The core contradiction in soldering immersion silver boards lies in the fact that Solder Paste must simultaneously satisfy strong wettability and low corrosivity. Conventional medium- and high-temperature solder pastes often cause localized dissolution of the immersion silver layer and pad blackening due to excessively strong organic acid activation, while low-temperature solder pastes are prone to insufficient wetting, bridging, or excessive voiding rates on devices with a pitch below 0.4 mm. The JJY GJ-812A solder paste was developed specifically to address this production line pain point. Its rosin-based weak acid system begins active activation release at 155°C, paired with spherical Sn96.5Ag3.0Cu0.5 alloy powder with a particle size D50 of 25±3μm. Under conditions of a stencil aperture ratio of 0.78, squeegee pressure of 0.8 MPa, and printing speed of 25 mm/s, it can stably achieve a thickness uniformity CV ≤ 8.2%.

Wetting performance must be evaluated in combination with interfacial reaction dynamics

We once used the same GJ-812A solder paste to compare the soldering effects of OSP and immersion silver boards, and found that the solder joint spread rate of the immersion silver board reached 89.3% at a peak temperature of 235°C, slightly lower than the 92.1% of the OSP board, but the shear force instead increased by 6.4%. This is because silver atoms partially diffuse into the Sn-Ag-Cu melt during reflow to form Ag-Sn solid solution strengthening phases. If the peak temperature rises above 245°C, the shear force decreases by 11.7%, indicating that excessive dissolution of the silver layer has begun to weaken the interfacial bonding strength. Therefore, the TAL of the reflow profile must be controlled at 60±5s, and the peak temperature must be locked within the range of 232~236°C.

Corrosion resistance is reflected in the integrity retention of the silver layer after soldering

A common problem after soldering immersion silver boards is not dull solder joints, but grayish-white corrosion marks appearing at the pad edges. Under a microscope, the silver layer thickness is observed to decrease from the original 0.35μm to 0.12μm. After three reflow cycles with the JJY GJ-812A solder paste, the pad silver thickness remains above 0.28μm. XPS analysis shows that the Cl⁻ content in its residue is <38 ppm and Br⁻ <12 ppm, far below the halogen limits specified in IPC-J-STD-004B. This is directly related to its use of phosphate esters to replace traditional bromide activators, and the Flux residue shows no electrolytic migration phenomenon after 1000 hours of aging at 85°C/85%RH.

Stencil design and solder paste matching cannot be separated

When using GJ-812A solder paste, if the 1:1.2 aperture width-to-thickness ratio commonly used for OSP boards is retained, the immersion silver board will experience excessive solder paste volume at the 0.3 mm QFN, and the tombstoning rate after reflow will rise to 3.7%. After we adjusted the aperture width-to-thickness ratio to 1:1.05 and added a 0.05 mm stepped chamfer, the tombstoning rate dropped to 0.4%. Meanwhile, SPI inspection showed that the solder paste volume deviation narrowed from ±12.3% to ±6.8%, indicating that the rheological properties of the solder paste and the geometric parameters of the stencil must be optimized simultaneously, and matching defects cannot be covered up solely by increasing viscosity.