Why SAC305 Solder Paste is prone to insufficient solder and misalignment during printing on 0201 components, while Sn99.3Cu0.7 is more stable, the root cause is that the 3.0% silver content in SAC305 increases the alloy's melting viscosity, resulting in poor shear flowability of the Solder Paste under the squeegee. In addition, its powder sphericity is generally slightly lower than that of Sn99.3Cu0.7 batches. The practical solution is to enlarge the stencil aperture by 5% according to the IPC-7527 standard, reduce the squeegee pressure from 4.5kg to 3.8kg, and lower the printing speed from 40mm/s to 32mm/s. The key preventive measure is to clean the bottom of the stencil with IPA + ultrasonic cleaning every 300 boards, otherwise the residual Flux film will aggravate the agglomeration of SAC305 powder.

Microcracks frequently appear at the edge of QFN solder joints after reflow of Sn99.3Cu0.7. The symptom is that the AOI false alarm rate increases, but no voids are found by X-ray. The reason is that the solidification temperature range of this alloy is only 1.2℃, and a slightly faster cooling rate will cause inter-dendritic stress concentration. The practical solution is to add a slow cooling section of 210℃～180℃ after the Peak zone of reflow soldering, control the time at 45 seconds, and reduce the chain speed by 12% synchronously. At the same time, the oxygen content of nitrogen is relaxed from 100ppm to 300ppm to slow down the growth rate of Cu6Sn5 intermetallic compound (IMC). The key preventive measure is to prohibit the use of Sn99.3Cu0.7 solder paste for thick copper PCBs with thickness >1.6mm and copper thickness ≥2oz.

After the high temperature and high humidity aging test (85℃/85%RH/1000h), the shear force attenuation of SAC305 solder joints reaches 18%, while that of Sn99.3Cu0.7 is only 9%. The symptom is that the contact impedance fluctuation of ICT test increases, and some BGA solder joints have cold solder joints after -40℃ thermal shock. The reason is that the Ag3Sn strengthening phase in SAC305 is easy to react with chloride ions to form soluble complexes in a hot and humid environment. The practical solution is to switch to no-clean SAC305 solder paste with halogen content <0.1%, complete the conformal coating of all PCBs within 2 hours after reflow, and pass the ionic contamination test (≤0.78μg/cm2 NaCl equivalent) before coating. The key preventive measure is that the humidity of the storage environment must be locked at 30%RH±5%, and avoid storing in the same cabinet with PVC-containing packaging materials.

After continuous printing for 2 hours, the viscosity of Sn99.3Cu0.7 solder paste increases by 1200cP, resulting in solder bridging starting from the 500th board, while SAC305 only increases by 450cP. The symptom is that residual flux drawing between pads and obvious dried paste at the bottom of the stencil can be seen under a microscope. The reason is that the softening point of rosin resin in the Sn99.3Cu0.7 carrier is low (68℃), and if the workshop temperature exceeds 25.5℃, solvent evaporation will be accelerated. The practical solution is to extend the solder paste stirring time from 60 seconds to 90 seconds, immediately tighten the aluminum foil sealing cover after each use, and wipe the mesh openings with a wet lint-free cloth if the stencil printing interval exceeds 90 seconds. The key preventive measure is that the temperature of the solder paste refrigeration warehouse must be stable at (0±1)℃, the package must not be opened during the temperature return process, and the temperature return time must be strictly controlled at exactly 4 hours.

When using the same reflow profile, the IMC thickness of SAC305 solder joints reaches 3.2μm, while that of Sn99.3Cu0.7 is only 1.9μm. The symptom is that metallographic section shows that the former IMC layer is discontinuous, while the latter is denser but brittle. The reason is that the diffusion activation energy of silver atoms in SAC305 is low, and the conversion of Cu6Sn5 to Cu3Sn is accelerated when holding above 235℃. The practical solution is to reduce the reflow peak temperature from 245℃ to 242℃, compress the holding time to 65 seconds, increase the chain speed by 8%, and add a 0.8mm thick thermal insulation gasket for BGA devices separately. The key preventive measure is that Sn99.3Cu0.7 is prohibited from being used for connector pads with Ni/Au surface treatment, because it is easy to induce abnormal growth of interfacial Ni3Sn4 near the 217℃ eutectic point.