Poor Solder Bridging: Should You Adjust the Solder Bar or the Flux?

Solder bridging after wave soldering is a common issue in the soldering process. When bridging occurs, the first thought is often: Is the wrong solder bar being used? Should we adjust the solder bar type or switch to another solder bar supplier? Or should we consider whether the flux type is compatible? In practice, most soldering processes use wave soldering machines, though some use small solder pots for dip soldering or tinning. In PCBA wave soldering, lithium battery protection board soldering, and electronic component tinning (e.g., power cords), "bridging" (also known as short-circuiting or icicles) is one of the most common and troublesome defects. Excess solder connects adjacent pads, potentially causing short circuits, functional abnormalities, or even component burnout and safety hazards. Many engineers instinctively want to "adjust the solder bar" first—change the alloy, modify the formula, or blindly increase the soldering temperature.

To resolve lead-free solder bar bridging issues, you can try the following methods to determine whether the solder bar or the flux is the main cause:

1. Observe the solder joint surface: If the joints appear dull, rough, with visible oxidation or graininess, this may indicate excessive impurities or an imbalanced alloy composition in the wave soldering bar, leading to poor solder fluidity, high surface tension, and "tail-like" bridging. In this case, consider replacing the bar with a high-quality anti-oxidation or lead-free wave soldering bar. If the joints are smooth but bridging is severe, appearing "stringy" or "water-bead like," the problem likely lies with the flux—insufficient activity prevents the solder from effectively retracting and separating.

2. Check flux spray volume and activity: Flux not only removes oxide films but also reduces the surface tension of molten solder, helping it "pull back" properly after joint formation. If flux spray volume is insufficient or its active temperature window is too narrow, the solder may remain "stuck" together after exiting the wave. In this case, simply switching to a more expensive solder bar is ineffective. The correct solution is to use a higher-activity, better-wetting flux specifically designed for such applications, or to increase the flux application amount.

3. Verify process parameters: Exclude wave soldering equipment factors—insufficient preheat temperature prevents full flux activation, and excessive conveyor speed reduces solder separation time. If process parameters are normal but bridging persists, return to the "solder bar vs. flux" diagnostic logic.

In summary, while the choice of wave soldering bar does affect solder fluidity and surface tension, in most cases the root cause of bridging is not the bar itself. Different verification steps are needed. If the flux lacks sufficient activity or wetting/spreading ability, the solder cannot quickly retract and separate from the pads, leading to "stringing" or "bridging." Conversely, a high-activity lead-free wave soldering bar combined with the correct flux allows solder to naturally retract and form clean, individual joints. If, after confirming the flux is correct and process parameters are normal, persistent bridging remains, then consider adjusting the wave soldering bar formula—for example, using a silver-containing or microelement-added anti-oxidation bar to reduce surface tension and oxidation tendency, thereby improving separation. This logic also applies to manual drag soldering and automated soldering: when using lithium battery soldering wire or lead-free iron soldering wire and bridging occurs, first check whether the flux content is sufficient and the iron temperature is appropriate, then consider changing the alloy composition.

Singway Solder Wire, a company focused on solder material R&D and manufacturing for over a decade, has extensive experience solving soldering problems. When bridging occurs with solder bars, Singway can quickly verify the root cause and provide an overall optimized solution for solder bar soldering. Dongguan Singway Metal Products Co., Ltd. has specialized in the R&D and production of solder bars, solder wire, and solder paste for 19 years, accumulating solutions for various electronics industry soldering processes. Its lead-free solder bar products range from high-temperature resistant bars to high anti-oxidation bars, low-temperature bars, and silver-containing bars, all available for custom production. If you have any questions about wave soldering bars, feel free to consult Singway!