Controlling Workstation Discharge Times

The control of electrostatic discharge (ESD) is an important aspect in manufacturing, assembling, and repairing electronic products. If not controlled, ESD can damage a sensitive device in a product at any stage of its production or application.

The primary method of control is to ground (or bring to the same potential) all conductors that come in contact with or in close proximity to the electronic device. These conductors include humans, tools, ESD mats, other electronic devices, boards, connectors, and packaging.

The time rate of a discharge is of specific interest in controlling ESD. A discharge will occur much quicker for a conductor with a surface resistance of 102 W than for a conductor with a surface resistance of 109 W.

How fast or slow should the controlled discharge be? By understanding the importance of discharge times, you can choose the right ESD-control materials for building, maintaining, or auditing your own ESD-safe workbench.

The upper and lower boundaries of an ESD-safe discharge rate are determined by the application and materials used. This discussion is limited to testing per the ESD Association Standard S5.1-1993 Electrostatic Discharge Sensitivity Testing—Human Body Model. The potential energy sourced from the Human Body Model (HBM) is applied into an electrostatic discharge-sensitive (ESDS) work area or ESD mat.

Body and Movement

To handle the upper limit of the controlled discharge, you should know the timing of the movements of the human body relative to handling or working near ESDS devices. To reduce the likelihood of discharging onto ESDS devices, operators should drain any charges before touching the device or bringing it in contact with other conductors, whether floating or grounded.

Table 1 depicts averaged times for the movement of tools or devices from one location to another at a workbench. The shortest time of 153 ms, or worst case, is the time that we will use to design our ESDS workbench tabletop.

You want to be sure that your device is fully discharged before the 153-ms grabbing time. A good rule of thumb is to use a 2× safety factor. Therefore, your device should be fully discharged before reaching 76.5 ms (76.5 ms x 2 = 153 ms). The time of 76.5 ms for body movement defines the upper boundary of the controlled discharge rate, not including the standard deviation of 11 ms.

Energy Considerations

Table 2 shows calculated discharge rates for the HBM onto an ESD grounded mat with surface-to-ground (RTG) resistances from 102 W to 1011 W using the equationt = RC ln
. The more conductive the ESD mat on the workbench is, the faster the discharge. But there are other considerations: How fast is too fast? And when does the discharge energy reach a critical level that can damage a semiconductor?

The answers depend on several variables relative to the semiconductor’s construction, such as line spacing, composition, density, and packaging. All these variables lead to an ESD component classification sensitivity as specified in S5.1-1993.

For simplicity’s sake, let’s use Class 0 which has a 0 to 249-V tolerance. Applying the HBM, a conservative worst-case capacitance would be 200 pF, twice that specified, and a resistance of 10 kW . The maximum power (P) level based on Ohm’s Law is
and the worst-case HBM is
= 6.2 watts or joules per second (J/s).

The maximum energy (E) stored in a worst-case HBM capacitance (C) of 200 pF and at a maximum voltage (V) of 249 V, 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 of time, in a natural response system, for the discharge current to decrease to a negligible value (assume 1% of the original signal).

Discharging the energy stored on C upon touching a conductor at zero volts yields a current, using
, of
or 24.8 mA. To avoid damaging a Class-0 ESDS device, the discharge current must be below 24.8 mA. Using a 2× 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.

The rate at which 6.2 m J of energy discharges is very important. Too fast a discharge will lead to an ESD event, which can be measured using a simple loop antenna attached to the input of a high-speed DSO.

Table 3 depicts the discharge current for 6.2 m J at varying discharge times. We are assuming lossless conditions during the discharge. To keep the discharge current below 12.4 mA for our example, the discharge rate (from Table 3) must be no quicker than 2.01 m s. This energy-based time constraint forms the lower boundary of the controlled discharge rate.

Mat Materials

The upper and lower boundaries of our controlled discharge rate now are defined and can be used to help choose the correct ESD mat for a workstation. ESD mat materials come in many variations.

Mats are made from vinyl or rubber material and can be homogeneous or multilayered. In general, rubber mats have good chemical and heat resistance, but vinyl tends to be more cost-effective.

The electrical properties of a mat are important to know in controlling ESD. An ESD mat either will be 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 resistance of the materials, which affects the speed of the discharge. By definition, a conductive material has a surface resistivity of less than 1×105 W /sq, and a dissipative material is greater than 1×105 W /sq but less than 1×1012 W /sq per ESD ADV1.0-1994. Anything with a surface resistivity greater than 1×1012 W /sq is considered insulative and essentially will 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 2 and assuming that the mat has a negligible capacitance relative to the HBM, the mat resistance must be greater than 2.2×103 W or 2.2×104 W /sq and less than 8.3×107 W or 8.3×108 W /sq. In other words, a conductive mat for some applications may be too quick to discharge and yield more dangerous ESD events whether properly grounded or not.

Figure 1 shows the natural response of a 249-V discharge in an RC circuit using a capacitance of 200 pF (HBM) into mat resistances of 104, 105, and 106 W . The natural response of the 104W curve is below 1% of its initial voltage in less than 10 m s. However, the 106W curve takes about 1 ms to discharge to less than 1% of its initial value.

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 fits snugly, making proper contact with the skin to reduce contact resistance.

The wrist strap, since it is connected to ground, quickly discharges any charge the body generates through tribocharging or becomes exposed to through induction. Any time the body directly touches a charged conductor, a discharge occurs because the body is at a different potential (0 V).

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 initiates a discharge at the rate of the time constant of the skin.

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 the skin to the device may be necessary. Some solutions are static-dissipative gloves or finger cots which, if worn properly, may add from 1 MW to 10 MW to the RC circuit of the skin. In turn, this slows down the discharge rate to well over 2 m s.

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 the upper boundary and the maximum energy as defined by the ESDS component classification defines the lower boundary. We want an ESDS workbench to control the discharge rate of our grounded or conductive materials within these limits.

For the HBM and a Class-0 device, the materials chosen for an ESD-safe workbench should have electrical properties to support discharge rates between 2.01 m s and 76.5 ms. These discharge rates, using worst-case assumptions, equate to an ESD mat surface with an RTG between 2.2×103 W and 8.3×107 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 for this example 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.

About the Author

Ryne C. Allen is the technical manager at ESD Systems. Previously, he was chief engineer and lab manager at the Plasma Science and Microelectronics Research Lab at Northeastern University. Mr. Allen is a NARTE-certified ESD control engineer, and the author of 23 published papers. He graduated from Northeastern University with B.S.E.E., M.S.E.E. and M.B.A degrees. ESD Systems, 261 Cedar Hill St., Marlboro, MA 01752, (508) 485-7390, www.esdsystems.com.

Table 1

Reaching

Grabbing

Lifting

Relocation

Landing

Time (ms)

455

153

231

924

247

Std. Dev. (ms)

48

11

61

137

73

Table 2

R

102 W

2.2×103 W

106 W

107 W

8.3×107 W

108 W

109 W

1011 W

Time

92 ns

2 m s

920 m s

9.2 ms

76.5 ms

92 ms

920 ms

92 s

Table 3

Current

24,900 A

24.9 A

12.4 mA

2.49 mA

249 m A

24.9 m A

2.49 m A

249 nA

Time (s)

1×10-12

1×10-9

2.01×10-6

1×10-5

1×10-4

1×10-3

1×10-2

1×10-1

Copyright 1998 Nelson Publishing Inc.

January 1998

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