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手工焊接場景，伸長率對焊接性能的影響更為直觀。當焊錫絲伸長率在30%～35%範圍內時，焊錫絲在烙鐵頭拖曳過程中能自然延伸成均勻的液膜，焊點圓潤飽滿。一旦伸長率降至28%以下，在相同烙鐵溫度下焊錫流動性急劇下降，必須提高溫度或延長停留時間才能完全潤濕。然而，這增加了 PCB 焊盤上銅箔氧化和元件熱損壞的風險，特別是對於 0402 封裝電阻器和電容器等溫度敏感元件。

焊錫絲的拉伸強度和伸長率也共同限制了含銀焊錫絲的成本效益實施。例如，如果Sn96.5Ag3.0Cu0.5焊絲的抗拉強度達到145MPa，伸長率穩定在32%，則可以在不犧牲送絲可靠性的情況下降低銀含量降低替代方案的轉換成本。但如果為了追求高伸長率而進行過度退火，則抗拉強度可能會下降到115MPa以下，反而使自動線的故障率倍增。這時，所謂的「焊接好」只是一種幻想，實際的綜合成本反而上升。

在為生產線選擇材料時，我們不能只看供應商提供的單點抗拉強度數據。我們必須要求同一批次下0.5mm、0.8mm、1.2mm三種常見直徑的實測拉伸強度和伸長曲線。因為冷拔製程的變形率差異，會使細徑焊錫絲的伸長率一般比粗徑焊錫絲低3~5個百分點。如果只用1.2mm的資料來估算0.5mm的效能，在導入初期很容易遇到高頻斷線問題。這是我們三年來在5條SMT線上反覆驗證的艱辛經驗。