When selecting Solder Paste, first check whether the alloy composition and reflow peak temperature match the production line equipment capabilities. For example, Sn96.5Ag3.0Cu0.5 lead-free solder paste corresponds to a peak temperature of 235℃～245℃. If the furnace temperature bandwidth is insufficient or the conveyor belt speed is set to 70cm/min, the actual peak temperature will be low, resulting in cold solder joints and false soldering. At this time, the preheating zone time must be synchronously adjusted to 90～120 seconds and the thermocouple attachment position must be calibrated. Before printing, it is necessary to confirm that the stencil tension is ≥35N/cm2, the squeegee hardness is Shore A80±5, and the angle is controlled between 45°～60°, otherwise the solder paste transfer rate will be lower than 75%, especially for 0201 and 0.4mm pitch QFN devices.

The use of Solder Wire must be dynamically adjusted in combination with the soldering tip shape and pad size. Solder wire with a diameter of 0.3mm is suitable for rework of 0402 resistors and the soldering iron temperature is set to 320℃. When solder wire with a diameter of 0.8mm is used for large-area ground pads, the temperature needs to be raised to 350℃ and matched with rosin core Flux with a content of 2.2%～2.8%. If pinholes or balling are found, it is most likely that the oxidation of the soldering tip leads to a decrease in heat conduction. At this time, the machine should be shut down immediately for cleaning and re-tinning. We once encountered a customer who used solder wire containing halogen activator to solder ENIG surface PCB. After continuous operation for 3 hours, white corrosion residue was observed at the edge of the solder joint under a microscope. The root cause was that Cl ions were not completely volatilized and migrated in a hot and humid environment. Therefore, it is necessary to switch to no-clean low-halogen solder wire and the flux residue is <0.1mg/cm2.

Solder bars are mainly used for wave soldering tank maintenance and solder replenishment. The common Sn63Pb37 eutectic solder bar has a melting point of 183℃, while the lead-free system mostly uses Sn99.3Cu0.7, with a liquidus temperature of 227℃ and a solidus line of 220℃. When the wave soldering tank temperature is set to 255℃, if the impurity Fe content of the solder bar is >0.02%, the frequency of nozzle clogging will increase by more than 3 times. Therefore, samples must be taken every shift to detect three elements: Cu, Fe, and Al. If the Cu concentration exceeds 0.8%, copper removal treatment must be started, otherwise the solder joint wetting angle will deteriorate from the normal 20° to more than 45°. In addition, Solder Bars must be added in small batches, no more than 2% of the tank capacity at a time. During the melting process, maintain a nitrogen protection flow rate of 15～25L/min, otherwise the oxidation slag generation rate will increase by 40%.

Abnormal troubleshooting cannot only focus on the appearance. For example, solder paste printing offset of 0.15mm seems to be a visual alignment problem, but in fact it may be caused by the sinking of the stencil support pin by 0.08mm causing local deformation, or the ambient temperature and humidity exceeding the range of 23±3℃, 50±10%RH causing Solder Paste Viscosity drift. Another example is that solder balls are densely distributed at the four corners of the QFP after reflow, which is most likely because the area ratio of the stencil opening to the pad is less than 0.65, rather than the reflow curve itself being wrong. The uneven thickness of the IMC layer after Manual Soldering often stems from fluctuations in the solder wire feed speed exceeding ±0.5cm/s. At this time, it is necessary to check whether there is a jump in the feedback signal of the wire feed motor encoder.

Experience and skills come from repeated verification. For example, before a new batch of solder paste is put into production, three sets of comparative tests must be done: one set uses standard parameters, one set reduces the squeegee pressure by 10%, and one set extends the demolding time by 0.2 seconds. The stability is judged by SPI to detect whether the solder paste volume coefficient of variation CV value is <9%. Another example is that solder wire storage must be protected from light and moisture. After opening, it should be placed in a drying cabinet with a relative humidity of <30%, otherwise the precipitation of rosin crystals will directly lead to difficulty in soldering. Also, when replacing Wave Soldering Solder Bars, be sure to empty the old material before injecting new material. It is strictly forbidden to mix old and new materials, because the Cu segregation tendency of different batches of SnCu alloys can differ by up to 0.15%. After mixing, the Solder Composition in the tank is out of control, which can easily cause solder joint cracking.