Sidactor Protection Thyristor Products Page 28 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 Sn-Pb Soldering Recommendations When placing surface mount components, a good solder bond is critical because: The solder provides a thermal path in which heat is dissipated from the packaged silicon to the rest of board A good bond is less subject to thermal fatiguing and results in improved component reliability. Reflow Soldering The preferred technique for mounting the DO-214AA package is to reflow-solder the device onto a PCB-printed circuit board, as shown in Figure 1.15. 1. Screen print solder paste (or flux) 2. Place component (allow flux to dry) 3. Reflow solder For reliable connections, the PCB should first be screen printed with a solder paste or fluxed with an easily removable, reliable solution, such as Alpha 5003 diluted with benzyl alcohol. If using a flux, the PCB should be allowed to dry to touch at room temperature (or in a 70C oven) prior to placing the components on the solder pads. Relying on the adhesive nature of the solder paste or flux to prevent the devices from moving prior to reflow, components should be placed with either a vacuum pencil or automated pick and place machine. With the components in place, the PCB should be heated to a point where the solder on the pads begins to flow. This is typically done on a conveyor belt which first transports the PCB through a pre-heating zone. The pre-heating zone is necessary in order to reduce thermal shock and prevent damage to the devices being soldered, and should be limited to a maximum temperature of 165C for 10 seconds. After pre-heating, the PCB goes to a vapor zone. The vapor zone is obtained by heating an inactive fluid to its boiling point while using a vapor lock to regulate the chamber temperature. This temperature is typically 215C, but for temperatures in excess of 215C, care should be taken so that the maximum temperature of the leads does not exceed 275C and the maximum temperature of the plastic body does not exceed 260C. (Figure 1.16) During reflow, the surface tension of the liquid solder draws the leads of the device towards the center of the soldering area, correcting any misalignment that may have occurred during placement and allowing the device to set flush on the pad. If the footprints of the pad are not concentrically aligned, the same effect can result in undesirable shifts as well. Therefore, it is important to use a standard contact pattern which leaves sufficient room for self-positioning. After the solder cools, connections should be visually inspected and remnants of the flux removed using a vapor degreaser with an azeotrope solvent or equivalent. Time (Seconds) 0 20 40 60 80 100 120 140 160 180 200 220 240 30 60 90 120 150 180 210 240 270 300 Temperature - C 0.5-0.6 C/s 1.3-1.6 C/s <2.5 C/s Peak Temperature 220 C-245 C 60-90 s typical (2 min MAX) Reflow Zone Pre-heating Zone ( 2-4 min MAX ) 30-60 s typical (2 min MAX) 260 Soaking Zone Wave Soldering Another method for soldering components to a PCB is wave soldering. After fluxing the PCB, an adhesive is applied to the respective footprints so that components can be glued in place. Once the adhesive cures, the board is pre-heated and then placed in contact with a molten wave of solder with a temperature between 240C and 260C and permanently affixes the component to the PCB (Figures 1.17 & 1.18) Although a popular method, wave soldering has drawbacks: A double pass is often required to remove excess solder Solder bridging and shadows begin to occur as board density increases Wave soldering uses the sharpest thermal gradient PC board Insert leaded components Turn over the PC board Apply glue Place SMDs Cure glue Turn over the PC board Wave solder Apply glue Screen print glue Place component Cure glue Wave solder Figure 1.15: Reflow Soldering Procedure Figure 1.16: Reflow Soldering Profile for SN-Pb Soldering Figure 1.17: Wave Soldering of Surface Mount Components Figure 1.18: Wave Soldering of Surface Mount and Leaded Components

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