Sidactor Protection Thyristor Products Page 24 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 PCB Pad Pattern The dimensions of the PCBs solderable pads should match those of the pads on the package (see Fig. 1.10 A-D below). PCB Surface Finishes The key factor in selecting an acceptable surface finish is to ensure that the land pads have a uniform coating. Irregular surface plating, uneven solder paste thickness, and crowning of the solder plating can reduce the overall surface mount yields. There are two common surface finishes which are used for PCB surface mount devices. The first consists of an organic solderability preservative (OSP) coating over a copper plated pad. The organic coating assists in reducing oxidation in order to preserve the copper metallization for soldering. The second recommended surface finish consists of plated electroless nickel over the copper pad followed by immersion gold. Of all the coating and plating options available, Ni/Au is the most versatile, providing the gold thickness is controlled. Typically, 5um nickel, and between 0.05um and 0.1um gold are needed to prevent gold embrittlement which may affect the reliability of the solder joint. Board Mounting Considerations Solder Paste The quality of the paste print is an important factor in producing high-yield assemblies. The paste is the vehicle that provides the flux and solder alloy necessary for a reliable and repeatable assembly process. A low-residue, "no-clean" type 3 solder paste should be used in mounting QFNs. Typically, the choice of solder paste determines the profile and reflow parameters. Most paste manufacturers provide a suggested thermal profile for their productsand must be referenced prior to manufacturing. Solder Stencil The stencil thickness, as well as etched pattern geometry, determines the precise volume of solder paste deposited onto the device land pattern. Stencil alignment accuracy and consistent solder volume transfer are critical for uniform reflow-solder processing. Stencils are usually made of brass or stainless steel, with stainless steel being more durable. Apertures must be trapezoidal to ensure uniform release of the solder paste and to reduce smearing (see Fig. 1.11). The solder joint thickness of QFN lead fingers must be 0.050mm to 0.075mm. Thickness of the stencils is usually in the 0.100mm to 0.150mm. The blade angle and speed must be fine-tuned to ensure even paste transfer. An inspection of stenciled board is recommended before placing the parts, because proper stencil application is the most important factor with regards to reflow yields further on in the process. As a guide, stencil thickness of 0.125mm for QFN components is recommended. Figure 1.11: Solder Stencil Profile 2.54mm 0.100" 1.50mm 0.059" 1.27mm 0.050" .0200 .0500 .2920 .2050 .1180 .0279 .0100 .2362 Figure 1.10 B:. 5x6 QFN PCB Layout Figure 1.10 A: 5x6 OFN Footprint Figure 1.10 D:. 3x3 QFN PCB Layout Figure 1.10 C: 3x3 OFN Footprint PCB Placement Guidelines for QFN - Quad Flatpak No-Lead Package (continued)

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