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Accuracy Measurements for Modern Dispensing
December 31, 1969 |Estimated reading time: 5 minutes
Modern PCBs require more miniaturization and previously unattainable component densities. This is forcing current assembly equipment to offer increased accuracy without sacrificing high yield and maximum throughput.
Typically, current dispensing platforms specify X/Y repeatability and/or dot-placement accuracy, expressed as a sigma value. These specifications do not account for errors associated with positional mechanisms, which contribute to final accuracy and repeatability. Many factors influence positional accuracy, including the X/Y gantry, camera calibration, fiducial teaching errors, and camera-to-needle offset calculations. To determine true machine capability, including positional accuracy, and express it as a Cpk value, a test method must be developed to account for all influencing factors. One system* for a dispensing platform includes material placement accuracy on a substrate relative to a defined target. With this system, it is possible to determine true platform capability and provide a better understanding of how the product will work for various applications.
This test uses a stable substrate with well-defined fiducial features etched onto the surface. The substrate is optical-quality float glass, which offers high clarity and thermal stability, and measures 240-mm wide by 330-mm long. Black fiducials, measuring 1.5 mm in diameter, are etched in each of the four corners. Their positions relative to each other are known and verified by the optical inspection system. The glass is loaded into the system, lifted, and clamped in the same manner as a printed wiring board (PWB). A pump suitable for dispensing adhesive dots is mounted, and the usual calibration routines are carried out. The fiducials are taught by the system and their precise distance is entered into the pattern. A uniform grid of dispensed dots eight rows high by nine columns wide is programmed relative to the lower fiducials, where the lower left fiducial is the anchor point and the lower right fiducial aligns the pattern angularly. Dot characteristics (lift height, size, dwell) are typical for high-speed dispensing.
To ensure that the results would be the same in an actual production environment, there is no programming that would not otherwise be in a typical pattern. An SMT adhesive that generally has high thixotropic and adhesive properties is used. This means that the dots will retain their shape and position on the glass during the measuring process. After the pattern is dispensed onto the glass substrate, it is placed onto the measuring system. The measuring system locates the fiducials and measures the adhesive dots in relation to the fiducials. Below is an example of results from this type of test and measurement.
Interpreting the Results
The test method facilitates error diagnosis:
Figure 1. The worst condition (blue) - the machine neither dispenses repeatably, nor on-target. The red area shows high Cp or low standard deviation value and low Cpk value. The most desirable outcome (green) - the system is highly repeatable and on-target.
Low Cp and low Cpk (Figure 1, top left, blue dots) - This is the worst condition. The machine can neither dispense repeatably, nor on target. Causes for these results can typically be traced to:
• Flimsy gantry,
• Dispense technology used,
• Poor settling,
• Poor board clamping,
• Poor camera-to-needle offset calibration,
• Poor fiducial-finding techniques or models.
High Cp or low standard-deviation value and low Cpk value (Figure 1, middle, red dots) demonstrates:
• Poor camera-to-needle calibration,
• Poor fiducial alignment.
These conditions lead to a well-grouped pattern that is not well-targeted. While some of the system is working well, parts of the system that affect targeting require improvement.
High Cp and Cpk (Figure 1, bottom right, green dots) - This is the most desirable outcome. The system is highly repeatable and well-targeted. Table 1 shows that the Cpk value is less than the Cp value of the X-axis. X-axis Cp values indicate that the machine is capable of being accurate. The Cp and Cpk for the Y-axis indicate the conditions are in control and capable of six sigma reliability. The histograms in Figure 2 show individual distribution for each axis. Note that the bell curves are steep and narrow. This is a good indicator of axis performance.
Figure 2. X- and Y-axis histograms.
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Why are Cp and Cpk Important?
To illustrate how Cp and Cpk can be used to indicate how well the equipment can accomplish a task, we can examine a typical high-accuracy dispensing task. In many high-density applications, there is little room between a flip-chip die and nearby passive components. The dispensing challenge is to get a needle or a stream of underfill material into this gap without touching the die because that will lead to top-die contamination. If the gap is 0.5-mm wide, and the needle is 0.25 mm in diameter, then 0.25 mm of total clearance remains before the needle hits. This 0.25 mm (or ±0.25 mm) becomes the process-spec limit. A greater deviation will result in the needle or stream striking the die or contaminating nearby components. The Cp number will indicate, given a target location, how much repeatability can be expected. The Cpk includes this repeatability, and adds targeting errors. It is the only true representation of system capability.
Conclusion
There are a number of mechanisms that can contribute to liquid placement inaccuracies. A testing method can be used to determine which combination of mechanisms exists in a platform. Platforms and gantry systems that are conservatively designed with massive supporting structures, stiff bearing systems, and high-resolution linear feedback devices achieve higher stiffness and response. This results in placement errors that are small and tightly grouped. Well-designed and implemented vision systems and flexible algorithms provide high-quality position information to the controller, resulting in high targeting accuracy. This is true whether you dispense dots of adhesives relative to fiducial marks or lines of underfill relative to die edges.
Assembly equipment users do not need to purchase and implement specific equipment and methods, but instead should ensure that their equipment providers have access to such equipment and methodology. There also are third-party services available to provide the same level of information.**
* Total System Accuracy test method, Speedline Technologies, Franklin, Mass.
** CeTaQ Americas, Hudson, N.H.
Brian Prescott, senior engineer, advanced development group, Speedline Technologies, may be contacted at (508) 520-0083; e-mail: bprescott@speedlinetech.com.