Selecting a Fluid Dispensing Solution: The Basics
The opening act or the headliner?
If you ask an equipment manufacturer to demonstrate dispense accuracy, and they report the resolution of the encoder(s) on their system, they fail. And unfortunately, you might too. A one-dimensional encoder specification does not define overall accuracy and repeatability.
Think about it this way.
When you attend a concert, who are you more excited to see? The opening act or the headliner? If you’re like most concert-goers, it’s the headliner. That’s who you came to see. The opening act is there to warm up the crowd.
This analogy is useful when considering how encoders contribute to overall dispensing accuracy. Simply put, the encoder is the opening act. It warms up the process by reporting position, speed and direction of movement. It might even be a high-resolution encoder. But that’s it. That’s all it does.
The real accuracy happens when the headliners take the stage. And in this case, it’s a festival. The three headliners are the motion, vision and fluid systems. These are the main systems that come together to deliver comprehensive, system-level accuracy and repeatability. So, before you approve that purchase order, make sure these systems are fine-tuned for accuracy.
The True Measures: Cp/Cpk Values and Application Results
To truly understand how a dispensing system will perform, a standard test method that reflects normal operating conditions, should be used to test the main systems. And test results should be reported in Cp and Cpk values. With Cp indicating repeatable performance, and Cpk indicating how accurate and repeatable the systems perform, relative to upper and lower specification limits. As a final step, your application should be tested on the dispensing system.Accuracy and Repeatability on a Larger Scale
Picture this: The dispense valve moves to a set target on the substrate, dispenses a precise dot or line path within target, moves to the next location, and completes another precise dispense – this is what you’re after. The ultimate goal is to have both a highly accurate and highly repeatable system. Let’s take a closer look at the main system components to understand some of the dynamics involved.When it comes to the motion system, what matters most is how well the system processes encoder data, adjusts, and quickly and accurately moves to a commanded location. For example, with accurate Z-axis positioning, non-contact dispensing and perpendicular dispense angles can be achieved.
The ability to accurately locate a fiducial and identify where to dispense on the substrate, is also critical to overall system accuracy. This is the vision system’s domain. Most equipment manufacturers do not report how accurate or repeatable their systems are when using camera resolution and a fiducial algorithm to determine a target position. For ASYMTEK products, these elements are included in our wet dispense specification to demonstrate overall dispensing accuracy.
Finally, valve and fluid type factor into overall accuracy. Valve performance, settling factors, ambient temperature variation, valve and reservoir mass, and dispense gap requirements must be optimized and compensated for to achieve accuracy.
We’ll Be There When the House Lights Go Up
You now have a wider view of the factors involved in dispensing accuracy. And you won’t accept a one-dimensional encoder specification as a full-scale illustration of system accuracy and repeatability. After all, you’re trying to buy a dispensing system, not an encoder. Recalling our concert analogy, would a concert promoter sell the opening act over the headliner? No. That just wouldn’t make sense.
In all seriousness, we know there’s a lot riding on equipment purchase decisions. You need accuracy, repeatability and maximum throughput. But to achieve that, you need to know more than what the machine’s encoder can do. Don’t settle for incomplete information or insufficient test results. When comparing possible solutions, ask for wet dispense accuracy metrics. And look for a supplier with in-depth engineering and applications expertise who can fine-tune your application to maximize outcomes. Let us know how we can help. We’re here to assist, and we have the expertise to do it. When you leave our labs, you’ll be ready to rock.
For a stage-dive into this topic,
Download the Whitepaper:
Making Sense of Accuracy, Repeatability, and Specification for Automated Fluid Dispensing Systems.
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