The phenomenon of difficult solder wetting after Reflow Soldering with high-temperature Solder Paste is relatively common in SMT production lines, especially when using high-melting-point solder paste, which is often related to mismatched temperature profile settings, insufficient Flux activity, or contamination on the substrate surface.

First, it is necessary to check whether the reflow soldering temperature profile meets the melting point requirements of the solder paste. If the heating rate in the preheating zone is too fast, the solvent in the solder paste will volatilize prematurely, affecting wettability, while insufficient time in the soaking zone will fail to fully activate the flux, causing severe oxidation on the solder joint surface, which leads to poor solder wetting.

Secondly, attention should be paid to the performance of the solder paste itself. High-temperature solder paste usually contains a high silver content or special alloy components, and its flux activity is weaker than that of ordinary solder paste. Therefore, during the printing process, it is necessary to ensure that the squeegee angle, pressure, and speed of the solder paste all meet the process specifications; otherwise, problems such as uneven solder paste distribution or slumping are likely to occur.

Meanwhile, the cleanliness of the substrate is also one of the key factors. If rosin, oil stains, or oxide layers remain on the PCB surface, it will significantly reduce the wettability of the solder. Especially in high-temperature environments, these contaminants are more likely to react with the solder, forming cold solder joints or bridging phenomena.

In actual operation, it is recommended to conduct a detailed analysis of the problematic batch, including the shelf life, storage conditions, and placement time after printing of the solder paste. Because when high-temperature solder paste is exposed to air for too long, its flux components may gradually lose effectiveness, leading to a decline in soldering performance.

In addition, attention should also be paid to the heating uniformity of the reflow soldering equipment. If the temperature difference inside the furnace is too large, it may cause some areas to be too hot or too cold, thereby affecting the formation quality of solder joints. Especially in the case of multi-layer PCBs or dense component layouts, this temperature difference effect is more pronounced.

To address such problems, you can try adjusting the reflow soldering temperature profile, increasing the time of the soaking zone to allow the flux to fully exert its effect, and appropriately raising the peak temperature to enhance the fluidity of the solder, but it is necessary to avoid excessive temperature that causes component damage or substrate deformation.

Finally, it is recommended to establish a comprehensive solder paste usage record system, including the opening time, usage amount, and remaining amount, so as to detect changes in solder paste performance in a timely manner and reduce poor solder wetting problems caused by material aging.