Gauging true speed and accuracy: the authors propose new standards for measuring throughput based on production floor environments.

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Typically, dispensing platforms specify only x-y repeatability or dot placement accuracy (expressed as a Sigma value). These specifications fail to cover all the error mechanisms associated with the true accuracy and repeatability of a given platform.

Many factors influence positional accuracy, including x-y gantry design (sidebar), drive electronics, motion controllers, vision system, camera calibration, camera-to-needle offset calculations and even the dispense pump or valve used. Different modes of operation can affect accuracy, and each should be associated with a separate specification. For example, a jetting technology would show differences in capability when the valve is in continuous dispense mode versus when the valve settles before dispensing. This would also be apparent if a high-resolution mode is used.

Current accuracy specifications tend toward the general, vague and inconclusive. They rarely give a solid feel for what platforms are capable of or how tests are run. They are of limited value when users attempt to qualify and compare different machine specifications (Table 1). The specifications shown refer to some type of accuracy, but it is impossible to draw a quantitative conclusion as to how they compare, or what qualities are actually being referred to. The data and descriptions are quoted directly from various dispensing equipment vendors.

Gray areas abound:

* Does dispensing of actual material occur during tests? If not, how are they tested?

* Are specifications at 3-Sigma or 1-Sigma when not otherwise stated?

* Is there a difference between positional accuracy and x-y accuracy?

* Are tests run at full system speed?

* Is a machine vision system used?

To clearly determine a given platform's capability, a new test method needed to be developed. Such a measurement should consider all influencing factors and express them as CpK around stated specification limits. (Note: A measurement specification stated at 3-Sigma represents CpK >1.0.) A new metric calculates the total system accuracy (TSA) of a dispensing platform.

TSA measurements are taken from dispensed material on a substrate relative to a defined target. An array of dots are dispensed at full system speed relative to circular fiducials. Full system speed indicates the standard x-y-z speeds and accelerations used in a high-speed production environment.

Hugh Read is product manager, Camalot Dispensing Products, Speedline Technologies (speedlinetech.com); [e-mail].

Table 1. Various Means for Describing Accuracy and Speed

Manufacturer  Accuracy Description  Specification

Speedline     X-Y dot placement     [+ or -]0.0015"@3-Sigma
                accuracy              (38 [micro]m)
Vendor 1      Positional accuracy   [+ or -]0.001" ([+ or -]0.025 mm)
Vendor 2      X-Y accuracy          [+ or -]0.076 mm (0.003")@3-Sigma
Vendor 3      Positioning accuracy  [+ or -]0.0015" ([+ or -]37.5
                                      [micro]m)