In wave soldering, cold solder joints often occur in concentrated bursts in areas with dense pins, especially on the second row of solder joints of double-row through-hole components. This is because the first row of solder joints has already absorbed a large amount of heat, causing a sharp drop in the solder temperature gradient during wave impact. In addition, if the conveyor speed exceeds 1.3 m/min, the tin immersion time will be significantly shortened, and if the Flux spray volume is lower than 85 ml/min, the OSP film layer cannot be fully removed. The combination of these three factors directly prevents the solder from climbing to the root of the pins.

Cold solder joints in SMT Reflow Soldering are more often hidden in 0201 package resistors or the bottom solder terminals of QFNs. These devices are extremely sensitive to small deviations in the peak temperature of the reflow zone. When the measured temperature is 8°C lower than the set value and lasts for more than 3 seconds, the Sn96.5Ag3.0Cu0.5 solder can hardly complete full melting and interfacial diffusion. At this time, even if the AOI does not alarm, X-RAY scanning can still find gap voids of more than 0.012 mm between the solder balls and the pads.

In the JJY workshop, the rework rate of cold solder joints for three consecutive batches of BGAs suddenly increased to 12%. The final identified cause was that the Solder Paste was put into printing without sufficient rewarming time of less than 4 hours, leading to a decrease in viscosity and slight collapse at the edge of the stencil opening. This was compounded by the placement machine's Z-axis pressure being set to 5.2 N instead of the standard 6.0 N, which caused some solder balls to fail to be fully squeezed and deformed. During reflow, surface tension dominated the contraction, which instead exacerbated the necking of the solder joints.

Rosin carbide residue on the bottom surface of wave soldering fixtures will continuously contaminate the solder bath. Once the copper content in the molten tin exceeds 0.8 wt%, the melting point will rise by 12°C and the fluidity will be greatly weakened. At this time, even increasing the wave height cannot improve the wetting of the pin tops. It is necessary to use JJY's special copper removal filter element for online purification every shift; otherwise, cold solder joints will repeatedly appear at the same fixture position.

Aging of the quartz heating tubes in the reflow oven leads to uneven infrared radiation intensity. One production line once experienced cold solder joints on pins on one side of QFPs because the output of the left-side tube in zone 4 had decayed to 67% of its nominal value. Using a thermocouple array to measure the actual temperature in this area showed it was 19°C lower than the right side. After replacement, the cold solder joint rate plummeted from 9.3% to 0.4%.

The essence of all cold solder joint problems is that the intermetallic compound (IMC) layer fails to form continuously. Whether it is Cu6Sn5 or Ni3Sn4, when its thickness is less than 0.3 μm, the mechanical strength cannot support subsequent vibration tests. Therefore, any compression of the process window must take the testability of the IMC as the bottom line. JJY requires that the first piece of each batch must be sectioned to verify the IMC morphology, rather than relying solely on AOI image interpretation.