Sidactor Protection Thyristor Products Page 23 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 PCB Design Guidelines There are two different types of PCB pad configurations commonly used for surface mount leadless QFN packages: 1) Non Solder Mask Defined Style (NSMD) 2) Solder Mask Defined Style (SMD) The NSMD contact pads have the solder mask pulled away from the solderable metallization, while the SMD pads have the solder mask over the edge of the metallization. With the SMD pads, the solder mask restricts the flow of solder paste on the top of metallization that prevents the solder from flowing along the side of the metal pad (see Fig. 1.8 below). This is different from the NSMD where the solder will flow around both the top and sides of the metallization (see Fig. 1.9 below). NSMD pads are recommended over SMD pads since the copper etching process is capable of a tighter tolerance than the solder masking process. Additionally, NSMD pads with solder mask opening larger than the metal pad size also improves the reliability of the solder joint as solder is allowed to wrap around the sides of the metal pads. NSMD Pad Design Considerations The solder mask should be located at least +/-3 mils (0.076mm) away from the edge of the solderable pad when dimensionally possible. This allows for solder mask registration tolerances and ensures the solder is not inhibited by the mask as it reflows along the side of the metal pads. Introduction This document is written to serve as a guideline to help the user in developing the proper PCB design and surface mount process. Development effort and actual studies may still be needed to optimize the process in order to meet individual specific requirements. Littelfuse's Q2L Quad Flatpak - No Lead Package (QFN) is a near Chip Scale Package (CSP) that uses conventional copper leadframe technology. Mechanical, thermal, and electrical connections are made through the exposed lands on the bottom of the package. This construction enables the use of a stable thermal path and electrical ground through a robust mechanical solder connection to the PCB. Its miniature dimension and low profile (1.0 mm height on PCB) requires less board area which increases board density compared to traditional leaded surface mount packages. The QFN packaged product allows for a decreased package size without sacrificing performance. This package platform is ideal for high density circuits and for handheld electronic products. Package Design The QFN packages are designed in MAP (Matrix Array Package) leadframe format and individually singulated by using a saw process (see Fig. 1.7 below). It can provide customized body size and customized land format design for specific design needs and applications. Figure 1.9: NSMD pad PCB Placement Guidelines for QFN - Quad Flatpak No-Lead Package 3 x 3 QFN 5 x 6 QFN Figure 1.8: SMD pad Figure 1.7: QFN package images

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