The selection of Solder Paste and Solder Bars for SMT Soldering of communication motherboards is not a simple matter of matching melting points or brands; it must simultaneously satisfy the triple constraints of high-frequency signal integrity, thermal cycle reliability, and production line printing yield. For example, for 0.3mm pitch BGA devices, the D50 particle size in the solder paste must be controlled within 20±2μm; otherwise, bridging or insufficient solder is likely to occur during printing and demolding. For Sn96.5Ag3.0Cu0.5 solder bars during wave soldering touch-up, if the preheating temperature is below 120°C and the duration is less than 60 seconds, it will cause carbonization and blackening of residual Flux on the bottom of the PCB.
During the preparation phase, it must be confirmed that the solder paste is refrigerated at 0～5°C and has not exceeded its 90-day shelf life. After opening, it must be allowed to warm back to temperature under nitrogen protection for more than 4 hours and stirred for 3 minutes. At the same time, a viscometer must be used to measure that its rotational viscosity at 25°C is maintained at 650±80Pa·s. For solder bars, the surface oxide layer thickness must be checked to ensure it is less than 0.5μm; if it appears grayish-white and hazy to the naked eye, it must be rejected, because the oxide film will cause the wetting angle to be greater than 90° during wave soldering and obvious solder spikes on the joints.
In terms of core parameters, the no-clean solder paste commonly used for communication motherboards must meet the J-STD-004B Class L0 level halogen content (Cl?+Br?＜0.5wt%), with the metal content strictly limited to 88.5±0.3%. If it is too high, edge collapse during printing will worsen; if it is too low, the solder joint height after reflow will be less than 120μm. For solder bars, the measured liquidus temperature must be 217.5±0.5°C and the solidus temperature 217.0±0.3°C; the temperature difference between the two must be compressed to within 0.5°C. Otherwise, in Selective Wave Soldering, it will easily cause the internal microvoid rate of solder joints to exceed the 15% upper limit allowed by IPC-A-610E Class 2.
In terms of operation steps, the stencil aperture must be enlarged at a 1:1.05 ratio. For 0.4mm CSP devices, a stepped aperture design is adopted, with the aperture area in the 0.35mm bottom region reduced by 18% compared to the top. The solder paste printing speed is set to 25mm/s, the demolding speed to 1.2mm/s, the squeegee angle fixed at 60°, and the pressure value calibrated to 4.8±0.3kg based on stencil tension. The peak reflow temperature must be stable at 238±2°C, with the duration controlled at 45±5 seconds. If using Sn96.5Ag3.0Cu0.5 solder bars for local touch-up soldering, the soldering tip temperature must be set to 340±5°C and the contact time must not exceed 3 seconds; otherwise, local delamination will be triggered near the glass transition temperature of the FR4 substrate (Tg=170°C).
During anomaly troubleshooting, if solder balls are found concentrated on the outer periphery of the BGA array, first check whether the solder paste has been stirred more than 5 times after warming back to temperature, because excessive shearing will break the molecular chains of the flux resin and unbalance the volatilization rate. Next, confirm whether the stencil cleaning cycle has exceeded the IPA solvent replacement interval of every 12 boards; otherwise, accumulation of mesh residue will lead to poor solder paste filling. Furthermore, verify whether the O? concentration inside the nitrogen protection furnace is higher than 100ppm; exceeding this value will directly intensify oxidation of the Sn-Ag-Cu alloy and cause the solder joint surface roughness Ra to exceed 0.8μm.
Experience and techniques have been repeatedly verified on the production line. For example, when handling 5G base station main control boards with a 24-layer stack and copper thickness ≥3oz, it is recommended to switch to solder paste containing a nano-scale ZnO activator, which can improve the wetting speed on copper surfaces by 37% in the 220°C soaking zone. Before daily startup of the solder bar melting furnace, three empty firing operations must be performed, with a 15-minute interval between each and the temperature raised to 280°C, to thoroughly remove rosin cracking residues left from the previous shift. In addition, before all solder paste batches are put into production, a 3g sample must be taken for a thermal shock test, i.e., 5 cycles of -40°C/1h→25°C/1h→125°C/1h, after which it is observed whether delamination or caking occurs. If any item fails, the entire batch is returned to the warehouse.
