The penetration rate of through-hole reflow process in automotive electronics and high-density board fields has exceeded 35%. The driving factor is customers' rigid demand for double-sided placement efficiency and collaborative soldering of peripheral plug-ins around BGA. Out-of-control Solder Paste usage directly causes the production line first-pass yield to fluctuate by more than 8 percentage points. A German Tier1 supplier was forced to stop the line to adjust stencil openings last year due to consecutive bridging of connectors with 0.3mm pin pitch, which forced the entire industry to tighten the solder paste volume control accuracy from ±15% to within ±8%.
The stencil opening shape is shifting from a unified rectangle to differentiated design. For example, for plug-in pins above 0.8mm, elongated openings with 0.1mm lateral extension are adopted, which not only ensures solder paste transfer rate but also suppresses lateral flow during reflow. For 0.4mm fine-pitch pin headers, circular openings are used with 25μm step thickness. This change stems from AOI inspection data feedback: if the solder paste volume dispersion of different devices on the same board exceeds 12%, the probability of cold solder joints increases by 3.7 times. However, simply reducing squeegee pressure will lead to insufficient edge filling. It is necessary to simultaneously increase the printing speed to 45mm/s and shorten the release distance to 0.15mm to maintain overall consistency.
The rheological properties of the solder paste itself cause implicit interference to dosage control. When the humidity in the workshop exceeds 65%RH in summer, the viscosity of some no-clean solder pastes decreases by 18%, leading to an increase of 0.04mm in post-printing slump. If the stencil tension is not adjusted from 45N/cm² to 48N/cm² at this time, solder paste overflow will be observed on the plug-in pads next to 0603 components. We have verified this phenomenon on three production lines. The corresponding measure is to extend the solder paste warm-up time from 2 hours to 2.5 hours, and add a 15-second vacuum standing step before printing, which can reduce the volatilization rate of the Flux by 9%, thereby stabilizing the paste structure.
Equipment-level compensation is replacing manual experience-based adjustment. The newly delivered DEK Horizon model has a built-in solder paste volume prediction module, which generates dynamic compensation coefficients based on real-time collected stencil tension, squeegee angle, and ambient temperature and humidity. When a solder paste batch change is detected, it automatically calls the historical database to match the optimal parameter combination. An EMS factory measured that this function increased the first-piece pass rate from 62% to 89%. However, the premise is that XRF must be used to spot-check the solder thickness of three typical pads every shift, otherwise the algorithm output will fail due to accumulated errors from stencil wear. Currently, 7 foundries have incorporated this into mandatory SOP clauses.
The process window is being compressed. The previously allowed solder paste thickness tolerance of ±25μm has been narrowed to ±12μm by mainstream automotive-grade customers. This means that the etching accuracy of a 0.127mm thick stencil must reach ±2μm, and there are fewer than 5 domestic manufacturers that can stably deliver stencils of this grade. This has given rise to a local nickel plating thickening solution: plating an 8μm nickel layer in the plug-in area to form a physical limit. Although this practice increases stencil cost by 35%, it reduces the solder paste volume coefficient of variation from 11.3% to 6.8%. A new energy electronic control project passed the AEC-Q200 certification two weeks ahead of schedule as a result. However, uneven plating will cause a sudden increase in local solder volume, so a confocal microscope must be used to spot-check 3 cross-sections per roll.
The ultimate constraint on solder paste dosage comes from the melting behavior during the reflow stage. When the peak temperature reaches 235°C, the actual wetting height of the solder paste around a 0.5mm diameter pin only accounts for 62% of the pin length. If the initial printing volume deviation exceeds 10%, void clusters will form at the pin root. We tracked 12 mass-produced models in 2023 and found that 92% of samples with a void rate >15% had out-of-tolerance solder paste volume. The solution is not to simply reduce the dosage, but to lower the heating slope of the reflow zone from 1.2°C/s to 0.8°C/s, and control the oxygen content to within 100ppm under a nitrogen atmosphere. This can reduce the surface tension of the solder paste by 23%, promoting the molten state to more evenly wrap around the pin sides.
