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www.us-tech.com
September, 2020
Drag Soldering: Touching Up and Reworking High Pin Count SMDs
By Bob Doetzer, President, Circuit Technology, Inc., and Bubba Powers, Manager of Technical Services, Weller Tools P
oint-to-Rework is a necessary evil. No matter how hard we try to perfect the PCB assembly
process, “you-know-what” hits the fan sometimes. We will always need good soldering and rework tools at the benchtop. Some devices, like QFPs require a great deal of time, training, skill, and dexterity to rework. High pin counts (often up to 256 leads) and fine pitch (down to 0.015 in. [0.38 mm]) can present challenges even for experienced sol- der technicians. Just as the introduction of
SMDs in the 1980s revolutionized automated PCB assembly, so too was the introduction of QFPs to the SMT automated process. QFPs enabled very high I/O counts in a small foot- print. But as always, new technolo- gies bring new challenges. The most common source of
problems in any automated SMT process is during paste printing. It has always been the single greatest point of origin for SMT defects. Problems with the paste printing process can result in misalignment, skips, bridges, slump, etc. As the lead count increased and
the lead pitch decreased, QFPs became more problematic to process. Not only are there variables in the printing process, QFPs add the extra
variables of alignment and coplanari- ty. Then add the fact that QFP leads are so delicate, it raises the likeli- hood of problems. The more leads on a device, the more opportunities for misalignment
where the device has to be removed and replaced and other cases where the device only requires some portion of the leads to undergo some touchup. The technique of “drag soldering” is often the best option for QFPs and
manual soldering process in which individual solder connections are sol- dered or reworked one connection at a time. When using this method to solder or rework QFP connections, progressively smaller soldering iron tips and wire solder diameters are required. Since a technician will be sol-
dering each connection individually, this allows them to adjust their heat bridge, dwell time and solder volume to suit each particular connection. This gives the technician a lot of dis- cretion with regard to the rework process, but also reduces the unifor- mity and consistency of the solder connections. This process is time-consuming
and expensive — not only from a labor standpoint but also from a con- sumables standpoint. The smaller the solder tip geometry, the faster the tip burns out. This may not seem like much, but must be taken into consideration as soldering equipment and tips get better, more sophisticat- ed and, therefore, more expensive.
Soldering of individual leads with the point-to-point method.
and coplanarity problems. This results in a greater need to rework QFPs after initial manufacturing. When rework needs to be done, there may be cases
high pin count SMDs. Point-to-Point Soldering Point-to-point soldering is a
Drag Soldering QFPs Drag soldering is a manual sol-
dering process that uses a special sol- dering iron tip with a concave surface or “spoon” shape to hold molten sol-
Continued on next page
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