Uncontrolled placement pressure is a hidden primary cause of frequent bridging on SMT production lines. When the vertical force applied to the Solder Paste the moment the nozzle descends exceeds its yield strength, the solder paste will plastically flow along the pad gap to both sides. Especially in the placement of tiny components such as 0201 and 01005, an over-tolerance of 0.05N is sufficient to make the solder paste break through the pad solder mask boundary and form a metal bridge with adjacent pads.

Coupling Relationship Between Pressure and Solder Paste Rheological Properties

Solder paste is not an ideal fluid. Its thixotropy determines the characteristic that it maintains its shape when standing still and its viscosity drops sharply after being sheared, and the placement pressure precisely provides this shearing condition. If the pressure rise rate is too fast or the peak value is too high, the spreading behavior of the solder paste on the pad will change from controllable elastic recovery to irreversible plastic displacement. At this time, even if no bridging is detected by AOI before reflow, the degree of bridging will still be aggravated by the surface tension of the liquid metal during the high-temperature melting stage.

Pressure Closed-Loop Control Logic of JJY Equipment

JJY placement machines embed strain-gauge real-time pressure sensors inside the nozzle holder, with a sampling frequency of 2kHz. The Z-axis servo system dynamically adjusts the downward stroke according to the preset pressure curve, and starts flexible deceleration 50μm before contacting the solder paste, ensuring that the pressure slope does not exceed 1.2N/ms. This parameter has been verified by an automotive-grade factory in Shenzhen, and can reduce the bridging rate of QFN32 packages from 1800ppm to below 210ppm.

Collaborative Calibration of Stencil Aperture and Pressure Parameters

The required placement pressure for the same solder paste varies significantly under different aperture areas. For example, when using solder paste with a particle size D50=25μm to print 0.3mm×0.15mm pads, the recommended pressure is 0.18N. However, if the stencil aperture is enlarged to 0.32mm×0.17mm, the pressure must be synchronously reduced to 0.15N, otherwise excess solder paste will be squeezed out and form liquid solder balls in the early stage of reflow, which will roll to adjacent pads.

Pressure Drift Identification Method in Daily Spot Checks

A three-point loading test on the nozzle must be carried out weekly using standard weights (50g, 100g, 200g), recording the deviation between the actual feedback value and the theoretical value. If the maximum deviation of the same nozzle in three tests exceeds ±0.03N, or the difference between adjacent nozzles is greater than 0.04N, the Z-axis rail grease must be cleaned and the pressure zero point recalibrated, because carbonization of old grease will cause sudden changes in friction, which in turn leads to lagging pressure response.