What are the causes of cracking PCBA solder joints?
2026-03-02
PCBA solder joint cracking is one of the most difficult reliability problems in electronics manufacturing, and it is not caused by a single factor, but is the result of a long-term combination of thermal stress, mechanical stress, manufacturing defects and aging effects.
First, thermal cycle fatigue is the most common cause of cracking solder joints. Electronics undergo repeated temperature cycles when turned on and off or when the ambient temperature changes. Because materials on PCBA (such as chips, PCB substrates, and solder) have different coefficients of thermal expansion, they expand to different degrees when the temperature rises, resulting in periodic shear stress inside the solder joint. In the long run, lattice slippage and microstructure coarsening will occur inside the solder joint material, which will eventually cause fatigue cracks. Cracks typically originate at the root or interface of the solder joint where stress is most concentrated and gradually expand until they break completely.
Secondly, mechanical overload and vibration shock are also common triggers. When the product is subjected to drops, bumps, or strong vibrations, the solder joints are subjected to instantaneous impacts. If the design does not take into account sufficient mechanical cushioning, or if the component is too heavy (such as large inductors, electrolytic capacitors) and lacks auxiliary fixation, the solder joint may be brittle and fracture due to inability to withstand instantaneous tensile stress or shear force.
Third, welding process defects can directly weaken the initial strength of the solder joint, laying hidden dangers for subsequent cracking. For example, cold soldering is due to insufficient temperature or time for reflow soldering, resulting in the solder not completely melted and wetted, resulting in a gray surface and a loose structure of the solder joint. The void is due to the failure of the volatile gas of the flux to escape in time, leaving a hole inside the solder joint, which will reduce the effective stress area, become a stress concentration point, and accelerate crack propagation. In addition, the pad has insufficient adhesion to the substrate, which can cause the pad to peel off the substrate when subjected to stress.
In addition, the growth of intermetallic compounds is an irreversible physical process. During soldering, the tin in the solder reacts with the copper or nickel on the pad to form a thin layer of IMC, which is necessary for achieving electrical connections and mechanical bonds. But IMC itself has a hard and brittle texture. In high temperature or long-term service environment, the IMC layer will continue to thicken, and too thick IMC layer will reduce the toughness of the solder joint, making the solder joint more prone to brittle cracking along the IMC interface under the action of external force.
Finally, with the advancement of lead-free solders, lead-free solder (such as SAC305) has weak buffer ability to mechanical stress due to its high elastic modulus and poor ductility compared with traditional lead-free solder, which also makes the problem of solder joint cracking more sensitive in the era of lead-free technology.
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