Incoming material inspection is not a formality, but the first critical lifeline

For each batch of Solder Paste delivered, the consistency of three certificates, namely MSDS, COA and batch number, must be verified simultaneously. The deviation of Sn/Ag/Cu main element content measured by ICP-OES shall not exceed ±0.3%. Meanwhile, samples shall be taken and placed in a 25℃ constant temperature chamber for 4 hours before testing the viscosity change rate. If the decrease exceeds 8%, the thixotropy is judged to be invalid and the batch will be directly rejected, because the solder paste printing window of Jia Jinyuan SMT line only allows ±1.2Pa·s fluctuation, beyond which both missing printing and edge stringing will be caused.

Solder paste stirring is not timed by seconds, but judged by rheological response

When using a planetary mixer, the three parameters of rotation speed, time and vacuum degree must be set in a linked manner. For example, 60rpm for 120 seconds with -0.08MPa vacuum. The machine must not be stopped and the lid must not be opened midway, otherwise the micro-bubbles inside the solder paste cannot be completely discharged, and void clusters will form at the bottom of QFN after being put into production. After stirring, the printing release rate must be verified immediately on the machine. If edge serrated trailing appears on 3 consecutive pieces, it indicates insufficient shear thinning, and re-stirring is required instead of simply adding diluent.

Printing parameters are not fixed values, but dynamically compensated with solder paste batches

The thixotropic index of the same model of solder paste can vary by up to 0.15 between different batches. Therefore, all stencil opening data of Jia Jinyuan must be linked with the viscosity test value of the current batch of solder paste. When the measured viscosity is 0.7Pa·s higher than the nominal value, the squeegee angle is automatically reduced by 0.8°, the pressure is increased by 1.2N, and the speed is reduced to 45mm/s. Otherwise, obvious solder beads will appear around 0201 components, and the AOI false alarm rate will increase by 23%.

The melting and casting of Solder Wire and Solder Bar is not only about monitoring the melting point, but also about controlling the impurity precipitated phase

The smelting of Jia Jinyuan solder wire before drawing must undergo two refinements under nitrogen protection, and the peak value of the furnace temperature curve is strictly locked at 265℃±2℃. Insufficient holding time will lead to insufficient dissolution of Cu6Sn5 intermetallic compounds, and gray-white brittle residue will form at the edge of BGA pads during online welding. If the temperature of the solder bar casting mold is lower than 110℃, the segregation of Sn-Bi eutectic phase will be aggravated, and the drop resistance of solder joints after reflow will drop sharply from 120 times to 67 times.

Warehousing is not simply constant temperature, but coupled control of temperature and humidity

The solder paste refrigerated warehouse must be maintained at 2℃~8℃ with humidity ≤35%RH, because for every 5%RH increase in humidity, the moisture absorption rate of rosin resin in the solder paste increases by 1.8 times, which easily induces micro-short circuits at both ends of 0402 resistors after being put into production. The solder wire storage area needs to be controlled at 20℃±3℃ and kept away from direct air outlets of air conditioners, otherwise the thickness of the surface oxide film will increase from 1.2nm to 4.7nm within 72 hours, causing the wave soldering wetting angle to increase to more than 58°.