When troubleshooting corrosion issues on SMT production lines, our first instinct is not to check the reflow oven temperature profile, but to immediately remove the PCB and use an ion chromatograph to measure chloride and bromide ion residue levels around solder joints, as the types and contents of active components in Solder Paste Flux directly determine the initiation threshold for post-soldering corrosion. For example, weak organic acid systems containing adipic acid and sebacic acid have essentially no corrosion risk under conditions of a peak temperature of 235°C and a holding time of 60 seconds. However, if switched to a strong activation formulation containing hydroxylamine hydrochloride or zinc chloride, even if the peak temperature is lowered to 220°C and the oxygen content in a nitrogen atmosphere is controlled below 100 ppm, obvious white corrosion products will still appear after a 48-hour high-humidity and high-temperature test (85°C/85%RH).
During the preparation phase, it is necessary to verify the technical parameter sheet of the Solder Paste Flux, focusing on confirming whether the rosin softening point is within the 135±5°C range, whether the non-volatile content is 8.5±0.3wt%, and whether the total halogen content is below 0.05wt% (IEC 61189-5 standard). At the same time, check whether the 200°C weight loss rate in the TGA thermogravimetric analysis report of the solder paste batch is greater than 12%, as an excessively high weight loss rate indicates more low-boiling-point solvent residue, which easily forms microporous channels after reflow and accelerates electrolyte migration. In one mass production run, we found that a certain brand of solder paste had an excessively high proportion of added ethylene glycol monobutyl ether, leading to needle-point corrosion pits around solder joints. This was ultimately improved by reducing the stencil aperture area ratio from 75% to 68% and increasing the squeegee pressure from 4.2kg to 5.1kg to enhance printing density, which reduced the residual solvent amount by 37%.
During operation, the atmosphere dew point inside the reflow oven must be monitored in real time. When the nitrogen purity is ≥99.995%, the dew point must be stably below -40°C; otherwise, HCl in the flux decomposition products will combine with residual moisture to form highly corrosive hydrochloric acid. Especially in areas such as QFN bottom pads where heat dissipation is fast and the cooling rate exceeds 3°C/s, capillary action after solder joint solidification will draw chlorine-rich condensate into the interface gap. At this point, if the PCB surface cleanliness does not meet the ionic contamination level of <0.78μg/cm² NaCl equivalent (IPC TM-650 2.3.25.1 method), electrochemical migration can be observed within 48 hours. We once reproduced this problem on an automotive MCU board. By moving the cleaning process forward to within 15 minutes after reflow, using a hydrocarbon cleaning agent (KB value 68, initial boiling point 162°C), controlling the cleaning temperature to 52±2°C and the spray pressure to 0.28MPa, we successfully reduced the corrosion failure rate from 127 per million units to 2.3 per million units.
Abnormality troubleshooting cannot only focus on the green copper patina under the microscope; it is also necessary to simultaneously measure the insulation resistance attenuation slope of the PCB dielectric layer near the solder joints. When the resistance value drops by more than 40% within 10 minutes under a 100V bias voltage, it can basically be determined that the organic acids in the solder paste flux have undergone a complexation reaction with copper to form soluble copper salts. At this point, the current solder paste batch must be immediately discontinued, and its GC-MS test report must be retrieved to check whether the ratio of azelaic acid to undecylenic acid is imbalanced, as the chelating ability of these two acids differs by 3.8 times, and an imbalanced ratio will accelerate local copper dissolution. In addition, if corrosion is found concentrated on the top of BGA solder balls rather than the root, it is most likely that rosin oxidation products in the flux (such as dehydroabietic acid) crystallize and precipitate during the cooling phase, blocking the solder joint stress relief channel, causing microcracks and becoming corrosion initiation points. At this time, the reflow cooling zone rate should be slowed from 6°C/s to 3.5°C/s to allow the rosin to fully rearrange and crystallize.
In terms of practical tips, we insist that all newly introduced solder pastes must complete three-stage verification. The first is a viscosity change rate test after 7 days of storage in a 25°C/60%RH environment, requiring Δη≤8%; otherwise, it indicates that the thixotropic agent system is unstable and there is a high risk of post-printing slump. The second is to simulate the pre-baking process before wave soldering (105°C×30min) to observe whether the flux volatilizes excessively in advance, leading to poor wetting during soldering. The last is a copper mirror corrosion test 72 hours after soldering (IPC J-STD-004B Appendix A), requiring no fog-like or spot-like erosion on the mirror surface. Only when all three items are passed is small-batch trial production allowed. In one instance, we found that a certain solder paste claimed to be "zero halogen" had edge etching in the copper mirror test. Investigation revealed that its surfactant contained trace chlorinated alkane impurities, and the peak area at a retention time of 12.73min in the GC-FID spectrum provided by the supplier exceeded the standard by 2.4 times, ultimately prompting them to upgrade their raw material purification process.
