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98 SMT007 MAGAZINE I SEPTEMBER 2019 There are numerous methods for getting the solder onto the right pads in the right volume during SMT rework of high pin count or very small footprint SMT devices. The most com- mon types of solder deposition include print- ing, dispensing, and hand soldering. Each of these methods has pros and cons, depending on a variety of factors in the rework process. Hand Soldering Rework Hand soldering of high lead count or very small package SMT components for rework is fast and requires a high degree of skill from the operator. The good news about using the hand soldering process for these types of SMT com- ponents is that a device during debug can be quickly reworked and passed back to test or design engineering. However, there are some deficiencies in using a hand soldering rework process for elec- tronic components. First, the lack of consistent solder volumes and consistent hand solder- ing uniformity makes this an uncontrolled pro- cess. When the process allows for adding flux, the lack of control over this flux volume means that flux residue and board cleanliness can be a potential reliability risk. For some of the fine-pitched parts in high-density areas, a high degree of rework technician skill is required, which limits the throughput (Figure 1) In addi- tion, nearby components can become dam- aged if the technician is not careful when plac- ing the soldering iron. Also, hand soldering is not a cost-effective option in some cases, as it can be very time-consuming. Stencil Printing Stencil printing for the rework of high pin count or very small package SMT components for PCB rework has some pluses and minuses. The advantages to using stencil printing for the rework of these devices are numerous and include this being a well-known technique with well-known materials, requiring a moder- ate degree of skill from the rework technician. The speed of the rework method is also fast compared to the other methods. The flexible, single-use stencils (Figure 2), while still being space-constrained, allow the rework technician to get into tight spaces. These adhesive-backed stencils have overcome the limitations of the metal stencils, including board coplanarity, the bending of the stencils during handling, getting into tight spaces, the necessity to tape around the device to prevent solder paste from contaminating the area, and the necessity for a holding fixture for align- ment and time it takes to clean the stencils. Process Methods for Reworking High Lead Count SMT Parts Knocking Down the Bone Pile by Bob Wettermann, BEST INC. Figure 1: Hand soldering for rework of fine-pitched components.

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