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88 SMT007 MAGAZINE I JULY 2019 Feature by Michael Gouldsmith and Zen Lee THERMALTRONICS The definition of failure is "the lack of success in doing or achieving something, espe- cially in relation to a particular activity." If the activity is concerning a soldering process, such a failure can have a downstream impact far beyond the actual solder joint. In this regard, it is first necessary to understand what consti- tutes a good solder joint because appearance is too often deemed a success. These challenges to solder joint reliabil- ity were exemplified when, in July 2006, the RoHS directive came into effect, and the higher thermal demands of lead-free solders forced all manufacturers of soldering irons to focus on improved heat transfer. This requirement was further complicated by the ongoing decrease in component sizes and the fact that many PCBs are becoming more like heat sinks due to multi- ple layers and other factors. The importance, therefore, is for soldering irons to provide: • A fast response (speed) • No overshoot (control) Certainly, most systems today offer good or even excellent performance in thermal energy capability, but difficulties emerge in those systems using conventional ceramic heater technology, especially concerning: • Tip-to-ground resistance (difficult to maintain) • Tip-to-ground voltage leakage (difficult to maintain) • Thermal transfer efficiency • The potential for solder splatter (due to temperature overshoot) • A requirement for calibration of the thermocouple In this article, we will explore the consider- ations necessary to achieve good solder joints and offer some practical rules for good solder joints and how to achieve them reliably. We will also discuss other thermal energy factors to keep in mind.