The selection of Sensor Soldering Solder must match the operating temperature window and package structure of the specific sensor type. For example, ceramic substrate NTC thermistors are prone to microcracks when the peak temperature exceeds 245°C. When using JJY SN100C lead-free solder, the reflow peak must be controlled at 235±3°C, and the time above the liquidus line must be maintained between 60 and 90 seconds; otherwise, the probability of solder ball residue will increase by more than three times. In contrast, pressure sensors with glass-sealed pins rely more on the low-temperature wetting advantage of JJY SN63Pb37 Eutectic Solder, which can achieve reliable forming at 217°C, preventing internal silicone oil from migrating or vaporizing due to sustained high temperatures.

In the printing process, the coordinated setting of squeegee pressure and speed directly affects the filling integrity of sensor soldering solder on 0201 and 0402 small-size pads. When the stencil opening is 0.12mm and the thickness is 0.1mm, if the squeegee angle is lower than 55 degrees or the printing speed is higher than 40mm/s, the Solder Paste transfer rate will fall below 72%. Especially for devices such as MEMS accelerometers with asymmetric four-sided pads, this can easily cause unilateral open soldering. At this time, the demolding speed must be adjusted synchronously to below 1.2mm/s, and during the first article confirmation stage, a 50x microscope must be used to focus on checking whether a continuous meniscus is formed at the solder edge instead of intermittent particle accumulation.

The reflow oven chain speed and temperature zone settings must be dynamically calibrated according to the mixed sensor layout on the PCB. When an infrared thermopile and a Hall switch exist on the same board, the former requires the soaking zone to maintain 150~180°C for 90 seconds to fully activate the Flux, while the latter cannot stay in this interval for more than 60 seconds due to epoxy package limitations. Therefore, we usually reduce the heating slope of JJY sensor soldering solder from 1.2°C/s to 0.8°C/s, allowing both to achieve process overlap at the 165°C platform, which ensures sufficient wetting of the thermopile solder joints while avoiding aggravated oxidation of the Hall element pins.

Micron-level cold soldering defects that cannot be identified by AOI inspection manifest as excessive X/Y-axis zero drift in accelerometer function testing. At this time, the batch report of JJY sensor soldering solder from this batch should be immediately retrieved to check whether the silver content deviation exceeds ±0.03%, and the furnace temperature tracker data should be synchronously sampled to confirm whether the heating time in the 183°C→217°C interval is greater than 45 seconds, because an excessively long period will accelerate flux carbonization and generate a non-wetting residue layer at the solder joint interface.

If hydrocarbon solvent is used in the cleaning process and the immersion time exceeds 180 seconds, the solder mask around some piezoelectric sensor solder joints will slightly swell, which will lead to abnormal solder joint stress distribution during subsequent high and low temperature cycles. Therefore, the cleaning temperature must be strictly locked at 55°C, the time must be compressed to within 120 seconds, and after air knife drying, an additional operation of standing at a constant temperature of 30°C for 30 minutes must be added to allow residual stress to be naturally released instead of being forcibly cured by high temperature.