At 2 AM last Wednesday, after Line A placed 0201 resistors, AOI continuously reported 27% cold solder joints and missing prints. We immediately stopped the line, took samples, and observed them under magnification. We found obvious smearing and slight whitening at the edges of the Solder Paste. Meanwhile, XRF scanning showed that bromine readings for the same can of solder paste fluctuated between batches from 1800 ppm to 3200 ppm, while the customer limit is 900 ppm. This indicates that the solder paste's halogen content itself has exceeded the definition of halogen-free SMT, and batch stability is seriously out of control.

The root cause is not a single point of failure, but that the solder paste supplier did not perform halogen testing according to the IEC 61249-2-21 standard, simply equated total halogen content with chlorine content, and ignored the residue of brominated flame retardants in the rosin carrier. In addition, our IQC only conducts sampling inspections and did not require a third-party SGS report, which led to bromine-containing rosin-modified solder paste entering the production line. This type of solder paste easily generates HBr gas after reflow, accelerates copper foil corrosion in high-temperature and high-humidity environments, and induces CAF growth.

We immediately activated the emergency plan at that time, isolated and sealed this batch of solder paste, and simultaneously switched to the low-halogen solder paste verified last month (measured Cl+Br=630 ppm). We adjusted the squeegee pressure from 0.45 MPa to 0.38 MPa to reduce shear heating, and lowered the reflow peak temperature from 245°C to 238°C to avoid excessive Flux decomposition. Two hours later, the AOI defect rate fell back to 0.3%, but 17 motherboards were scrapped that day because some PCB panels that had been placed but not yet reflowed already had invisible halogen migration traces.

Long-term resolution must break the siloed approach of the procurement, process, and quality control departments. Now, all newly introduced solder pastes must sign the "Halogen-Free SMT Material Commitment Letter", explicitly requiring suppliers to provide a full-halogen (F, Cl, Br, I) IC test report with an official CMA stamp for each batch, and ROHS reports cannot be used as a substitute. Meanwhile, our laboratory has been equipped with an ion chromatograph to conduct sampling tests on each incoming batch according to the GB/T 29335-2012 method, focusing on monitoring whether the peak area ratio of Br⁻ to Cl⁻ is abnormally elevated. Once Br⁻ is found to exceed 65% of total halogens, a secondary re-inspection process is initiated.

The printing process must recalibrate the stencil tension to ensure the tension value is stable at 38 N/cm ± 2 N, because insufficient tension will cause solder paste residue on the aperture sidewalls to aggravate halogen enrichment. The squeegee angle must be fixed at 55 degrees and inspected daily; an angle deviation exceeding 2 degrees will change the solder paste rolling distance by 12%, directly affecting the uniformity of flux distribution and consequently causing local halogen residue to exceed the standard. The heating slope in Zone 3 of the reflow oven is locked at 1.8°C/s ± 0.1; too fast will cause insufficient rosin cracking, while too slow will extend the high-temperature exposure time, both of which increase the risk of HBr release.

The cleaning process is no longer optional but mandatory. Currently, all halogen-free SMT products must undergo secondary cleaning. The first stage uses hydrocarbon solvent spray cleaning at 55°C for 90 seconds to remove the main flux. The second stage uses deionized water with 0.8% organic acid inhibitor for ultrasonic cleaning for 60 seconds. After cleaning, the ionic contamination level must be ≤ 0.78 μg/cm² NaCl equivalent, otherwise the entire batch will be reworked. This is because our actual measurements found that 72% of residual halogens adhere to the edge of the pads in the form of Br⁻, which cannot be removed by simple nitrogen purging.

Regarding supplier management, we have terminated cooperation with two solder paste factories that failed the halogen process audit. Newly admitted manufacturers must disclose their rosin synthesis process roadmap to confirm that precursors such as tetrabromobisphenol A or decabromodiphenylethane are not used. Meanwhile, their solder paste batch numbers must be bidirectionally linked with the rosin raw material batch numbers, so that once a halogen abnormality occurs, it can be quickly traced back to the specific reactor number, rather than vaguely locating it to a certain month's production as in the past.

On-site engineers must verify three sets of data at the daily morning meeting, including whether the previous shift's solder paste opening time exceeded 72 hours, whether the temperature and humidity in the constant temperature cabinet were maintained at 25°C ± 1°C and 40% RH ± 5%, and whether the RSD of Br elements in the last three XRF scans was below 8.5%. If any of these three values exceeds the standard, use must be suspended and the FA process initiated. This is because exceeding the opening time will cause the solder paste to absorb moisture and hydrolyze to produce HBr, out-of-control temperature and humidity will accelerate halogen ion migration, and an excessive RSD directly reflects that the supplier's internal quality fluctuation has exceeded the controllable threshold.