When the Solder Paste Viscosity is too high, the solder deposit volume during printing is significantly insufficient with obvious jagged edges. This is because high-viscosity Solder Paste cannot fully fill the stencil apertures under squeegee pressure, and local missing prints are extremely likely to occur especially on fine-pitch components such as 0201 or 0.3mm pitch QFN. Meanwhile, residual solder paste at the stencil bottom increases, forcing a higher cleaning frequency. After two hours of continuous production, the common aperture clogging rate reaches over 15%. When the solder paste viscosity is too low, it manifests as collapse at the moment of demolding, with the printed pattern height reduced by more than 20%. At this time, the probability of component offset during placement due to disturbance from the solder paste surface tension increases threefold. After reflow, not only does the bridging risk surge, but isolated solder balls with a diameter exceeding 0.15mm will also form at the root of QFP pins. These solder balls are often misjudged as cold solder joints during AOI inspection, triggering manual re-inspection.

We once conducted actual tests with the same lead-free solder paste in workshops at 25℃ and 32℃, and found that for every 1℃ increase in temperature, the measured viscosity drops by about 80cP. If the squeegee speed and demolding rate are not adjusted synchronously, the process window with a 99.2% pass rate at 25℃ drops to 94.6% at 32℃, among which 78% of the defects are concentrated in excessive voids at the bottom of BGA. This is because the Flux in low-viscosity solder paste volatilizes too quickly in the early stage of reflow, and the gas escape channels are blocked in advance. Similarly, a certain batch of solder paste absorbed moisture due to improper storage. Although the nominal viscosity in the TDS is still within the range of 1200±150cP, the measured thixotropic index plummeted from 0.68 to 0.41, resulting in poor pattern retention after printing. The first-print pass rate on 0.4mm pitch SOP components is only 83%, and forced standing for 45 minutes is required before the second printing to recover to over 96%.

On-site adjustment cannot only focus on the viscometer reading; it is necessary to synchronously observe the stencil rebound state and the spreading morphology of the solder paste at the squeegee edge. When the viscosity is appropriate, the solder paste adheres in a uniform film at the stencil bottom, and the pattern edges are sharp without burrs after demolding. When the viscosity is abnormal, either severe trailing occurs behind the squeegee, or intermittent stripes appear at the stencil apertures. Before replacing a new batch of solder paste, small-batch verification must be carried out, focusing on checking the aperture filling rate of pads below 0.5mm and the solder paste distribution uniformity of QFN thermal pads, because these two positions are most sensitive to changes in solder paste viscosity, and any slight deviation will cause warpage or cold solder joints after reflow.