Cold solder joints in PCB soldering often manifest as dull solder joint surfaces, a wetting angle greater than 90 degrees, and incomplete coverage of the pad edges by solder visible under X-ray inspection. This is usually related to the attenuation of Flux activity in the Solder Paste, failure to promptly clean the oxidized residual solder paste layer on the stencil bottom, and insufficient placement pressure that prevents component pins from making full contact with the solder paste. On the Jia Jinyuan Dongguan production line, we once encountered this issue in three consecutive batches of QFN devices, which was ultimately determined to be caused by a 12% drop in printing pressure due to squeegee wear, coupled with the peak temperature of the third reflow zone being 8°C lower than the standard value.

Solder beads mostly occur on the sides of 0201 and 0402 resistors and capacitors or under the IC body. Their formation is not simply due to excessively fast reflow speed, but rather the combination of an out-of-spec particle size distribution of the solder paste (D90 > 28μm) and a stencil opening aspect ratio less than 1.5, which causes the solder paste to break and splash onto non-pad areas during the demolding process. In a nitrogen environment, if the oxygen content is higher than 100ppm, it will further exacerbate the oxidation and agglomeration of solder beads. We once tested different stencil tensions with the same solder paste on the Jia Jinyuan Hefei line, and found that when the tension was lower than 35N/cm², the solder bead occurrence rate increased by 3.7 times.

Excessive void rates are commonly found at the bottom of large-pad QFNs and BGAs, especially when the pad size is larger than 3mm × 3mm and there is no thermal via design. In such cases, if the time above the liquidus line in the reflow profile exceeds 65 seconds, or the cooling rate is lower than 3°C/s, the gas inside the solder cannot escape in time, resulting in circular voids with a diameter > 150μm forming at the center of the solder joint. After the Jia Jinyuan Suzhou line increased the nitrogen purity from 99.99% to 99.999%, the average BGA void area decreased by 42%, but the prerequisite is that the stencil openings must be simultaneously modified to a stepped design to reduce the solder paste volume.

Improvement plans must be implemented as a complete set. For example, for cold solder joints on 0.4mm pitch TSSOPs, one cannot only adjust the reflow temperature, but must simultaneously reduce the stencil thickness from 0.12mm to 0.10mm, increase the squeegee angle from 55° to 62°, stabilize the Solder Paste Storage temperature at 22±1°C, and compress the post-opening usage time to within 18 hours; otherwise, single-point optimization will instead induce an increase in solder beads. Similarly, when replacing with a new batch of solder paste, its viscosity change value must be measured. If the thixotropic index deviates from the original parameters by more than ±0.15, stencil opening simulation must be redone and the squeegee pressure compensation coefficient updated.

All parameter adjustments must be accompanied by first-article X-ray cross-sectional scanning and thrust testing after 50 cycles of -40°C~125°C thermal shock. Jia Jinyuan's internal acceptance criteria are that the thrust of cold solder joints must not be lower than 85gf, the solder bead diameter must not exceed 15% of the adjacent pad spacing, and the total void area ratio must be controlled within 12% of the solder joint projected area. Any out-of-spec item will trigger the process freeze procedure, and release is only allowed after completing closed-loop FMEA verification.