It is indeed common practice in actual production lines to use both Solder Paste and red adhesive processes on the same PCB, but the prerequisite is that the red adhesive is only used to secure through-hole pins that cannot withstand SMT placement impact, extra-large thermal components, or bottom-side components during double-sided mixed assembly, while solder paste is exclusively responsible for the soldering connection of surface-mount devices. The two must be applied in a staggered sequence along the time axis, and spatially, the edge of the red adhesive dot must be at least 0.3 mm away from the solder paste pad; otherwise, the red adhesive will thermally expand during reflow, creep toward the pad, and encapsulate the solder balls, causing poor wetting or short-circuit risks.

We once encountered a power board where a 0.15 mm offset in red adhesive dispensing caused the tombstoning rate of 1206 resistors to surge to 8%. The root cause was that the red adhesive prematurely softened during the 150°C preheating stage and flowed laterally toward the pad, compressing the spreading space of the molten solder during reflow. This issue was completely invisible during AOI inspection, and the batch abnormality was only discovered through visual inspection after reflow. Therefore, before dispensing, the XY offset must be measured with a graduated magnifier, and the first-article data must be entered into the SPC system for real-time monitoring of fluctuation trends.

There is a hard window constraint between the curing temperature of red adhesive and the melting point of solder paste. For example, commonly used epoxy-based red adhesive requires 150°C × 60 seconds to complete preliminary crosslinking, while the peak temperature of SAC305 Solder Paste must reach 235°C ± 5°C. This forces the reflow oven zone settings to ensure that the 150°C soaking zone is long enough to meet the red adhesive curing requirements, while preventing premature oxidation of the solder paste. Therefore, we typically set Zone 5 as a constant temperature zone of 148°C to 152°C, extend the duration to 75 seconds, and then enter the rapid heating ramp, which preserves the red adhesive strength without sacrificing solder paste activity.

The selection of stencil thickness must also yield to the presence of red adhesive. When 0201 components coexist with red adhesive on the PCB, even if the solder paste section only requires a 0.1 mm thick stencil, we mandatorily use 0.12 mm to increase the solder paste volume margin. This is because slight collapse after red adhesive dispensing raises the Z-axis reference of the placement head; if the solder paste volume is insufficient, insufficient solder or voids are more likely to occur. This detail is often overlooked during the NPI phase, and it was not until after a trial production run of 100 boards that we found the IMC layer thickness of 0201 capacitor solder joints in the BOM was generally below 3 μm.

The most troublesome part is the cleaning process. Water-based solder paste residues are incompatible with the red adhesive body; cleaning with pure DI water would cause the red adhesive to absorb water, swell, and delaminate from the substrate. Therefore, we uniformly use hydrocarbon solvent for segmented cleaning: first, low-temperature soaking to dissolve the Flux, then raising the temperature to soften the red adhesive surface layer, and finally blowing dry with nitrogen. The entire process takes 47 seconds longer than conventional cleaning, but it can prevent subsequent ICT probe contact failures caused by red adhesive detachment.