Although syringe dispensing Solder Paste and stencil Printing Solder Paste appear to be the same material, they have essential differences in thixotropic index, viscosity decay curve, metal content, and Flux volatilization rate. For example, the thixotropic index of printing solder paste is usually controlled between 0.75 and 0.85 to adapt to squeegee shear, while dispensing solder paste needs to maintain 0.60 to 0.70 to ensure stable dispensing in the syringe. If printing solder paste is forcibly filled into a syringe for dispensing, it is highly prone to delayed start-stop response and severe stringing due to high thixotropy, accompanied by accelerated particle accumulation inside the needle that causes needle clogging.

We encountered a problem on an automotive ECU production line in Dongguan where syringes of the same model were clogging centrally after 10 AM for three consecutive days. Investigation revealed that the clogging was not caused by the needle, but by slight stratification of the solder paste after standing for more than 4 hours in a constant-temperature workshop at 25°C. The flux floated upward, causing the metal powder at the bottom to agglomerate and the viscosity to rise sharply. If 0.3 MPa air pressure was still applied for dispensing according to the original parameters at this time, the residual solder paste in the syringe would form cold-welding-like retention at the valve core. Even if new solder paste was replaced afterward, normal operation could not be restored immediately. The valve body had to be disassembled and cleaned with anhydrous ethanol plus ultrasonic cleaning for more than 15 minutes to completely remove the residue.

The interchangeability of dispensing solder paste and printing solder paste cannot be judged solely by whether the metal content on the TDS parameter table is both 88.5%. More attention should be paid to the viscosity change slope at three temperatures: 25°C, 30°C, and 35°C. For example, a certain Japanese brand of printing solder paste has a viscosity decrease rate of 12%/°C at 30°C, while the dispensing solder paste from the same factory is only 6.5%/°C. This means that a 1°C temperature fluctuation in the summer workshop may cause the printing solder paste to enter a low-viscosity state prematurely in the syringe, resulting in uncontrolled dispensing volume, while the dispensing solder paste remains stable. Therefore, before production line switching, the dispensing consistency of this batch of solder paste under the current ambient temperature must be actually measured, rather than relying on the supplier's factory data.

Improving needle clogging cannot be solved solely by increasing the cleaning frequency. We verified that cleaning the needle every 2 hours actually resulted in a higher failure rate than cleaning every 4 hours. The reason is that frequent disassembly and assembly cause slight deformation of the sealing ring and micro-leakage in the air circuit, which in turn causes the dispensing pressure to fluctuate by ±0.05 MPa. This fluctuation is sufficient to cause 0.12 mm diameter solder ball deviation in high-precision 0201 component dispensing. The correct approach is to bind the cleaning cycle with temperature and humidity records. Forced cleaning should be initiated only when the workshop RH > 65% and temperature > 28°C for more than 90 minutes, and the dispensing pressure should be reduced by 0.03 MPa and the dispensing time extended by 0.15 seconds to compensate for viscosity changes.

All solder paste stored in a syringe for more than 6 hours must be re-stirred for 30 seconds. This is not simple shaking, but using a special stirring rod to rotate clockwise at a constant speed along the cylinder wall, press down to the bottom and then lift up, to ensure that the settled silver-zinc alloy powder and the upper flux are fully redispersed. Otherwise, even if nominally compatible dispensing solder paste is used, after 8 hours of continuous operation, the phenomenon of qualified first piece and solder bridging on the last piece will occur. This has nothing to do with the solder paste itself, and is purely a process execution deviation.