We do not perform Solder Paste thermal cycling tests just to meet report requirements; our goal is to identify solder pastes that show no issues after hundreds of hours on the production line but develop cold solder joints once installed in vehicles. Test conditions must strictly align with the customer's end-use scenarios. For example, automotive electronics commonly use -40℃ to 125℃, 1000 cycles, with heating and cooling rates controlled within 10℃/min. Any oven with a temperature deviation exceeding ±2℃ must be shut down for calibration; otherwise, no matter how good the data looks, it is useless.

During thermal cycling, solder joint failures often initiate at the interfacial IMC layer. In particular, after repeated thermal expansion and contraction, SnAgCu systems tend to experience accelerated transformation of Cu6Sn5 to Cu3Sn and grain coarsening, which in turn triggers microcrack propagation. Solder pastes modified with nickel or bismuth exhibit more stable intermetallic compound growth rates. However, when the nickel content exceeds 0.05%, the risk of stencil clogging during printing increases significantly. Therefore, during selection, thermal cycling data must be evaluated together with stencil aperture design, squeegee pressure, and reflow peak temperature in DOE verification.

Measurements show that the thermal cycling performance of the same solder paste varies greatly across different PCB surface finishes. The solder joint failure cycle on OSP boards is generally more than 30% shorter than on ENIG boards. This is because uneven OSP film thickness leads to inconsistent local wetting during reflow, causing cracks to initiate early at thermal stress concentration points. Although ENIG is more expensive, the nickel layer can buffer CTE mismatch between the copper substrate and solder joint. This difference is particularly pronounced in wide temperature range tests from -55℃ to 150℃.

Before testing, it must be confirmed that all samples have completed standard Reflow Soldering with no cold or insufficient solder joints. X-ray sampling coverage must be no less than 3% per batch. Batches with an AOI miss rate higher than 0.5% are directly rejected, because thermal cycling only amplifies initial defects rather than masking them. We once encountered a batch of solder paste where all joints cracked after 500 thermal cycles. Investigation revealed that the actual temperature in the third zone of the reflow oven was 8℃ lower than the set value, causing the internal void rate of solder joints to exceed the standard. Such problems cannot be root-caused by thermal cycling data alone.

Cross-section analysis after thermal cycling cannot rely solely on macroscopic solder joint morphology; SEM+EDS must be used to quantitatively measure IMC thickness, compositional uniformity, and crack propagation direction. In particular, diagonal cracks at 45° along the pad edge are often the result of coupled thermal stress and mechanical vibration. Such failures rarely appear in high-temperature storage tests alone; only thermal cycling can expose them. Therefore, when customers require automotive-grade certification, thermal cycling has always been a mandatory threshold.

We recommend embedding thermal cycling testing in the second stage of the solder paste introduction process, starting immediately after basic printability and reflow verification are completed. At least three parallel sample groups should be run simultaneously each time, with one group retained as a retest backup to avoid disrupting the entire verification schedule due to a single abnormal interruption. After all, the production line cannot afford to wait, and a single thermal cycling test takes as little as 7 days or as many as 20 days, so the time window must be strictly controlled.