The continuous wire feeding process of Solder Wire on automatic soldering machines is highly dependent on its tensile strength index. When the tensile strength is below 120MPa, slight slippage between the feeding wheel and the pressure wheel will cause intermittent wire breakage. Especially in high-speed feeding of 0.3mm fine-diameter solder wire, the wire breakage frequency will soar from once every 8 hours to more than 3 times every 2 hours. If the elongation is below 25%, welders will obviously feel that the solder wire is brittle and difficult to extend during the tin dragging operation, and sharp burrs are easily formed at the end of the solder joint, which is extremely prone to bridging defects in double-panel wave soldering.

We once encountered a batch of virtual soldering problems on an automotive ECU production line. The investigation found that it was not the failure of temperature or Flux, but the measured tensile strength of the new batch of solder wire was only 108MPa, which was 9% lower than the nominal value, and the elongation rate fell to 21%. As a result, the automatic soldering head experienced 3 wire feeding interruptions after 45 minutes of continuous operation. After each restart, the first 3 solder joints had poor wetting, and the solder was not completely spread under the microscope. This shows that insufficient tensile strength not only affects mechanical continuity, but also indirectly destroys the thermodynamic equilibrium conditions for solder joint formation through wire feeding jitter.

In Manual Soldering scenarios, the impact of elongation on soldering performance is more intuitive. When the solder wire elongation is in the range of 30% to 35%, the solder wire can naturally extend into a uniform liquid film during the dragging process of the soldering iron tip, and the solder joints are round and full. Once the elongation falls below 28%, the solder fluidity drops sharply at the same soldering iron temperature, and the temperature must be increased or the residence time extended to complete wetting. However, this increases the risk of copper foil oxidation on PCB pads and thermal damage to components, especially for temperature-sensitive devices such as 0402 package resistors and capacitors.

The tensile strength and elongation of solder wire also jointly restrict the cost-effective implementation of silver-containing solder wire. For example, if the tensile strength of Sn96.5Ag3.0Cu0.5 solder wire reaches 145MPa and the elongation is stable at 32%, the switching cost of silver content reduction alternative can be reduced without sacrificing feeding reliability. However, if excessive annealing is performed to pursue high elongation, the tensile strength may drop below 115MPa, which instead doubles the failure rate of the automatic line. At this time, the so-called "good soldering" is only an illusion, and the actual comprehensive cost rises instead.

When selecting materials for the production line, we cannot only look at the single-point tensile strength data provided by suppliers. We must require the measured tensile strength and elongation curves of three common diameters of 0.5mm, 0.8mm, and 1.2mm under the same batch. Because the deformation rate difference of the cold drawing process will make the elongation of fine-diameter solder wire generally 3 to 5 percentage points lower than that of coarse-diameter solder wire. If only 1.2mm data is used to estimate the performance of 0.5mm, it is very easy to encounter high-frequency wire breakage problems in the early stage of introduction. This is a hard experience we have repeatedly verified on five SMT lines in the past three years.