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56 SMT007 MAGAZINE I JULY 2025 K N O C K I N G D OW N T H E B O N E P I L E Preheat Considerations for Re-tinning For many through-hole and surface-mount com- ponents—particularly plastic-bodied parts—brief contact of the leads or terminations with solder for about five seconds is sufficient for re-tinning without preheating, simplifying the process. How- ever, it's essential that components with ceramic or glass bodies, or other thermally sensitive mate- rials, be preheated to reduce thermal shock and prevent damage. Infrared preheating systems are effective, providing rapid, uniform heating, but can pose a risk of damage if not carefully controlled. Forced convection preheating offers a more uni- form and controllable approach, minimizing ther- mal stress on sensitive components. Additional Testing for High-reliability Applications In certain high-reliability environments, supplemen- tal solderability testing is often recommended to verify the robustness of the re-tinned surfaces, par- ticularly after processes like steam aging, which simulates long-term storage conditions, exposing components to elevated humidity and tempera- ture, accelerating the aging process. This method assesses how well the solderable surface main- tains its integrity over extended periods, effectively reproducing a typical shelf life, often equating to about five years. To ensure re-tinned components meet reliability standards, manufacturers recommend a minimum solder thickness of approximately 150 microinches (3.81 µm) before aging, which provides sufficient intermetallic layer growth to withstand environmen- tal stressors. Non-destructive measurement tech- niques, such as X-ray fluorescence (XRF), verify sol- der thickness and composition, while scanning elec- tron microscope (SEM) analysis can be employed to examine the intermetallic layer, if necessary. Conclusion Solderability directly affects the reliability and lon- gevity of electronic systems. Since the recogni- tion of high failure rates linked to porous finishes in the 1980s, standards and testing protocols have evolved to ensure high-quality, reliable joints. Indus- try-standard methods like dip-and-look and wet- ting balance testing play vital roles in verifying sur- face conditions. Re-tinning is an effective reme- dial technique for restoring solderability to oxidized or aged components, especially in high-reliability applications. Proper process control, thorough pre- cleaning, and appropriate testing, together with aging simulations such as steam aging, are essen- tial to validate the long-term performance of sol- dered joints. Together, these practices contribute to robust, dependable electronic assemblies that meet stringent industry requirements. SMT007 Nash Bell is president of BEST, Inc. To read past columns, click here.