Sidactor Protection Thyristor Products Page 25 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 Lead Finger Stencil Design The optimum and reliable solder joints on the perimeter pads should have 50 to 75 microns (2 to 3 mils) standoff height. The first step in achieving good standoff is the solder paste stencil design for perimeter pads. The stencil aperture opening should be designed so that the maximum paste release is achieved. This is typically achieved by considering the following two ratios: 1) Area Ratio = Area of Aperture Opening / Area of Aperture Wall 2) Aspect Ratio = Aperture Width / Stencil Thickness For rectangular aperture openings, as required for this package, these ratios are: Area Ratio = LW/2T(L+W) Aspect ratio = W/T where L & W are aperture length and width, T is stencil thickness. For optimum paste release the area and aspect ratios should be greater than 0.66 and 1.5 respectively. It is recommended that stencil aperture should be 1:1 to PCB pad sizes as both area and aspect ratio targets are easily achieved by this aperture. The stencil should be laser cut and electropolished. The polishing helps in smoothing the stencil walls resulting in better paste release. It is also recommended that the stencil aperture tolerances should be tightly controlled, especially for 0.4 and 0.5mm pitch devices, as these tolerances can effectively reduce the aperture size. Package Placement and Alignment The pick and place accuracy governs the package placement and rotational (theta) alignment. This is equipment/ process dependent. Slightly misaligned pads (less than 50% off the pad center) automatically self-align during reflow due to surface tension of the solder (see Fig. 1.12). Grossly misaligned packages (greater than 50% off the pad center) must be removed prior to reflow, as they may develop electrical shorts resulting from solder bridges, if they are subjected to reflow. Figure 1.12: Self-Assignment at Reflow Solder Reflow There are no special requirements when reflowing QFN components. As with all SMT components, it is important that profiles be checked on all new board designs. In addition, if there are multiple packages on the board, the profile must be checked at different locations on the PCB. Component temperatures may vary because of surrounding components, location of the device on the board, and package densities. Figure 1.13 is an example of a standard reflow profile for a lead-free solder paste. The paste manufacturer will determine the exact profile, since the chemistry and viscosity of the flux may vary. In general, the temperature of the part should not be raised more than 2 C/sec during the initial stages of the reflow profile. The soak zone then occurs when the part is approximately 150 C up to 190 C and should last for 90 to 120 seconds. Extending the time in the soak zone will typically reduce the risk of voiding within the solder. The temperature is then raised and will be above the liquidus of the solder for 60 to 90 seconds depending on the mass of the board. The peak temperature of the profile should be 30 C to 40 C above the melting point of the solder. However, the temperature during reflow should not exceed the maximum temperature the package is qualified for according to Moisture Sensitivity Level Testing. Finally, Ramp Down Rate from peak temperature to room temperature should not exceed 4 C/sec. PCB Cleaning If a low-residue, "No Clean" solder paste is used, PCB cleaning is not required, and has little effect on QFNs. "No Clean" solder paste simply means that there are no harmful residues left on the board that could cause corrosion or damage to the components if left on the board. However, some types of "No Clean" solder paste may not be satisfyingly free from contamination on the final board, so it is recommended that an experiment should be conducted to examine whether eventually the flux residues are required to be removed. Peak Temp Liquidus 60-90 sec Soak Stage Time (Seconds) Temperature (C) 300 255 220 190 150 0 50 100 150 200 250 Figure 1.13: Typical Profiles for Lead-Free Solder PCB Placement Guidelines for QFN - Quad Flatpak No-Lead Package (continued)

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