In actual production lines, tin-bismuth Solder Paste is mainly used for heat-sensitive devices such as COG liquid crystal driver chips, LED flexible strips, wearable modules with MEMS sensors, and multilayer ceramic capacitors (X7R/X5R). The glass transition temperature of the plastic packages of these devices is generally below 150°C. If conventional 217°C Reflow Soldering is used, it can easily cause body blistering, pin oxidation, or substrate warpage. With its 138°C eutectic melting point and narrow melting range window, tin-bismuth solder paste can achieve stable wetting under a nitrogen atmosphere. However, the prerequisite is that the stencil aperture must be designed to 0.85 times the original pad size and used with solder paste having a particle size below 30 μm; otherwise, the printing release defect rate will sharply increase to over 12%.

We found that the cold solder joint problem of low-temperature solder paste on large-size QFN48 packages is particularly prominent. The root cause is that the large-area copper foil at the bottom of the device absorbs heat too quickly, causing the local peak temperature to lag behind the surrounding small pads by more than 8°C. At this point, the tin-bismuth solder paste has already started to solidify and cannot complete secondary reflow compensation. The solution is not simply to increase the furnace temperature, but to reduce the furnace chain speed corresponding to this area by 15% and increase the residence time in the preheating zone, so that the overall thermal balance of the PCB is improved to within ±2.5°C. Otherwise, even using high-activity RMA Flux cannot improve the void rate.

The reliability shortcomings of solder paste are concentrated in the -40°C to 125°C temperature cycle test. After 200 cycles, the tin-bismuth alloy phase exhibits obvious grain boundary slip, and the solder joint shear strength attenuates by 37%. Under the same conditions, SAC305 still maintains 82% of its initial value. What is more troublesome is that tin-bismuth solder paste is extremely sensitive to PCB surface treatment. If the OSP film thickness is below 0.3 μm or the ENIG nickel layer thickness fluctuation exceeds ±0.05 μm, it will induce discontinuous growth of the interfacial IMC, causing a cold solder joint illusion. This type of defect is almost unrecognizable in AOI inspection and must be located by X-ray tomography.

Production line verification shows that tin-bismuth solder paste has serious compatibility risks on double-sided mixed assembly boards. When the A-side uses low-temperature solder paste to solder FPC connectors and the B-side uses SAC305 to solder the main control SOC, the second pass through the furnace will cause the A-side solder joints to repeatedly undergo a melting-solidification process, intensifying Bi element segregation and forming a brittle bismuth-rich phase. In a certain medical monitor project, we had to rework 176 main boards in batches due to this issue. The final solution was to change the B-side to a step reflow process and reduce the solder paste printing thickness from 120 μm to 95 μm to shorten the remelting time window.

Currently, all stencil suppliers claiming to be compatible with solder paste have not disclosed their micron-level burr control standards after laser cutting. In actual production, burrs above 0.03 mm will directly scratch the Bi-Sn intermetallic compounds in the solder paste particles, causing jagged trailing at the printing edge. This type of defect manifests as insufficient unilateral wetting after 0402 resistor soldering, but all functional tests pass. It is not until the whole machine undergoes HALT testing that it is concentratedly exposed as intermittent open circuits. Therefore, solder paste application must not be separated from the closed-loop verification of actual measurement data from specific stencil manufacturers.