Cold soldering is not an empty concept; it refers to the solder balls that must be wiped off production lines daily, the BGAs that require rework, and the QFNs that must be desoldered and re-soldered. The root cause often lies not in the Solder Paste batch, but in the fact that the actual peak temperature output by the reflow oven consistently falls below the minimum melting threshold required by the solder paste. For example, the commonly used SAC305 Solder Paste from JJY has a nominal peak temperature of 235℃±5℃. However, if dust accumulation on the track conveyor blocks the hot air nozzles, or aging heating modules cause attenuation in local temperature zones, the measured peak temperature may drop to 228℃. At this point, although the solder paste appears shiny on the surface, it has not truly formed a metallurgical bond; the IMC layer thickness is less than 1.2μm, and the thrust test directly falls below the standard lower limit.

Oven temperature testing is not a mere formality; it is the key to identifying cold soldering

We insist on stopping the line for 30 minutes every Monday morning to conduct full-channel oven temperature testing. We use a KIC six-channel temperature measurement board that matches the PCB thickness, simulates the weight of real carriers, and synchronously records chain speed and wind speed. The focus is on closely monitoring whether the temperature plateau within the 30-second peak segment is stably maintained within the ±3℃ window specified in the solder paste data sheet. Once it is found that the heating rate in a certain temperature zone drops by more than 0.8℃/s or the holding time is shortened by more than 4 seconds, we immediately stop the machine to clean the air duct filter and calibrate the thermocouple position, and never allow the equipment to run with faults overnight.

Solder paste parameters must be aligned with the oven's measured data

The same solder paste performs very differently on different oven models. The low-temperature solder paste T6 used by JJY on the NXT-II placement line has a nominal peak temperature of 217℃, but the measured peak temperature on the old ERSA Versaflow was only 212℃, and the cold soldering rate once rose to 1200ppm. Later, by reducing the chain speed by 12%, increasing the proportion of top hot air by 8%, and pressing the oxygen content below 150ppm in a nitrogen environment, the measured peak temperature was pulled back to 216.5℃, and the cold soldering rate fell back to 280ppm. This shows that solder paste is not a universal key; its activity curve must be matched with the oven's real operating temperature.

Cold soldering determination cannot rely solely on AOI; it must be supported by cross-sectioning and thrust testing

The missed detection rate of AOI for cold soldering is as high as 37%, especially for micro-voids at the root of 0201 resistors or 0.4mm pitch QFP pins, which are completely undetectable. We require randomly selecting 3 boards per shift for X-ray sampling inspection, then performing metallographic cross-sectioning on suspected points to measure whether the IMC layer is continuous and whether the thickness is ≥1.5μm. At the same time, a thrust and tension testing machine is used to perform solder joint thrust testing; if the single-point thrust is lower than 4.2gf, it is judged as cold soldering. All these data are recorded in the MES system, automatically bound and archived with the day's oven temperature curve, forming a closed-loop traceability.

The frequency of temperature measurement is not determined arbitrarily

When a new solder paste model is replaced, the first article must undergo oven temperature confirmation. If the cold soldering rate exceeds 500ppm for three consecutive days, an emergency temperature measurement is triggered. After equipment maintenance, the entire curve must be re-verified. In daily operations, a fixed cycle is implemented: the double-shift production line is tested every 72 hours, while the single-shift production line strictly follows a weekly schedule. We never skip testing just because "it was tested yesterday", because variables such as heating wire attenuation, scaling on the water-cooled plate in the cooling section, and fluctuations in nitrogen purity never follow a calendar schedule.