The occurrence of stringing and adhesive breakage during Solder Paste dispensing is not caused by a single factor, but is the combined result of low viscosity, slow thixotropic recovery, excessively high metal content, overly fast Flux volatilization, and storage temperature fluctuations. During our three-week continuous tracking of 21 batches of dispensing defects on an automotive ECU production line in Dongguan, we found that 87% of samples with stringing length exceeding 0.8 mm corresponded to Solder Paste Viscosity below 650 Pa·s (25°C, 1 rpm), while the period of sudden increase in adhesive breakage rate coincided with a 3°C rise in ambient temperature caused by a workshop air conditioning failure and uneven solder paste warm-up.

Viscosity must be controlled within the range of 680–750 Pa·s to balance dispensing fluidity and demolding stability. If too low, the adhesive will spread uncontrollably and form stringing; if too high, the adhesive cannot break in time when the needle detaches. The thixotropic index must be maintained between 4.2 and 5.0. When below 4.0, structural recovery after shearing is too slow, and the adhesive continues to creep in the Z-axis direction after dispensing. When above 5.2, extrusion resistance is too high, leading to pressure compensation lag and adhesive breakage. We once tested a dispensing solder paste with a thixotropic index of 5.6 under the same air pressure, and the adhesive breakage rate soared from 1.3% to 6.8%.

Metal content is not the higher the better. When the metal content in the Sn96.5Ag3.0Cu0.5 alloy system exceeds 89.5 wt%, the increased powder proportion intensifies inter-particle friction resistance, significantly delaying the response of the dispensing valve. When used with a 0.3 mm needle, the frequency of adhesive breakage increases by 2.3 times. If the flux activity level exceeds L3, the solvent volatilization rate accelerates, the adhesive surface skins over prematurely, and stringing appears within 15 seconds after dispensing. After a certain customer switched to L4 activity level solder paste, the stringing defect rate on the same equipment rose from 0.7% to 4.1%.

After opening, solder paste must be refrigerated at a constant temperature of 2–8°C, and fully rolled and mixed for at least 3 minutes before each use. For batches not operated according to this procedure, thixotropic performance attenuates by 18% within 3 hours after warming up to 25°C. We compared two groups of samples aliquoted from the same batch: one group followed standard procedures, and the other was left to warm up statically. The latter had an adhesive breakage rate 3.2 percentage points higher than the former after 1200 dispensing cycles. This shows that parameter optimization cannot discuss material indicators in isolation from actual operating habits.

All parameter adjustments must be synchronously verified for the minimum dispensing volume stability of the dispensing valve. When solder paste viscosity is increased to 730 Pa·s, if the originally set 45 ms jetting time and 0.18 MPa air pressure are still used, the CV value of the minimum dispensing volume will deteriorate from 6.4% to 11.7%. At this point, the jetting time must be reduced to 38 ms and the air pressure finely adjusted to 0.16 MPa to reconverge; otherwise, the so-called optimization will instead amplify solder paste defects.