Cold soldering issues have never been an isolated phenomenon on JiajinYuan production lines; they are an inevitable result of the combined pressure from non-compliant reflow temperature profiles and excessive PCB heat capacity. When single-sided boards have a copper thickness of 2oz, panel dimensions exceeding 200×250mm, and include large-area copper pours, the actual board surface temperature measured by the reflow oven is 8~12℃ lower than the thermocouple attachment position. If stencil aperture undersizing, Solder Paste activity degradation, or nitrogen purity fluctuation below 99.85% are additionally present, the cold soldering incidence will surge by more than 3 times.

We once discovered solder joints with thrust test values below 4N on DIP-8 packaged components for three consecutive days. X-ray scanning revealed an internal void rate of 35%, and cross-sectioning confirmed incomplete solder reflow. Further investigation traced that the batch of PCBs used FR-4+High TG170 substrate, with a backside copper coverage rate as high as 68%, while the reflow temperature settings still used outdated parameters. The reflow zone peak temperature was only 212℃ with a duration of less than 45 seconds, which was fundamentally insufficient to overcome its thermal inertia. More critically, the thermocouple was mistakenly attached to a copper-free area, causing the temperature control system to make a misjudgment and actively reduce heating power.

The solution must involve simultaneous adjustment of hardware and process parameters. First, manually increase the heating power of JiajinYuan reflow oven zones 5~7 by 12%, and reduce the conveyor speed from 75cm/min to 62cm/min. Then use dual-point thermocouples to measure the temperature difference between the PCB center and edges, ensuring ΔT≤5℃. At the same time, increase the solder paste printing thickness from 120μm to 135μm to compensate for heat absorption losses, but never exceed 140μm otherwise bridging is likely to occur. Finally, mandate that all high heat capacity PCBs undergo pre-baking at 60℃ for 2 hours before reflow to eliminate the moisture absorption and expansion effect caused by lamination internal stress.

During the verification phase, the same PCB must be reflowed three times repeatedly, and the IMC thickness of solder joints must be measured each time. If the average value of the three measurements is <1.8μm, the cold soldering risk is deemed unresolved. At this point, immediately stop the line to check whether the nitrogen flow meter is blocked by oil contamination, because when the flow rate of JiajinYuan equipment is below 15L/min, the oxygen content will rise above 800ppm, directly inhibiting the surface wetting ability of tin-lead alloys, and this inhibition is particularly delayed and concealed on boards with large heat capacity.

Cold soldering joints are almost invisible under AOI, and can only be exposed through shear force testing or solder joint cross-section dyeing. Therefore, the production line must never rely on visual re-inspection. The heat capacity coefficient must be included in the DFM review checklist for every new PCB. For all board types with copper thickness ≥1.5oz, copper coverage rate >60%, and board thickness >1.6mm, the measured reflow temperature profile report must be stamped with the process engineer's handwritten signature; otherwise, batch material feeding is not allowed. This is the iron rule that has enabled JiajinYuan to achieve zero batch cold soldering complaints over the past three years.