The only 100% effective way to guarantee the performance of static-control systems is to use continuous multifunction workstation monitors.
Over the last 20 years, the line width used in typical ICs has decreased from 10 to 20 microns to less than 0.2 microns—a reductiondecrease of 100:1 with a corresponding increase in susceptibility to static damage. As a result, Eelectrostatic discharge (ESD) test equipment and workstation monitors now play a very important role in the battle against damage from ESD/static forinto expensive components and equipment.
Wrist-strap testers almost always are of the manual touch-to-test type. The strap is worn on the wrist, the coil cord is plugged into the tester, and one or more fingers are pressed against a plate to close the resistance measurement circuit. This type of tester always uses a simple go, no-go measurement circuit that activates a green LED when the resistance is inside a safe range and a red LED if the resistance is excessive.
Surface Resistance Testers
Like wrist-strap testers, all the circuitry of a surface resistancetester is contained in a small enclosure. But that’s where the difference ends. The surface resistance tester has two contact strips on the bottom of the unit that are spaced to read surface resistance or resistance per square. Also, the simple go, no-go LEDs are replaced by a bar-graph-like row of LEDs that indicates the measured surface resistance in 10 to 1 steps, such as 105 , 106 , 107 , 108 , 109 , and so on.
This type of tester obviously has very limited resolution and accuracy. An indication of 108 or 100 MW means that the actual resistance could be anywhere between 100 MW and 1,000 MW. As a result, these testers are, at best, suitable for roughly classifying a material to be within a 10- to- 1 range.
A much more sophisticated variety of surface-resistance testers uses a 3½ digit readout and normally is supplied with a simple resistance-per-square probe. Typical accuracy for such testers falls in the range of 1% to 5%. Although often slightly more expensive than the LED bar-graph type, they offer a significantly better value.
Continuous Multifunction Workstation Monitors
OneThe only 100% effective way to guarantee the performance of static-control systems is to use continuous multifunction workstation monitors. Ideally, these unitsmonitors should continuously monitor/observe these functions:
Monitor wrist straps for continuous conductance from the system ground to the operator’s wrist. The operator’s wrist and the 1-MW resistor(s) in the coil cord must be part of the measurement being performed by the monitor.
Continuously mMonitor workstation surfaces for acceptable resistance from the surface to system ground. It is not acceptable to only monitor the near- zero resistance from the ground wire of the workstation surface of the workstation to the system ground.
More often thant not, the snap attached to the ground wire makes a poor very high-resistance contact to the mat material. This may cancel out the performance of an otherwise acceptable workstation surface.
To properly verify an acceptable contact resistance between the workstation surface and the system ground, use two contact points on the surface. Then the tTest current willmust enter into the mat material from one of the contact points, and out of, and to ground through the second contact point. Rather than useusing two snaps, the two contact points should be two sets of strips made from either stainless steel or nickel-plated brass that can be fitted to the mat with two screws.
Since mat materials range from a few kilohms to thousands of megohms per square, the monitor should trip at about 50 MW. The contact strips should be 3″ to 4″ long. Using the resistance-per-square principle, the actual resistance between the pairs of contact strips can be tailored to the trip point of the monitor by increasing or decreasing the spacing between the contact strips.
If the strips are 3″ long and spaced by 3″, the measured resistance will equal the material’s resistance per square. Each time the spacing is halved, the measured resistance is halved. The spacing between the strips may be reduced to as little as 1/8″ for materials with very high resistance per square. Some monitors on the market today have a trip point for workstation surfaces of less than 4 MW and can only monitor carbon-filled materials.
If a conductive floor mat is part of theany workstation, monitor thethis floor mat using two sets of contact points and circuitry identical to that used for monitoring a workstation surface. Most monitors are not equipped with the additional monitoring circuit required for a floor mat. Some modular monitors, however, will accept up to eight dedicated remote units for monitoring wrist straps, workstation surfaces, floor mats, conductive footwear, and the grounding of machinery and test stations.
Continuously tTest and verify the ground connections and ground quality. Some monitors only confirm that the workstationunit’s ground wire connects to the same ground point as the ground wire for the workstation mat. This shared ground point typically is the nearest electrical outlet or conduit.
This arrangement does not verify that the ground point actually conducts into the soil where the ground stake supposedly is located. With this arrangement, it actually is possible to simply connect the two ground wires in mid air and still get a “green- ground- OK” indication, without any part of the monitor being connected to ground.
More advanced monitors measure the actual resistance into the soil of both the electrical ground stake and the earth ground stake that is used for the static-control system only.
In the early days of static control, all wrist straps connected to ground with a straight or coiled cord with aone single conductor. This worked just fine when the cord’s banana plug simply plugged into a grounded banana jack. When it became desirable to continuously monitor these single-conductor wrist straps, it became necessary to devise an indirect method to verify the integrity of the wrist strap.
The method that now is used, with some minor variations,s measures the complex impedance of the human-body capacitance in series with the 1-MW resistor in the coil cord. Most often, this is accomplished with a form of an AC-exited impedance or capacitance bridge,. bBut other methods also are used, including methods that those ensureing zero AC or DC voltage at the wrist strap.
Since all these methods were indirect and not as reliable as a simple DC resistance measurement, the dual-conductor wrist strap was developed. Using two electrically isolated half sections in the wristband and two wires in the coil cord, a simple and foolproof resistive-loop measurement can be made with clearly defined test limits.
The test limits typically are set up as a “pass” window. The pass condition is defined for loop readings of more than 0.5 MW to 1.5 MW but less than 10 MW to 35 MW.
Conductive footwear is defined as specially made shoes with a conductive soles or a conductive heels, or a shoe strap that may be used with ordinary shores. Footwear testers fall into a class by themselves and typically consist of a wall- or pedestal-mounted unit that connects to a metal foot plate.
The person testing the footwear stands on the foot plate with one or both feet and touches a plate or grabs a “handle” on the test unit. This closes a measuring circuit that reads the resistance from the foot plate through the footwear and through the person’s hand into the plate or the handle. The resistance of the person’s body and the contact resistance to the plate or handle normally areis a minor additions to the resistance from the foot plate into the person’s feet.
As a result, the test will be an accurate measure of how well the person conducts into a conductive floor surface. Typically, footwear testers also are set to indicate a pass condition whenfor reading within an acceptable window.
The trip limits normally are set from 0.5 MW to 2 MW for the low limit and from 50 MW to 100 MW for the high limit. Testers often use yellow, green, and red LEDs to indicate low, pass, and high test results.
Footwear testers also haveare available with additional capabilities, such as testing both footwear and wrist straps or wrist straps only. Testers even are available to checktest footwear and single-conductor and dual-conductor wrist straps.
Automated Static-Control Systems
Today, fully automated static-control systems can collect data from a large number of workstation monitors. One such system can accumulate data from more than 7,000 workstations and process it this data into printed reports or display the datea on a computer monitor. In addition to moment-to/by-moment displays of all operational status of data for anywhere from any number of a few to many workstations, information can be stored for more than 10 years and recalled as history or statistical reports for any time periods as specified.specified by the user.
The available data from each workstation or an from the entire system is:
All functions OK.
Wrist-strap failure, station number, time, and date.
Failed wrist-strap repaired, station number, time, and date.
Benchtop failure, station number, time, and date.
Benchtop repaired, station number, time, and date.
Ground failure, station number, time, and date.
Ground failure repaired station number, time, and date.
Station placed on standby, station number, time, and date.
Station removed from standby to operation, station number, time, and date.
Station not in use, station number, time, and date.
Station returned to use, station number, time, and date.
Historicaly data, with the most current event listed first,: followed by earlier events by time and date. Listed are the type of failure, the workstation number, the time and date of the failure, and the time and date of repair of the failed function.
Statistical failure data by function.: This report lists the percentage of wrist-strap failures, benchtop failures, floor-mat failures (if used), and grounding failures and the statistical impact on the entire system.
Statistical failure data by workstation.: This report lists the number of failures and the duration of each failure for each workstation and calculates the statistical impact the downtime of the failed workstations had on the entire system.
Data communications from each workstation to the central data acquisition unit use inexpensive telephone cords. Either system will pay for itself from datalogging savings and reduced failure rates in a surprisingly short time.
About the Author
Jan C. Hoigaard has served as president of SpectraScan International since 198-0 when he founded the company. Previously, he was a program manager on NASA and DoD space and satellite programs and was affiliated with TRW, Singer, Varil, and Watkins-Johnson. Mr. Hoigaard received an E.E. degree from O.T.S. in Oslo, Norway. SpectraScan International, 2812 E. Bijou St., Colorado Springs, CO 80909, (719) 447-0170.
Copyright 1998 Nelson Publishing Inc.