Thyristor Semiconductor Products Page 43 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1005 Surface Mount Soldering Recommendations Introduction The most important consideration in reliability is achieving a good solder bond between surface mount device (SMD) and substrate since the solder provides the thermal path from the chip. A good bond is less subject to thermal fatiguing and will result in improved device reliability. The most economic method of soldering is a process in which all different components are soldered simultaneously, such as DO-214, Compak, TO-252 devices, capacitors, and resistors. Reflow Of Soldering The preferred technique for mounting microminiature components on hybrid thick- and thin-film is reflow soldering. The DO-214 is designed to be mounted directly to or on thick-film metallization which has been screened and fired on a substrate. The recommended substrates are Alumina or P.C. Board material. Recommended metallization is silver palladium or molymanganese (plated with nickel or other elements to enhance solderability). For more information, consult Du Pont's Thick-Film handbook or the factory. It is best to prepare the substrate by either dipping it in a solder bath or by screen printing a solder paste. After the substrate is prepared, devices are put in place with vacuum pencils. The device may be laid in place without special alignment procedures since it is self- aligning during the solder reflow process and will be held in place by surface tension. For reliable connections, keep the following in mind: (1) Maximum temperature of the leads or tab during the soldering cycle does not exceed 280 C. (2) Flux must affect neither components nor connectors. (3) Residue of the flux must be easy to remove. Good flux or solder paste with these properties is available on the market. A recommended flux is Alpha 5003 diluted with benzyl alcohol. Dilution used will vary with application and must be determined empirically. Having first been fluxed, all components are positioned on the substrate. The slight adhesive force of the flux is sufficient to keep the components in place. Because solder paste contains a flux, it has good inherent adhesive properties which eases positioning of the components. Allow flux to dry at room temperature or in a will depend on flux used. With the components in position, the substrate is heated to a point where the solder begins to flow. This can be done on a heating plate, on a conveyor belt running through an infrared tunnel, or by using vapor phase soldering. In the vapor phase soldering process, the entire PC board is uniformly heated within a vapor phase zone at a temperature of approximately 215 C. The saturated vapor phase zone is obtained by heating an inert (inactive) fluid to the boiling point. The vapor phase is locked in place by a secondary vapor. (Figure AN1005.1) Vapor phase soldering provides uniform heating and prevents overheating. Transport Cooling pipes PC board Heating elements Boiling liquid (primary medium) Vapor phase zone Vapor lock (secondary medium) Figure AN1005.1 Principle of Vapor Phase Soldering No matter which method of heating is used, the maximum allowed temperature of the plastic body must not exceed information on temperature behavior during the soldering process, see Figure AN1005.2 and Figure AN1005.3. Figure AN1005.2 Reflow Soldering Profile Time Temperature T P T L T S(max) T S(min) 25 t P t L t S time to peak temperature Preheat Ramp-up Ramp-down Ramp-do

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