Quantifying Solder Paste Transfer Efficiency across High Density Stencil Apertures
Optimizing stencil area ratios above 0.55 using electropolished foils and Type 5 paste maximizes volume transfer efficiency while minimizing SMT assembly defects.
Paste Volume Capability
Performance Index
Performance indices measure the consistency and accuracy of the solder deposition step in surface mount technology. A high paste volume capability indicates that the printing system and stencil are producing deposits within the specified tolerance. This metric is usually expressed through statistical process control values like Cp and Cpk.
Measurement System
Automated solder paste inspection machines scan the entire board to calculate the volume of every individual deposit. These systems track the paste volume capability over time to identify trends such as stencil wear or environmental changes. Data from these inspections allows for real-time adjustments to squeegee pressure and speed.
If the volumes begin to drift, the system can trigger an automatic cleaning cycle to remove residue from the bottom of the stencil. High-resolution sensors ensure that even the smallest deposits for micro-scale components are captured accurately for analysis.
Aperture Design
The ratio of the aperture area to the wall area is a primary driver of the amount of paste released. Improving the paste volume capability often involves optimizing the stencil thickness for the smallest components on the assembly. When the design and process are aligned, the risk of solder balls or starved joints is greatly reduced.