When we encounter solder joint cracks on the production line, our first reaction is not to replace equipment but to check the Solder Paste batch and reflow profile, because more than 85% of hidden cracks originate from the mismatch between the thermal expansion coefficient of the solder paste itself and the substrate, combined with an excessively fast cooling rate during the cooling stage that generates shear stress inside the solder joint. When this stress accumulates to a critical value at the solder joint edge or the IMC interface, microcracks will form. Such cracks are often invisible under X-ray, but they break out centrally during the customer's high-temperature and high-humidity aging test.

The selection of solder joint solder paste must closely follow three hard indicators: the Tg temperature must be more than 20℃ higher than the customer's maximum operating temperature, the elongation must be greater than 45% to absorb thermal deformation, and the silver content between 0.3% and 0.7% can not only ensure wettability but also avoid reduced fatigue resistance caused by excessive brittle Ag3Sn phases. We once used the same 0402 resistor for comparison after 500 cycles of thermal shock from -40℃ to 125℃, and found that the cracking rate of solder joints using low-modulus solder paste was only 1/6 of that of high-modulus models, and the crack initiation position shifted from the pad edge to the middle of the solder body, indicating that the stress release path has undergone an essential change.

The stencil aperture size in the printing process must strictly match the solder paste particle size. When using Type 4 solder paste, if the Type 3 stencil aperture is still used, it will lead to insufficient solder paste volume and insufficient solder joint height after reflow. During cooling and shrinkage, there is a lack of redundant plastic deformation space, which is extremely likely to induce transverse cracks at the root of the solder joint. Although Type 5 solder paste is suitable for 01005 devices, it is more sensitive to ambient temperature and humidity. When the workshop RH exceeds 60%, its viscosity attenuation rate is 3 times faster than that of Type 4, which directly causes poor demolding and an increased risk of microcracks associated with solder balls.

The peak reflow temperature is not the higher the better. When the Flux residue in the solder paste lasts for more than 60 seconds above 230℃, its carbonized products will embed into the IMC layer and weaken the bonding force between Cu6Sn5 and the solder matrix. At this time, even if the solder joint appears full in appearance, it is easy to peel off along this interface during the drop test. We tracked a batch of medical motherboards and found that after reducing the peak temperature from 245℃ to 238℃ and shortening the holding time to 45 seconds, the fluctuation range of solder joint shear strength narrowed from ±12% to ±5%, and the thrust attenuation rate after high-temperature storage decreased by 40%.

The reliability of solder paste cannot only rely on the elongation and melting point in the data sheet, but also must verify its actual performance under the real PCB laminated structure. For example, the combination of 1.6mm thick FR4 + 2oz copper thickness will make the solder joint cooling rate 1.8 times faster than that of the standard board. At this time, it is necessary to match the solder joint solder paste with a lower glass transition temperature and faster stress relaxation response, otherwise no matter how good the process parameters are, they cannot suppress the inherent defects at the material level.