When we actually replaced the solder on a certain 5G small base station RF board on the production line, we found that after the original halogen-containing Solder Paste underwent 72 hours of 85℃/85%RH damp heat aging after reflow, microcracks began to appear at the edges of BGA solder joints. After switching to JJY halogen-free high-reliability solder for 5G equipment, the solder joint integrity remained intact under the same conditions. This indicates that its Flux residue has lower hygroscopicity, smaller ionic contamination, and better thermal expansion coefficient matching; this solder adds 0.03wt% Ni and 0.015wt% Ge microalloying elements to the SAC305 matrix, increasing the grain refinement degree of solder joints by 40%, so no brittle fracture was observed in the -40℃ cold bending test.

The printing process must simultaneously adjust squeegee pressure and demolding speed, because the thixotropic index of JJY's solder for 5G equipment is 4.8, which is 0.6 higher than that of conventional halogen-free solder paste. If the demolding speed is lower than 25mm/s, it is easy to cause solder paste drawing or bridging, which is more sensitive especially on 0.3mm pitch 5G filter package pads; we once failed to calibrate the stencil tension in time, resulting in a local solder volume deviation of 18%. Subsequently, we uniformly controlled the tension at 45N/cm and measured it with a tension meter every 4 hours, keeping the CPK stable above 1.33.

The reflow oven temperature zone settings must take into account the characteristic that the activation temperature of halogen-free systems is on the high side. The heating ramp rate in the preheating zone must be controlled within 1.2℃/s, otherwise the violent volatilization of flux in advance will cause solder ball splashing. When we were debugging at a factory in Dongguan, we found that when the peak temperature of the 3rd temperature zone reached 158℃, the solder ball rate of 0201 devices soared from 0.3% to 2.7%. Finally, we lowered the temperature of this zone to 142℃ and extended the holding time by 12 seconds to completely solve the problem; it is worth noting that the oxygen content under nitrogen protective atmosphere must be pressed below 100ppm, otherwise the thickness of the Cu6Sn5 intermetallic compound layer will abnormally increase by 35%, directly weakening the drop resistance of solder joints.

AOI inspection cannot only look at solder paste coverage, but also pay attention to the burr height at the edge of solder paste. JJY's solder for 5G equipment uses ultra-fine powder with sphericity >92%, which can still achieve edge collapse <8μm under a 30μm stencil opening. However, if the stencil laser polishing process is not in place, the collapse amount will exceed 15μm and induce tombstoning; we recommend using 3D SPI to scan the stencil opening cross-section every 500 printed boards to confirm whether the hole wall roughness Ra value is maintained within 0.25μm. Once it exceeds the standard, replace the stencil immediately to avoid batch cold solder joints.

It is strictly prohibited to use ordinary soldering iron tips for direct heating during repair and rework, because the melting range of this solder is 217～220℃, with a window of only 3℃. Ordinary soldering iron temperature fluctuation of ±8℃ can easily cause solder joint overheating oxidation or poor wetting. A constant temperature soldering iron with real-time temperature feedback must be used and the contact time must be ≤3 seconds; when repairing a certain millimeter-wave antenna array once, due to two consecutive re-soldering operations, the IMC layer of the solder joint thickened to 4.2μm, and the third device showed a 22% drop in thrust. Subsequently, all repairs were changed to hot air local rework, and the number of heating times per point was strictly recorded.