Drag Soldering: Your Answer to Rework and Touchup

Rework is like taxes – an unavoidable eventuality. No matter how precise operators try to be, the printed circuit board (PCB) assembly process by its nature creates the need for rework and touchup. And it’s only getting more challenging. PCBs are getting ever smaller due to the evolution of modern electronics and micro components. Devices, like quad flat packs (QFPs), require time, training, skill, and dexterity. High pin counts (often up to 256 leads) and fine pitch (down to 0.015″) can present challenges even for experienced solder technicians. Because rework is so common and often time-consuming, it’s critical to explore all of your soldering method and tool options.

Technique Essentials

Drag soldering works by dragging a bead of solder across fine-pitch pins so you can quickly solder an entire row. This replaces manual point-to-point soldering where individual solder connections are soldered or reworked one connection at a time. The traditional point-to-point process is very time-consuming and costly, in terms of both labor and materials. The smaller the solder tip geometry, the faster the tip burns out. This may not seem like a major concern, but as soldering equipment and tips get better, more sophisticated, and more expensive, it can be a factor, especially over time.

Drag soldering is most effective when using a special soldering iron tip, called a “gull-wing” tip, with a concave surface or spoon shape to hold molten solder. The operators drag this molten solder across the leads, letting the surface tension and natural wetting forces of the solder deposit the correct amount on each lead. It takes experimentation to control the pressure and speed. Ideally, you should glide across the leads with little to no pressure, with the speed determined by the thermal mass of the board. Surface tension physics help, resulting in consistent, uniform solder volume and appearance. Typically, the quality of these solder joints will meet IPC Class 3 inspection criteria.

The right tools

Because solder only flows to surfaces that are above solder melt temperature, the speed will be determined by how long it takes to get the PCB to land up above solder melt temperature or 217°C for SAC305. This is why it’s imperative to have a quality solder station and iron with active tip temperature monitoring and control.

The good news is that like a lot of production tools, recent advances have made soldering tools more efficient, more precise, and easier to use. The Weller WX2021 two-channel soldering station is an excellent example, offering good value and the updated features needed for effective drag soldering. The WX model soldering irons heat extremely fast so you can get started almost immediately. Modern design and components include a capacitive touchscreen made of anti-static and temperature-resistant glass and a turn-and-click wheel with a confirm button for easy operation. A large graphic LCD display can be viewed from all angles, which is always helpful for greater visibility. The ability to act as a benchtop controller for the WHP1000 / WHP3000 hot plates and Zero Smog 4V / 6V filtration units, as well as a multi-purpose USB port for parameter configuration and data logging, add to its versatility.

One size doesn’t fit all

The WX2021 is ideally suited to drag soldering, which is most often, the easiest, most consistent, and repeatable way to solder or touchup a QFP or any multi-lead SMD. But this isn’t always the case. If there are only a few leads that need rework, it might be best to use the traditional point-to-point technique. If there are a large number of leads to be soldered or the entire device has to be placed and soldered, then drag soldering is much more efficient. It’s also important to remember that external liquid flux is highly recommended and will make drag soldering easier and more effective. So robust cleaning after rework is essential, even with no-clean fluxes.

Get started

Just like equipment evolves, so do techniques. If you aren’t familiar with drag soldering, give it a try. With the right tool, a little practice, and an experienced trainer or technician to demonstrate the finer points, anyone can become proficient at it. You’ll see the results of your work on the bench every day. The high quality, high yield, and low cost of the final product will also bring significant long-term benefits.

Experienced with drag soldering? Add a comment with your success story or check out Weller’s full line of soldering products.