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JULY 2019 I SMT007 MAGAZINE 93 2. Thermal energy—which passes through the tip, pad, and joint—activates the flux and depletes the oxidization from the pad and joint (thermal energy at this phase is a mixture of stored and heater energy) 3. Any loss in thermal energy due to the joint is compensated by the heater energy that provides the reflow necessary to form a reliable solder joint. The ideal joint temperature should be solder melt temperature plus 40°C to form a good intermetallic bond. In this phase, it is critical that the heater provides the correct level of response and does not overshoot 4. The final phase is the cool-down period, which takes place 2–5 seconds after the joint is formed It should also be noted that while tip idle temperature is often referred to as a measure of potential solder joint reliability, it can only indicate that the tip temperature is in range (e.g.,±5°C). It does not indicate what happens upon contact with the load (joint). Basically, it is only there as a part of the stan - dard, and the absence of proper joint testing is a simple method of checking tip temperature. So, what is the best way to achieve a reliable solder joint? Other Thermal Energy Factors The achievement of reliable solder joints does not just depend on the tip idle tempera- ture but requires consideration of other ther- mal energy factors: 1. Tip mass (energy is stored in mass) 2. Heater power and control 3. Tip-to-pad contact area 4. Thermal energy bridge (heater g copper tip g joint) 5. Tip geometry 6. Plating thickness on the tip 7. Tip idle temperature 8. Operator skill To overcome the potential problems asso- ciated with conventional heater technology (Figure 9) and to ensure a more reliable solder connection, it is recommended that soldering systems with the Curie Heat Technology (CHT) principle be used wherever possible.* The use of CHT ensures (Figure 10): • Tip-to-ground resistance is maintained • Tip-to-ground voltage leakage will not occur • Thermal transfer will be efficient • No calibration is necessary • No overshoot of temperature Figure 8: Hand soldering profile showing energy phases and transitions.

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