Investigating how alloy, application, and operator experience influence solder wire performance
by Gayle Towell
On many production floors, operators use whichever spool of alambre de soldadura is provided, often with little information about how its aleación, diameter, or melting behavior may affect the task. But solder wires are not interchangeable, and the differences may matter in ways operators can feel even when they cannot immediately identify the cause.
A recent customer question gave us an opportunity to investigate this variability more closely. Through our Applied Industry Research program, we collaborated with EPTAC and soldering expert Leo Lambert to study how different solder alloys behave in wire form and how operator experience may influence perceptions of their performance.
A Practical Question from the Production Floor
The investigation was inspired by a practical observation from one of AIM’s technical support engineers. Based on both customer feedback and his own experience during a technical demonstration, one CX18 cored solder wire appeared to melt more quickly and easily than another when tested using the same soldering iron and temperature settings.
The observation raised several questions. Was the apparent difference primarily due to the alloys’ inherent melting behavior? Would it remain noticeable during actual joint formation rather than when feeding the wire directly to the soldering iron tip? Would an experienced operator and a novice perceive the difference in the same way? And would the result change between through-hole and surface-mount applications?
Rather than draw conclusions from a single comparison, we used the observation as the starting point for a broader blind evaluation designed to examine how alloy, application, and operator experience interact.
Collaborating with EPTAC and Designing an Experiment
To explore these questions, AIM partnered with EPTAC, an internationally recognized leader in solder training and IPC certification.
For more than 35 years, EPTAC has helped electronics manufacturers improve workmanship, productivity, and process knowledge through training offered across 24 North American locations, online, and at customer facilities.
The evaluation was coordinated with Leo Lambert, EPTAC’s Vice President and Technical Director. Leo has contributed to the electronics manufacturing industry for more than three decades and is the author of Soldering for Electronics Assemblies. His contributions have earned honors including the IPC Raymond E. Pritchard Hall of Fame Award and the IPC President’s Award.
In other words, Leo knows his stuff.
The blind evaluation compared three lead-free solder alloys, all produced as cored wire using AIM’s CX18 flux chemistry. Keeping the flux system consistent helped focus the comparison on alloy-related differences.
Each alloy was tested in two wire diameters. The thicker wires were used on through-hole assemblies, while the thinner wires were used for surface-mount soldering.
Two operators participated: Leo as the expert and a novice who had soldered personal projects but had no electronics manufacturing experience.
This comparison was important because solder wire performance can be highly user dependent. A wire that feels fast and efficient to an expert may feel difficult to control for someone still developing technique. A material that a novice finds predictable may seem unnecessarily slow to an experienced operator.
What Makes a Solder Wire “Easy to Use”?
Some of the most interesting preliminary observations came from the participants’ rankings.
For each assembly type, the expert and novice agreed on which alloy was easiest to use. However, they disagreed about second and third place. The expert placed greater value on speed. An alloy that melted quickly and allowed the joint to be completed efficiently suited his practiced workflow. The novice emphasized predictability. A wire was easier to use when its melting and flow behavior felt consistent and gave the operator confidence in judging when the joint was complete.
Neither perspective is wrong. They reflect different production needs.
Experienced operators may prioritize throughput, while newer operators may benefit from materials that provide greater control and consistency. The result also illustrates why evaluating a solder wire only with expert users may not reveal how it will perform across the full range of production personnel.
Different Applications, Different Preferences
The preferred alloy also differed between the through-hole and SMT evaluations.
This is likely related to differences in thermal mass and solder volume. Through-hole joints generally require more solder and may demand greater heat transfer, while many SMT joints involve smaller volumes and lower thermal loads.
Because the alloys have different melting temperatures and melting behaviors, the application can significantly change how each one feels to the operator. The result suggests that there may not be one universally easiest alloy for hand soldering. The best choice may depend on the assembly, thermal demand, wire diameter, and operator.
Correlating User Experience with Alloy Properties
The hands-on evaluation was only one part of the project.
We are compiling participant observations related to melting, wetting, flow, speed, predictability, and ease of use. The alloys are also being evaluated through wire-tinning trials and slow-motion video of fixed-length wire segments melting.
These observations will be compared with differential scanning calorimetry data and measurements of the total thermal energy required to fully melt each alloy.
This matters because liquidus temperature alone may not explain how a solder wire behaves at the workbench. Two alloys with similar melting temperatures may require different amounts of energy to fully melt, and the shape of the melting transition may affect how quickly and predictably the wire responds.
By combining operator feedback with visual and thermal data, we hope to better explain why the alloys were experienced differently.
The End Goal: Useful Guidance for the Industry
While the analysis is still underway, and the findings discussed here remain preliminary, the goal of the project is not simply to rank three alloys. A ranking without context has limited value.
Instead, we aim to clarify the advantages and tradeoffs of each alloy under different conditions and develop better guidance around alloy selection, wire diameter, thermal demand, equipment setup, and operator training.
When a wire is described as slow or difficult, the answer may not be as simple as changing the product. The facility may need to consider whether the alloy is appropriate for the assembly, whether sufficient heat is reaching the joint, whether the wire diameter matches the solder volume required, and whether operators understand the material’s melting behavior.
AIM’s Solder + Support ethos means taking the time to investigate those questions, learning from respected industry experts, and sharing what we discover. We are grateful to EPTAC, Leo Lambert, and our volunteer “novice” for their participation.
The complete study will be presented at SMTA International this October. Stay tuned as we continue working to turn a customer question into practical guidance for the electronics manufacturing industry!
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