SMT007 Magazine

SMT007-Aug2022

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26 SMT007 MAGAZINE I AUGUST 2022 ere are multiple methods for hand sol- dering QFPs as outlined in the IPC 7711.21 Rework and Repair of Printed Circuit Assem- blies process guidelines document. QFPs have several challenges related to their hand solder- ing especially when the component has a high lead count. e methods for hand soldering QFPs listed under IPC 7711 5.5 include the fol- lowing: • Top of the lead • Point-to-point method • Solder paste method hot air system • Hook tip wire layover • Blade tip with wire solder • Adhesive-backed stencil with solder paste and hot air method While these are a head-spinning number of techniques, the following discussion will com- pare the more widely used techniques: point- to-point, drag, and adhesive-backed stencil. Point-to-Point Soldering e point-to-point method is the one with the most history behind it. Point-to-point soldering is a manual soldering process in which individual solder connections are sol- dered one connection at a time (Figure 1). e method usually consists of using a chisel tip connected to the soldering hand piece. First, locate the correct tip. e size of the chisel tip should be such that it can properly create the Soldering for QFPs and Other Gull Wing Leaded Parts "heat bridge" between the land and the com- ponent lead. A good rule of thumb is to have the tip 60–70% of the pad width. e com- ponent should be properly inspected to make sure the leads are coplanar and that they are not damaged and bent. Upon proper inspec- tion, the component should be aligned with the pads and be held with a pair of tweezers or an orangewood stick. Next, liquid flux appro- priate for the assembly is applied at two oppo- site corners and the component is tack-sol- dered in place such that it does not move off the pads. Flux is applied to the remaining lead/ land rows and creates a heat bridge between the land on the PCB, the soldering iron tip, and the component lead adding in solder to form the proper solder fillet. Finally, the fil- lets are inspected per the class required of the assembly. Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: Point-to-point soldering of a QFP.

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