Ryne C. Allen
ESD Systems
Desco Industries, Inc.
19 Brigham St., Unit#9
Marlboro, MA 01752-3170
Email: Ryne C. Allen" <ryne@esdsystems.com>

 

I INTRODUCTION

The control of electrostatic discharge is an important aspect in the manufacturing, assembling and repairing of devices that employ electronics. Electrostatic discharges can damage an electronic component at any stage of its production or application if not controlled. The primary method of control is to ground (or bring to the same potential) all conductors that come in contact or near proximity to the electronic device(s). These conductors include humans, tools, ESD mats, other electronic devices, boards, connectors, packaging, etc.

There are other components to a good ESD Control program including, removal of unnecessary insulators, shielding, ionization, environmental controls, training, education and top down compliance. This paper will talk about controlling discharges to a grounded ESD mat on a workstation.

Of specific interest in controlling an electrostatic discharge is the time rate of the discharge. A discharge will occur much quicker in/on a conductor with a surface resistance of 10² Ohms than in a conductor with a surface resistance of 10⁹ Ohms. How fast or slow should the controlled discharge be? Understanding the importance of discharge times will help you choose the right ESD control materials in building, maintaining, or auditing your own ESD Safe workbench(es).

The upper and lower boundaries of an ESD safe discharge rate are determined by the application and materials used. To limit the discussion, the potential energy sourced from the Human Body Model (HBM), [refer to ANSI EOS/ESD S5.1-1993], is applied into an ElectroStatic Discharge Sensitive (ESDS) work area or ESD mat.

II BODY & MOVEMENT

You should be familiar with the timing of the human body’s movements relative to handling or working near ESDS devices to have a handle on the upper limit of the controlled discharge. To reduce the likelihood of an operator discharging onto an ESDS device, they should drain any charges before bringing an ESDS device in contact with themselves or another conductor, whether floating or grounded.

 

Table I

Movement times (averaged) from typical operations.

 

Table I depicts averaged times, in milliseconds for the handing of tools or devices at a work bench with a corresponding standard deviation in millseconds. The shortest time of 153 ms, or worst case, is the time that we will design our ESDS workbench table top with. You want to be sure that your device is fully discharged well before the 153 ms landing time. A good rule of thumb would be to engineer a x2 safety factor. Therefore your device should be fully discharged before reaching 76.5 ms (76.5 ms x 2 = 153 ms). The time constraint of 76.5 ms for body movement defines the upper boundary of the controlled discharge rate (not including the standard deviation of 11 ms).

III ENERGY CONSIDERATIONS

Table II

Typical Discharge times [t=R*C*ln(V/V₀)] for an RC circuit

where C=200 pF and V₀=249 Volts

 

Table II shows calculated discharge rates for the human body model onto an ESD grounded mat with surface-to-ground (RTG) resistances from 10² to 10¹¹ Ohms. The more conductive the ESD mat on the workbench is, the faster the discharge, but there is another consideration too.

How fast is too fast? When does the discharge energy at any given time reach a critical level that can damage a semiconductor? The answer depends on several variables relative to the semiconductor’s construction such as line spacing, composition, density, packaging, etcetra, all leading to an ESD component classification [refer to table I in the ANSI EOS/ESD S5.1-1993 and the manufactures’ device specifications].

For simplicity’s sake assume the worst case, class 0, which has a 0 to 249 Volt tolerance. Applying the HBM, a conservative worst case capacitance would be 200 pF, twice that of the HBM and resistance of 10KW . Therefore the maximum power (P) level based on Ohm’s Law is P=V²/R (J/s) and the worst case HBM is ((249)²/10K)=6.2 Watts or Joules per second (Js^-1).

The maximum energy (E) stored in a worst case HBM capacitance (C) of 200 pF and at a maximum voltage (V) of 249 Volts, (using E=1/2 CV²), yields 6.2 m J. The next concern is to relate energy to time. The time constant (t ) is the measure of the length in time, in a natural response system, for the discharge current to die down to a negligible value (assume 1% of the original signal). For an RC circuit, the time constant (t ) is equivalent to the multiple of the equivalent resistance and capacitance. In this case, the time constant (t ) of our RC circuit is (10KW )(200pF) or t = 2 m s. Discharging this energy upon touching a conductor at zero volts yields a current, (using I=P/V), of (6.2Js^-1)/(249V) or 24.8 mA. To avoid damaging a class 0 ESDS device, the discharge current must be below 24.8 mA. Engineering in a "2x" safety factor, the maximum discharge current would be 12.4 mA. To maintain a discharge current below 12.4 mA, we need to look at our grounding equipment on the ESDS workbench.

Table III

Discharge currents from a 6.2 m J lossless energy source

(with C=200pF & V=249V) dependent on the discharge time.

 

The rate at which 6.2 m J of energy discharges is very important. To fast a discharge will lead to an ESD Event, which can electromagnetically be measured using a simple loop antenna attached to a high impedance input of a high-speed storage scope. The faster the discharge the greater the discharge current becomes as well as the emf (electromotive force) on the loop antenna from the EMI (ElectroMagnetic Interference). Table III depicts the discharge current for 6.2 m J at varying discharge times. We are assuming lossless conditions during the discharge for worst case. For our example, to keep the discharge current below 12.4 mA, the discharge rate [from Table III] must be no quicker than 2.01 m s. This energy-based-time constraint forms the lower boundary of the controlled discharge rate.

IV MAT MATERIALS

The upper and lower boundary of our controlled discharge rate are now defined and can be used to help in choosing the correct ESD mat for an ESDS workstation. ESD mat materials come in many variations. In general, mats are either made from vinyl or rubber material and can be homogeneous or multi-layered. Rubber mats, in general, have good chemical and heat resistance but vinyl tends to be more cost effective. The electrical properties of an ESD mat are important to know in controlling the electrostatic discharge.

An ESD mat will be either electrically conductive or dissipative. Both terms mean that the mat will conduct a charge when grounded. The difference in the terms is defined by the materials resistance, which effects the speed of the discharge. By definition [ESD ADV1.0-1994] a conductive material has a surface resistivity of less than 1x10⁵ W /sq and a dissipative material is greater than 1x10⁵ W /sq but less than 1x10¹² W /sq. Anything with a surface resistivity greater than 1x10¹² W /sq is considered insulative and will essentially not conduct charges.

Back to our example. If the maximum discharge current of our ESDS device is 12.4 mA, then the discharge time based on energy must be slower than 2.01 m s and based on body movement must be faster than 76.5 ms. Using the discharge times from Table II and assuming that the mat has a negligible capacitance relative to the HBM, then the mat resistance must be greater than 2.2x10^{3 W} or 2.2x10⁴ W /sq and less than 8.3x10⁷ W or 8.3x10⁸ W /sq. In other words, a very conductive mat for some applications may be to quick to discharge and yield more dangerous ESD events whether properly grounded or not.

 

Graph I

Graph I shows the natural response of a 249 Volt discharge in an RC circuit using a capacitance of 200 pF (HBM) into resistances (mat) of 10⁴, 10⁵, and 10⁶ Ohms. The natural response of the10⁴ W curve is below 1% of its’ initial voltage in less than 10 m s where the 10⁶ W curve takes less than 1ms to discharge to less than 1% (V<2.49 V) of its initial value (V₀=249 V).

V GROUND STRAPS

Another defense, and the most common method, to reduce the risk of creating an ESD event is wearing a grounded wrist strap at the workstation. The wrist strap connects the skin (a large conductor) to a common potential (usually power ground). Properly worn, the wrist strap should fit snugly, making proper contact with the skin, to reduce contact resistance. Refer to the manufactures specifications and instructions of the wrist strap.

The wrist strap, since it is connected to ground, will quickly discharge any charge the body either generates through tribocharging or becomes exposed to through induction. Anytime the body directly touches a charged conductor, a discharge will occur because the body is at a different potential (0 Volts). Controlling this discharge is important if the conductor is an ESDS device and in minimizing induced charges through EMI onto nearby ungrounded ESDS devices.

The electrical properties of the skin of an operator can have a wide range in both resistance and capacitance depending on several variables. An operator’s hand touching a charged device will initiate a discharge at the rate of the time constant of the skin before including the RL properties of the wrist strap. To reduce the potential of an unsafe discharge from a device to a very conductive operator, adding resistance to the operator at the interface from skin to device may be necessary. Some solutions are static dissipative gloves or finger cots, which if worn properly, may add from 1 to 10 MW to the RC circuit of the skin. This in turn slows down the discharge rate to well over 2 m s.

VI CONCLUSION

The upper and lower boundaries of a safe discharge rate are determined by the application and materials used. The movements of the operator define our upper boundary and the max energy, as defined by the ESDS component classification, dictates our lower boundary. We want to design an ESDS workbench to control the discharge rate (via the circuit’s time constant) of our grounded or conductive materials within these limits.

For the HBM, and a class 0 device, the materials chosen for a safe ESD workbench should have electrical properties to support discharge rates between 2 m s and 76.5 ms. These discharge rates, using worst case assumptions, equate to an ESD mat surface with an RTG (Resistance To Groundable point) between 2.2x10^{3 W} and 8.3x10⁷ W . This controlled discharge rate window will vary depending on the class of semiconductor components used (class 0 to class 3B) and the properties of production resources used (human vs. automated).

The numbers calculated were based on assumptions used to simplify the explanation of the material. Real world applications are much more complex and require a more detailed analysis, which was beyond the scope of this paper.