Varistor Metal Oxide Varistor Products Page 80 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Surface Mount Varistors > SM20 Series Lead (Pb) Soldering Recommendations The principal techniques used for the soldering of components in surface mount technology are IR Re-flow and Wave soldering. Typical profiles are shown on the right. The terminals of SM20 series devices are tin plated copper, and the recommended solder is 62/36/2 (Sn/Pb/Ag), 60/40 (Sn/Pb) or 63/37 (Sn/Pb). Littelfuse also recommends an RMA solder flux. Wave soldering is the most strenuous of the processes. To avoid the possibility of generating stresses due to thermal shock, a preheat stage in the soldering process is recommended, and the peak temperature of the solder process should be rigidly controlled. When using a reflow process, care should be taken to ensure that the SM20 chip is not subjected to a thermal gradient steeper than 4 degrees per second; the ideal gradient being 2 degrees per second. During the soldering process, preheating to within 100 degrees of the solder's peak temperature is essential to minimize thermal shock. Once the soldering process has been completed, it is still necessary to ensure that any further thermal shocks are avoided. One possible cause of thermal shock is hot printed circuit boards being removed from the solder process and subjected to cleaning solvents at room temperature. The boards must be allowed to cool gradually to less than 50C before cleaning. Lead-free (Pb-free) Soldering Recommendations The terminals of SM20 series devices are tin plated copper, and the recommended Lead-free solder is 96.5/3.0/0.5 (SnAgCu) with an RMA flux, though there is a wide selection of pastes and fluxes available that should be compatible. The reflow profile must be constrained by the maximums in the Lead-free Reflow Profile. For Lead-free Wave soldering, the Wave Solder Profile still applies. Note: the Lead-free paste, flux and profile were used for evaluation purposes by Littelfuse, based upon industry standards and practices. There are multiple choices of all three available, it is advised that the customer explores the optimum combination for their process as processes vary considerably from site to site. PREHEAT ZONE PREHEAT DWELL RAMP RATE <2C/s MAXIMUM TEMPERATURE 230C TEMPERATURE C TIME (MINUTES) 250 200 150 100 50 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 40-80 SECONDS ABOVE 183C TIME (MINUTES) 300 250 200 150 100 50 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 MAXIMUM WAVE 260C SECOND PREHEAT FIRST PREHEAT TEMPERATURE C MAXIMUM TEMPERATURE 250 C, TIME WITHIN 5 C OF PEAK 20 SECONDS MAXIMUM PREHEAT ZONE RAMP RATE <3 C/s 60 - 150 SEC > 217 C TEMPERATURE C TIME (MINUTES) 300 250 200 150 100 50 0 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Reflow Solder Profile Wave Solder Profile Lead-free Re-flow Solder Profile

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