The issue of silver layer migration during the soldering process of immersion silver boards is particularly prominent in high-humidity environments. Once the relative humidity in the workshop exceeds 60%RH and lasts for more than 4 hours, silver ions will undergo directional migration along the surface micropores of the PCB or the edges of solder mask openings under the action of bias voltage, leading to dendritic short circuits between adjacent pads. Statistics from JiajinYuan's Chengdu base in Q3 2023 show that such failures account for 38.6% of the total soldering rework volume for immersion silver boards.

Humidity fluctuations directly trigger a chain reaction of silver migration

When the dehumidification capacity of the air conditioning system is insufficient or the intake of fresh air suddenly increases, the workshop RH value rises from 48% to 63% within 20 minutes. At this point, the residual halide ions, rosin pyrolyzates, and water molecules on the surface of the immersion silver board rapidly form a conductive electrolyte film. The electrochemical potential difference of the silver layer at the interface between the pad copper substrate and the solder mask is amplified, and the migration rate increases exponentially. We have reproduced this phenomenon three consecutive times on the F12 line, each time accompanied by an 11.2% increase in the AOI missed detection rate.

Coupled temperature and humidity control is more effective than humidity control alone

Simply reducing the humidity to 35%RH instead exacerbates Solder Paste collapse and splashing. It is necessary to simultaneously stabilize the workshop temperature at 22℃±2℃, because every 1℃ increase in temperature will raise the saturated water vapor partial pressure by 7.2% under the same humidity. Moreover, when the temperature of the low-temperature solid solder paste certified by JiajinYuan is above 23℃, the activation speed of the Flux accelerates, the hygroscopicity of the residue increases, further deteriorating the interface stability of the silver layer.

Humidity monitoring at key points cannot rely on single-point readings

We have installed three calibrated temperature and humidity transmitters at the entrance of the reflow oven, the stencil printing area, and the ICT testing station respectively. It was found that due to frequent solder paste opening operations and intense personnel movement in the printing area, the actual RH value is on average 9.4% higher than the reading at the air conditioning return air outlet. If the dehumidifier is adjusted only based on the data from the return air outlet, the risk of condensation on the surface of the immersion silver board after printing remains as high as 27%.

Nitrogen-protected reflow must be implemented in coordination with humidity control

Even if nitrogen reflow is enabled, if the humidity in the pre-oven section exceeds the standard, the water molecules adsorbed on the surface of the silver layer will vaporize in the early stage of heating and impact the solder joint interface, causing microvoids and silver-rich bands. Verification data from JiajinYuan shows that when the pre-oven humidity is >58%RH, increasing the nitrogen purity from 99.99% to 99.999% is ineffective in suppressing silver migration. It is necessary to reduce the humidity to below 52%RH in advance to realize the true value of nitrogen.