How to Choose ESD Test Equipment

By:

Also View/Participate in the Survey

ESD environment is only as safe as it is proven to be.  The purpose of ESD test equipment is to do just that.  A good suite of ESD test equipment can help you assess the effectiveness of your ESD preventive measures and verify that they do their job for you.  In this article we will explore selection factors for Ionization, Grounding, Static Voltage and ESD Events.

Ionization

The two main properties of ionization are decay and balance, or offset.  Decay is a measure of how quickly the accumulated static charges are dissipated by an ionizer and balance indicates whether the ionizer introduces additional charges itself as a result of its operation.  All of that, of course, must be measured at the point where your sensitive components are being handled.  Ionization performance (decay and balance) is measured with a Charge Plate Monitor (CPM).  The critical properties of CPM to watch for are:

• Plate Size

• Accuracy

• Reaction Time

• Ionization Controllers

Plate Size

CPM built to meet ANSI 3.1 standard has a plate size of 6x6”, or 150x150mm.  The plate provides a universal way of correlating ionization parameters.  While quite satisfactory for many applications, the large plate has problems correlating with actual ionization performance for small components.  Large CPM plates also hide uniformity issues, e.g. where decay and/or balance of an ionizer are not distributed evenly across the work surface.  Plate size is important when dealing with small components, such as ICs or magnetic heads.  When working with small components, try to use instruments with smaller plate sizes, since they provide more meaningful results.  Good instruments will also provide accurate and correlatable data to standardized measurements.

Accuracy

If your components are sensitive to low voltage discharges, then it will be important for you to have sufficient accuracy of measurements at low voltages. When working with sensitive components, choose instrument with smaller scale, such as +/-100V or even +/-10V to make your measurements relevant, rather than CPM in the +/-1000 V range.

Reaction Time

When dealing with pulsed ionizers, with unstable DC ionizers or with AC ionizers, large capacitance of the 6x6” plate creates low-pass filter which causes CPM to read artificially low offset voltages while the actual exposure of the components to the imbalance of an ionizer can be significantly higher.  As an example, in order to make sure that you can accurately access the influence of the imbalance of a pulsed ionizer on your product, the bandwidth of the CPM has to be at least 50 Hz or better.  For AC ionizers the bandwidth should be 200Hz or better.  When working with DC ionizers and such sensitive components as magnetic heads, the bandwidth of the CPM has to be at least 50 Hz to capture the balance “noise,” or short-term deviations of balance.

Ionizer Controllers

For continuous control of ionizer balance in critical environments you would need to use an ionizer controller – a small sensor with feedback loop to the ionizer.  Together with an ionizer controller, the ionizer can deliver continuous balance under +/-1V or even lower without frequent manual adjustments.  This would result in reducing cost of ownership for the ionizer and maximizing its performance through its lifecycle.  There are two main properties that define performance of a good ionizer controller:  reaction speed and immunity to static voltage.

Reaction Speed 

The control speed defines how tightly the balance will be maintained.  If the controller is too slow, the ionizer can veer far off on its own before it gets corrective signal from the controller.

Immunity to Static Voltage 

A charged object, such as a tray with components, or an operator approaching the workbench, can be mistaken by the controller for static voltage and cause an ionizer imbalance.  This would throw an ionizer balance in the opposite direction, charging all the components on the workbench to unacceptably high voltages.

Grounding

Proper grounding is the most important ESD control measure you can apply in your factory.  Good grounding, however, should never be taken for granted.  Ground wires may go nowhere, they can break, they can be connected to different grounds, and so on. The proper way to verify ground connection is in accordance with the ANSI 6.1 standard.  In all cases, for proper measurements, you must always have known and reliable reference grounds.    Let us examine some of the grounding considerations.

Metal Tools

Ground of metal tools should be measured using AC impedance as specified by the ANSI 6.1 standard.  Regular multimeters make measurements using DC resistance method, which is sufficient in some cases but may give erroneous results in other cases, especially when noise is present on the ground.  When measuring low-value ground impedance, such as in the 1 Ohm range, take into account the resistance of the meter connections.

Dissipative (Soft) Ground 

Verifying grounding of static-dissipative mats presents certain challenges due to their high resistance.   Only special meters with high resistance scale should be used in these cases because they will allow time for signal integration to reduce influence of the ever-present 50/60Hz noise.  One must be careful not to measure dissipative surface resistance with high voltage (as is typical with a Megommeter) because the static voltage typically applied is fairly low and because resistivity of static dissipative materials may be non-linear with the applied voltage.

Wrist Strap Monitoring

Without grounding of operators it is impossible to achieve ESD protection in manual operation.  In the environment where sensitivity of components is not critical (500V CDM), heel straps and conductive floors may work, however when dealing with more sensitive components, the operators must wear wriststraps, and there needs to be a reliable monitoring method in place.  A wriststrap monitor can be used to assure proper operator’s grounding at all times.  The following wriststrap properties are required:

Safety

Safety resistors are needed to protect an operator in case he or she accidentally touches live wire while wearing wrist strap. Good monitor uses 1MOhms safety resistors to ground (for dual wrist straps –a safety resistor for each half).  Quality wrist straps should have such safety resistors built in, but an extra measure of precaution will not hurt.

Imposed Voltage

In order for a wrist strap monitor to detect the presence of an operator, it must supply a signal to the operator.  The larger the signal, the higher the exposure the components will experience.  There is also the possibility of skin irritation caused by small but prolonged current through the operator’s body when he/she is using dual wriststraps.  Today’s wriststrap monitors can offer less-than-1V imposed voltage and the best ones – use less than 50mV.

Assurance of Operator’s Connection

Some wrist strap monitors offer added measure of ESD protection by sounding an alarm if an operator is within reach of the workbench but is not plugged in.

Additional Features

A good wriststrap monitor also offers monitoring of grounds – metal and dissipative, thus becoming a complete workstation monitor.

Materials

Proper packaging and other materials present in tools on workbenches offer further protection of sensitive components in transit or in process.   The critical measure of static dissipative properties is resistivity.  There are a number of instruments that measure resistivity in accordance with ESDA standards.  When measuring dissipative properties of materials, one should measure volume resistiviy in addition to surface resistivity.

Static Voltage

The best tool to measure static voltage on the surface is a static voltmeter (not to be confused with static field meter which could also be useful under limited circumstances). The following are the limitations of some static voltmeters that you need to take into account:

Viewing Angle

Every static voltmeter has a viewing angle, meaning that it measures static voltage not just on the object of interest, but also on the nearby surfaces.  Thus, the accuracy gets significantly worse when measuring voltage on small devices such as IC inside the tool (e.g. IC handler).  The IC appears to the static voltmeter to be a tiny bright speck on the dark background of grounded metal, and since the meter averages the data, the resulting reading is much lower than the actual voltage on the device.

Reaction Time

Most static voltmeters have relatively long reaction time, so they provide relevant readings only when the component is stationary.  Attempts to measure voltage on the moving device are generally futile.

Voltage vs. Charge

It is important to note that the static voltmeter measures only surface voltage, not the charge of the object.  For measurements of charge on insulators you need to use a Faraday cup.  For measurement of charge on conductive objects you can use a nanocoulombmeter.

ESD Event s

The ultimate measure of ESD safety is not how well your ionizer works or how good your ground is, but rather the frequency of occurrence and the magnitude of ESD Events (discharges) in your environment.  After all, the only measure of success of your ESD preventive measures is not the balance and decay of ionizer or ground impedance, but whether you still have discharges.   ESD Events are measured using ESD Event monitors that capture signal from electrostatic discharges and provide you with the measurements of magnitude of each discharge and count of discharges.  The properties of a good ESD Event monitor are:

• Ability to distinguish between multiple discharges

• Real-time response

• Correlation with established ESD models, such as Human Body Model (HBM), Machine Model (MM) and Charge Device Model (CDM)

Periodic Verification vs. Continuous Monitoring

In critical environments it is not sufficient to have periodic assurance of performance of your ESD environment.  Continuous monitors provide you with the information about your ESD environment at all times giving you factual knowledge on ESD exposure of your components.  Selecting the proper ESD test equipment is an important way to protect your products. 

• If your friend forwarded this newsletter to you, please register as a member and receive The BestTest Newsletter -- absolutely free!
• If you wish to update your news preferences or cancel the subscription, please unsubscribe.
• If you have any questions, please email experts@BestTest.com