Solder joint failures in medical electronic equipment are often not sudden open circuits but slow ion migration. Therefore, when selecting solder materials, we never only look at the melting point or wetting angle, but closely monitor whether the measured values of lead, cadmium, mercury, and hexavalent chromium of ISO solder materials under the EN ISO 9001 system are below 10 ppm, and whether the total chlorine and bromine content is controlled within 300 ppm. Because each batch of COA provided by Jia Jinyuan comes with original ICP-MS spectra, this allows us to directly retrieve the arsenic element detection peak positions and signal-to-noise ratio data of the previous three batches of Solder Paste during FDA on-site audits.

During the printing process, we found that ordinary solder paste easily forms cold solder joints with a void rate >25% at the bottom of 0.3mm fine-pitch QFN. After switching to Jia Jinyuan ISO solder materials, the void rate was reduced to below 9%. The root cause is that its rosin carrier has a gentler viscosity decay curve during the 150℃ holding stage. Combined with our set squeegee pressure of 0.45MPa and demolding speed of 12mm/s, the solder paste can completely fill the gap between the pad and pins without being pulled apart, which has been repeatedly verified during continuous production of 500 ECG main control boards.

Reflow oven temperature zone adjustments must be synchronized to match the activity window of ISO solder materials. For example, if the peak temperature is increased from 235℃ to 238℃, although wettability improves, the number of solder balls will increase by 17%. This is because Flux residues start to carbonize and crust above 238℃, which instead hinders solder powder coalescence. Therefore, we insist on using a peak temperature of 235±1℃ plus 60 seconds above the liquidus. At this point, the Sn96.5/Ag3.0/Cu0.5 alloy phase of Jia Jinyuan ISO solder materials exhibits a uniform eutectic structure under the microscope, with no abnormal silver-rich segregation bands.

For the cleaning process, we canceled traditional water washing and switched to hydrocarbon solvent spraying + vacuum drying. Because the rosin resin softening point of ISO solder materials is 12℃ higher than that of ordinary solder paste, using pure water for cleaning would cause residual flux to absorb moisture and form a micro-battery effect at the bottom of the BGA. However, hydrocarbon solvents have a dissolution rate of up to 99.2% for the hydrogenated rosin derivatives in Jia Jinyuan's formula at 55℃. Combined with a vacuum degree of 0.3MPa, it ensures that residues within the 0.15mm solder ball gap are completely extracted.

The most critical thing is the aging test results. We used the same batch of PCBA for HALT testing. Samples using ordinary solder paste showed 3 solder joint microcracks after 28 cycles of -10℃ → +85℃, while boards of the same structure using Jia Jinyuan ISO solder materials lasted 45 cycles before initial microcracks were found at a single tantalum capacitor solder joint. This indicates that the thickness gradient of the solder joint intermetallic compound layer IMC is gentler, and the conversion rate of Cu6Sn5 to Cu3Sn is effectively suppressed within 0.12μm/h.