibrates, but these vibrations are more pronounced in our context. Electronics are used in transport, wearable gadgets, and critical operating conditions. Vibration gradually, rolling after rolling, weakens the coupling of many metals with each other, and sooner or later, the joints may be breaks. Reducing vibration through component support and proper PCB design can significantly reduce the number of cracks in solder joints caused by vibration.

High voltages and currents passing through the boundary of alloys and metals transfer molecules from one side to another, like on a car spark plug. In our case, there is no gas environment in the form of oxygen through which the transfer occurs, but there is heterogeneity of metals, different metals with different densities, pores or poor quality, and purity of connecting metals. Many have often observed fallen-off contact groups at the soldering point of the kinescope lamp of old TVs. Using high-quality soldering materials and correct manufacturing technology will extend the shelf life of soldered joints under high-voltage conditions.

The quality of solders made using old dirty technologies leaves much to be desired. The purity level of solder alloys is very poor, visible even by the color of cheap tubular solders - alloys of a matte lead color with inclusions of foreign particles. The problem is aggravated by incorrectly selected soldering flux, which can also be caused by gross violations of composition and technology. I recommend using only high-quality soldering fluxes made from high-quality materials and fully comply with IPC standards, such as Diamond Flux FN231 soldering flux, Diamond Flux FN232 liquid soldering flux, and Diamond Flux FA241 high active soldering flux.
Using LOW-quality solders and soldering fluxes, you get a problem of slow action. Such solders are highly short-lived and very susceptible to destruction from external influences. By using high-quality solders and fluxes, you will ensure lattice stability and reduce micro-voids, preventing early cracking of solder joints. Using IPC standards will give you a better understanding of material and soldering issues.

Temperature fluctuations reduce the operating time of electronics and equipment several times and poor-quality equipment by tens of times. Different metals have different coefficients of thermal expansion. In the heat, copper (coefficient 16.6) and tin (coefficient 23.4) will expand differently, which will inevitably lead to microscopic cracks that will expand over time.
Another example is the soldered aluminum component leg to Sn63Pb37 alloy.

I have collected the metals that are most often found in electronics on the table.

Coefficients of thermal expansion

It is easy to see that from 0 to 100 degrees their coefficients are different. Accordingly, a crack will gradually form at the boundary of the solder and the metal due to the different coefficients. One metal will always expand more than the other.
The complex of problems described above guarantees the appearance of cracks in the solder over time. Therefore, to create responsible electronics, it is necessary to use slightly different approaches both in development and in production. By designing printed circuit boards with thermal compatibility in mind, PCB designers can slow down cracking and extend the life of the device.

Minimization.
The smaller the component, the less it is subject to thermal expansion due to the reduced area of ​​metals. The smaller the component, the less the impact of vibration due to the lower weight of the component and solder. If the component is large and the currents on it are high, then to increase the life cycle of such components, another therapy is used - the quality of the component itself, its support and the right choice of soldering materials. And hence the next point...

It will be difficult for some to adapt, but the presence of lead in solders is not only harmful to ourselves and the environment, but also lead does not allow us to reach a higher quality level of the solders themselves. The elimination of the lead component in solders helped to solve the problem of low melting points. That is, it is possible to create devices with high operating temperatures, and at the same time, there is no degradation of the solder itself, and this is a stumbling block. The elimination of lead also made it possible to solve the problem of the quality of the crystal lattices after the solder had cured. The point is that the more homogeneous the metal, the more homogeneous it cools, and when cooling, its crystal lattice becomes relatively smooth and uniform. But it is necessary to remember that the transition time from liquid to solid form occurs quite quickly - within two degrees. It is very important to pass this point very slowly, but this is a topic for a separate article about production. At the same time, the more homogeneous the metal, the worse it copes with low temperatures, and an allotropic transformation occurs - tin plague - thin threads. To prevent this effect, silver and other metals are added to tin in low (up to 5%) proportions to give the alloy resistance to low and high temperatures without significant degradation while giving the alloy elasticity. Of course, any soldering must be accompanied by high-quality soldering flux, which must be washed off after production so that foreign chemical elements do not remain on the metal surfaces.
Thanks for reading!