If there’s one certainty in the print process, it’s that you have to have paste in front of the blade to print. Seems obvious, I know, but I could tell you countless tales of board defects, line stoppages and low yields because of insufficient solder paste in front of the squeegee blade. And, while it seems easy enough, in a high-volume manufacturing environment that’s producing assemblies with highly miniaturized devices, ensuring consistent volumes of paste in front of the squeegee is essential to high yields and optimized throughput.
What I’ve observed on many occasions is an operator who manually puts down a tremendous amount of solder paste so that it will last the entire shift. This approach might be sufficient in an operation that is fairly low volume and is assembling standard SMT boards. If, however, one is printing at a higher level with more miniaturized devices, more complex circuitry and geometries in high volume, having the correct quantity of paste in front of the blade is critical. Without it, print quality and, therefore, yield levels are affected.
The other factor to consider is the impact the environment is having on the solder paste. Laying down an entire jar of solder paste at the beginning of the shift may certainly lend itself to unintended consequences. With a material that is potentially changing with environmental exposure (i.e., the material may be susceptible to moisture; solvents could be pulled out by the atmosphere, etc.), the dynamics of the print can be upset. So, not only is it ideal to have the correct volume of paste in front of the blade, but the correct volume of fresh paste.
Automating paste deposition is the most effective method to ensure proper volumes of new, fresh material is placed in front of the squeegee blade. Most printing equipment manufacturers have such tools, and they are worth the investment. Automatic paste deposition technology can be combined with a paste roll height monitor for completely automated, hands-off, quality paste management. The paste roll height monitor functions just as the name implies; it uses a vertical laser to detect paste presence and roll height sensing, which saves significant operator monitoring and application time. Closed-loop paste level detection provides alerts for either manual or automatic paste replenishment and, therefore, avoids insufficient paste deposits and down-the-line defects. According to our company’s test data, combining the paste roll sensing functionality with automatic paste dispensing has numerous benefits, particularly in high-volume operations. Operator intervention is significantly reduced and can result in downtime savings of up to 40 min. per day. When your printer is churning out over 2,000 boards per day, that can result in tremendous throughput improvements.
Optimized, automated material replenishment has also been shown to reduce paste outside the print area by 80%, and solder paste wastage is significantly reduced, saving as much as 100g per day. In addition to the cost benefits from limiting paste wastage, perhaps the most significant benefits are defect reduction and process control improvements. When the volume of paste in front of the blade is inconsistent from side to side – particularly with smaller, high-density apertures – there will be defects. In fact, data from one of our customers showed that there were 685 defective parts per million (DPPM) fewer when using automatic paste replenishment. Finally, Cp and Cpk also showed substantive improvement when using the combination of paste roll height sensing and automatic paste replenishment. The Cp moved from 2.19 without these technologies to 2.57 with them, and the Cpk increased from 1.8 to 2.34 when the systems were incorporated.
At the end of the day, when variables are removed from the printing process, the better the results will be. Less operator intervention generally equates to more effective process control. Such is the case with machine-controlled paste roll height monitoring and automatic paste dispensing. The combination of these two technologies improves throughput, reduces material waste, dramatically increases yield and raises quality levels significantly. With an ROI payback of less than one year, it’s a quality investment worth making.
Clive Ashmore is global applied process engineering manager at DEK International (dek.com); firstname.lastname@example.org. His column appears bimonthly.