During reflow oven debugging, we found that the tombstoning rate of a batch of 0201 resistors suddenly rose to 3.7%. Simultaneously, under the microscope, we observed blackening at the edges of Solder Paste joints and an abnormal increase in wetting angle. Further measurement showed that the actual peak temperature was 23°C higher than the set value. The nominal tolerance limit of this solder paste is 245°C, while the highest point in the actual oven cavity reached 268°C. This indicates that the deviation in thermocouple attachment position or the aging of the K-type thermocouple after long-term use has caused a serious lag in temperature feedback.

When the solder paste exceeds its recommended peak temperature by more than 5°C for a duration exceeding 8 seconds, the carbonization of Flux residues intensifies. This not only weakens surface tension leading to solder paste collapse but also hinders the metallurgical bonding between the molten solder and the pad copper layer. At this point, even shortening the reflow time cannot remedy the uncontrolled thickness of the solder joint IMC layer. We once verified this with FIB cross-sectioning: at a peak temperature of 265°C, the thickness of the Cu6Sn5 layer was 1.8 times greater than that under the 240°C condition, directly reducing the joint shear strength by 42%.

If the production line does not re-run the temperature profile after switching to a new solder paste model, misjudgments are highly likely due to differences in alloy melting points and activation temperatures among solder pastes of different brands. For example, one morning meeting revealed that the BOM had been switched to a bismuth-containing low-silver solder paste, yet the original high-temperature profile was still being used. As a result, three consecutive batches showed excessive void rates in BGA solder balls and QFN bottoms. The root cause was that the liquidus temperature of this solder paste was only 217°C, but the original profile peak was set at 250°C, causing the solder paste joints to undergo prolonged overheating and oxidation.

Poor temperature uniformity inside the oven is also a hidden contributing factor. When the temperature difference between the center and edge of the conveyor track exceeds ±5°C, uneven heating occurs for double-sided mixed-mount devices on the same PCB. For instance, the root pins of large QFP packages often develop microcracks due to local overheating, while adjacent fine-pitch SOP packages exhibit poor wetting due to insufficient heat absorption. This phenomenon is particularly pronounced in reflow ovens using a combined infrared + hot air heating system. It is necessary to measure at least 5 cross-sectional points with a multi-channel temperature logger every shift.

We handled a case where solder paste joint tensile tests failed for five consecutive days. The final root cause was identified as the nitrogen protection concentration dropping from 99.95% to 99.2%. Although no alarm was triggered, the increased oxygen content caused rapid oxidation of the solder paste in the high-temperature zone, leading to an imbalance in molten surface tension. This prevented the solder from spreading sufficiently, and instead, it contracted during the cooling phase, tearing the joint structure. Such damage is difficult to identify with AOI; only X-ray or metallographic cross-sectioning can confirm the internal voids and IMC fracture morphology.