September, 2020
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Drag Soldering: Touching Up and Reworking High Pin Count SMDs
Continued from previous page
der. This ball of molten solder is then “dragged” across the leads of the QFP, letting the surface ten- sion and natural wetting forces of the solder deposit the correct amount of solder on each lead. This is a similar process to wave soldering, only the assembly and the solder source are inverted. Instead of the assembly traversing across the top of a wave of molten solder, the assembly is held stationary and the solder source is passed across the top of the leads. The drag soldering tip often is called a “gull
wing” tip for the old vernacular of the QFP being called a “gull wing device.” Like the wave soldering process, drag soldering must be performed with external flux added to the lead/land interface. Since drag soldering relies on
the physics of the surface tension of the solder to form the solder connec- tions, it results in consistent, uniform solder volume and appearance. Typically, the quality of these solder joints will meet the requirements of IPC Class 3 inspection criteria. The variables to control are pressure (very light) and speed. The operator should glide across the leads with lit- tle to no pressure, and the speed is determined by the thermal mass of the board. Solder will only flow to surfaces
that are above solder melt tempera- ture so the speed will be determined by how long it takes to get the PCB lands up above solder melt tempera- ture — 422.6°F (217°C) for SAC305. It is imperative to have a high-quali- ty solder station and iron with active tip temperature monitoring and con-
Drag soldering with a mini wave tip.
that the reworked component and all those around it must be cleaned, even when using no-clean flux. Drag soldering can be used for touching up,
for soldering a few leads or for an entire device. It is fast and relatively easy with some practice, and it results in uniform solder connections. The amount of solder that will be deposited at each con- nection is a function of the lead and land geome- tries, as well as placement accuracy and coplanari- ty).
One potential drawback is the fact that drag
soldering cannot be accomplished without the use of external liquid flux. As a matter of fact, more flux and higher solids typically result in greater
Continued on page 62
Page 59
WXMP iron with a GW tip.
trol. With the right tool, some prac- tice and preferably a good instructor, anyone can become proficient at drag soldering.
Comparing QFP Rework Methods
Point-to-point rework and sol-
dering of QFPs may be the best process if there are only a few leads that require rework. Typically, a defect such as bridges, insufficient solder or cold solder on a small num- ber of leads would be best remedied with a point-to-point process. If there are a large quantity of leads that need to be soldered or the entire device has to be placed and soldered, then point-to-point soldering will take too long. The use of external liquid flux is
highly recommended, no matter which method is used. This means
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