Abstract

In the past and currently, people have always been the main sources of electrostatic charges. All movements generate electrostatic body voltages. Employees of an electronic manufacturer must be aware of this. Normally, sufficient ESD control steps are already realized to discharge possible existing electrostatic charges and to avoid generation. People may be controlled. Machines, and the automatic handling process as well, become more and more important as another risk source of electrostatic charges. Caused by the permanent movement of electronic components, charges are divided (charge separation) and electrostatic charges are generated. The movement processes are realized quickly. PWBs are also significant charge sources. Their large surfaces and capacity may store large charges. New failure models (CBM, FICBM) are just the beginning.

Introduction

All electronic components and assemblies are at risk for electrostatic discharges. Producers, suppliers, distributors and users have to realize the ESD control system during the entire manufacturing process, measurements and application. All active electronic components, beginning with simple diodes, transistors or complex inner circuits, require an external ESD control system. In the next step, SMD resistors and condensers, and prospectively NEMS and MEMS, will be included in this danger category. Initial tests show that these passive components can be damaged through electrostatic discharges.

The structures of electronic components are becoming smaller. Already 5 volts of an electrostatic charge are enough to change the structures in small electronic components. The structures will achieve such small dimensions, so electrostatic charges can cause permanent damages. In the year 2020, the sizes of the electronic components will be less than 15 nm. Electrostatic charges of 0.04 nC and electrostatic fields of 15 V/cm will be enough to damage electrostatic sensitive devices (ESDS) permanently.

Basics

In the last few years, many directions and worldwide standards for the static control handling of ESDS have been created. The basic principles of all steps are the safe (and slow) electrostatic discharge, as well as the avoidance of electrostatic charge developments. These basic principals are necessary. They are included in all directions and standards. The international standards IEC 61340-5-1 and IEC 61340-5-2 contain these basic requirements for the protection of electronic devices and components against electrostatic discharges. Additionally, users can find an entire section with directions for an ESD control system in the American Standard ANSI/ESD S20.20.

Both standards are confined to the main reason--"the human being." The requirements to machines are not considered, although such special requirements must be created in the future. A start is the standard ANSI/ESD SP10.1-2007, "Automated Handling Equipment (AHE)." This standard only describes the requirements to the grounding of machines and the handling of materials, which may not be charged, as well. Another document is already prepared in the International Electrostatic Committee (IEC). This will describe special requirements.

Different failure models are used for the analyses of humans and machines. The Human Body Model (HBM) is always used for the electrostatic charge of a person. Otherwise the Charged Device Model (CDM) is applied for the charge analysis of machines or production lines. Nevertheless, both will not be sufficient in the future. New failure models like the Charged Board Model (CBM) or the Field Induce Charged Board Model (FICBM) become necessary. The CDM only considers a single electronic component; however the CBM is applied to analyze the entire PWB.

Failure Models

Reflecting the following considerations, single failure models are caused when:

• A person touches an electronic component and the stored electrostatic charges are transported from the person to the electronic component. These charges are grounded by the connection between the electronic component and the earth potential.
  
• An electronic component or an electronic device acts as capacitor plate and stores electrostatic charges. While contacting the earth potential, damages are caused by a discharge pulse.
  
• A charged object is in an electric field. A potential is generate over the gateoxid or the pn-junction of an electronic component. Electrostatic charges are generated and discharges cause damages (breakdown).

Already known failure models:

• Human Body Model (HBM);
• Machine Model (MM);
• Charged Device Model (CDM); and
• Field Induced Model (FIM).

The first failure model only considers the charged person. The second is a specialization of the HBM. The third failure model assumes that the electronic component charges itself electrostatically and discharges itself suddenly by contacting metal. However, a person does not influence directly the charge and discharge process any more at this third model.

Reflecting the fact that quick discharges must be considered more and more in the future, all previous statements about electrostatic charges will no longer be sufficient. Very fast discharges in very short terms already exist. Those are really energy loaded and damage ESDS, of course.

Nowadays, it is not enough to consider every single electronic component. One has to analyze the complete electronic assembly either. However, a suitable failure model is still missing. Two models are in preparation: the CBM and the FICBM. Both assume that the board (PWB) is electrostatic charged. The board has a higher capacity, so that it may store much more electrostatic charges. Those can also be grounded by a single electronic component. The high energy causes an early damage of the entire electronic component.

Human Body Model (HBM)

A person is able to generate a considerable charge by a simple movement. While just walking on a synthetic carpet, voltages of at least 5 kV are generated. If this person handles an electronic component or an electronic device afterwards, a great part of the stored charges are transported to the object. The electronic component either stores such charges or grounds it directly to the earth potential. A discharge impulse has enough energy to cause changes of the component parameters. Figure 1 shows a person who is already electrostaticly charged and just discharges himself at a machine. The resistances between the person and the floor, the person and the discharge path (grounding of the machine), the person's capacity in comparison to the floor (object capacity) and the actual discharge current must be considered. The circuit diagram of a person discharge and the HBM are shown in Figure 2.



Figure 1: Person touches a grounded machine.



Figure 2: Circuit diagram.

C_K = capacity of the person to floor grounding.
R_i = resistance of the person.
i = current to grounding point.
R = resistance of the machine to grounding.

Additionally to the discharge current the device/machine resistance and a parasitic pn-junction must be considered for the calculation of the electrostatic charge of a person. The parasitic pn-junction is explained later. The body resistance R_B ranges from 1,000 Ω to 2,000 Ω and such of the body capacity C_B from 100 pF to 250 pF. In case a person is charged up to 2,000 V, the stored energy of the body W_el is calculated as follows--equation (1) to 0.2 mJ (C_B = 100 pF, R_B = 1,500 Ω):

At an instrument resistance R of 10 Ω and an impulse period of a couple 1/10 μs outputs of several kWs may be calculated. Such short discharge periods and its depending high outputs cause melting of the silicon areas. One can describe such a melting as an explosion on the silicon surface, which can be seen as a crater through an electron microscope (see Figure 3). The crater's diameter is between 6 ... 8 μm. In comparison, the melting of silicon (crater with a diameter of 1μm) needs at least an energy amount of approximately 1μJ.



Figure 3: Crater in a silicon surface caused by electrostatic discharge.

Charged Device Model (CDM)

The electronic component acts as a condenser. It gathers charges, such caused while sliding through a magazine or while contacting another charged object. Additionally, electrostatic charges are generated by removing the electronic component from a conductive tray or a belt. Electrostatic charges are generally caused by taking an electronic component out of conductive material, because it is not equipped with a conductive chassis. Thus, an electronic component is always electrostatic charged after every mechanical process, independent of its actual handling like the movement in a pick-and­place-machine or another production line.

However, just a discharge damages the electronic component. The discharge can be realized directly or indirectly via further processes. It's just enough to bring the electronic component in the near of a dischargeable point or object. So, an electrical or electrostatic field may already provoke such a discharge. Damages of pn-­junction, dielectric and other components are caused by a discharge impulse and its discharge current, depending on the grounding via the chassis or the chip.



Figure 4: Typical CDM discharge.

An electronic component can store energies up to 100 μJ. However, at a very low contact resistance (< mΩ) and a conduction inductivity o f10 nH, such energy, depending on the charge amount, can realize a direct or an indirect connection to the earth potential. An output per impulse of several 100 W to 1,000 W is reached by an increase of the discharge current impulse of several ns. Such outputs are enough to change the component parameters considerably or to destroy the electronic component finally (see Wunsch und Bell [6]).

Charged Board Model (CBM)

The previous models HBM and CDM are not enough to describe ESD failures. There will be breakdowns caused by humans, but most ESDS are moved in automatic handling equipment. There is no direct influence of the human. The electronic components and assemblies charge themselves electrostatic. The capacity conditions of a PWB are absolutely different in comparison to humans.

One solution is the CBM. Here, the capacity proportions are very difficult. The value of the capacity is higher as the body of the person. The result is a bigger electrostatic charge on the board. We have a bigger energy at the discharge. This leads to the influence of ESDS and it damages the ESDS.



Figure 5: Typical CBM discharge.

Field Induce Charged Board Model (FICBM)

A different model is the FICBM. The influence of the electrical field on PCBs has been neglected or has been thought that it is not important, until now. An electrical field is able to produce electrostatic charges on a PCB, as demonstrated. These electrostatic charges will be stored by the larger capacity of the PCB or by the electronic components on it. The discharge process is not predictable. It can always happen when the PCB is grounded or contacted. That's why the description of FICBM is not easy, but the effect is very important. Electrical fields can appear everywhere, where machines, motorized plants stand or where circuit processes are produced.

Requirements

The basic conditions in an EPA are the following two basic principles:

Basic principal 1--Only a complete ESD control program will guarantee an adequate protection for electrostatic sensitive devices (ESDS).

Basic principal 2--The grounding of a person with a wristband is the best possibility for the discharge and the charge avoidance.

Steps for Personnel Grounding

The following grounding steps apply to all employees:

An employee is permanently grounded by a wrist band. While sitting at work, a wristband must be used generally. Another method for the realization of a potential balance does not exist. In the case of standing at work, an employee may be grounded alternatively by the system person-shoes-floor. Furthermore, an employee must always wear ESD clothing. The adequate requirements and limits are described in Table 1.

Table 1: Requirements to personnel grounding.

Table 1 shows the requirements to the product and its measurement methods (standardized methods), as well as the requirements to periodical inspections.

The new standards IEC 61340-5-1 (2007) and ANSI/ESD S20.20- 2007 differ between groundable clothing and "normal" ESD clothing. Such groundable clothing is usually made of a textile mixture and conductive fiber (mostly carbon). The other kind of clothing contains a so called core fiber, which is not conductive outside. However, both materials must cover electrostatic fields and must avoid electrostatic charges either

The requirements to the employees are also valid for visitors, workmen, etc.

If those foresaid personnel grounding steps and ESD control steps are kept in an EPA, the first risk sources of electrostatic charges are almost controllable. However, quite bigger problems are the electronic components or PWB. Both, electronic components with an isolating plastic chassis, as well as PWBs, can be electrostaticly charged. An isolating plastic material (e.g. F4) is used for those. Such ceramic materials, often used in the automobile industry for higher temperatures, are new risk sources of electrostatic charges.

Steps for Machine Grounding

Firstly, the grounding steps are explained. Thus, it is necessary to connect all ESD control steps to the same potential. This is just the only way avoiding electrostatic charges. Such a typical grounding system is shown in Figure 6: All ESD equipment, including floors, workstation equipments, the employee and the machines as well, must be connected to the same potential. Figure 6 shows a grounding system, where an earth or a safety ground exists.



Figure 6: Grounding system with equipotential lines--one wrist strap; two working surface; three potential lines; four floor mat; five floors.

The first and only requirements are the demand for a grounding of all metal parts, as well as the demand for the avoidance of plastic usage, which could generate electrostatic charges and fields. Experiences show that it is not enough for the protection of ESDS in automated machines and systems. ESDS won't be damaged by the operator, but through the machines. The transport operation of an ESDS in a machine can happen as follows:

1. Removal of the ESDS out of packaging. This is the first partition act. The ESDS has an isolating case, so it will be electrostatic charged during the removal out of the reel or the tray.
   
2. The electrostatic charged ESDS will be transported to the PWB where a further electrostatic charge can happen. The movement at high-speed pick-and-place system should be enough of the generation of electrostatic charges.
   
3. Through the placing on the PWB, different potential between the ESDS and the PWB exist. The potential difference leads to a discharge, which will damage the ESDS.

These examples show that electrostatic charges always develop when ESDS are parted or moved. Electrostatic charges will always be generated because the components, as well as the PWBs, are made of an isolating material. Other acts and production steps show that this is not the only possibility for the generation of electrostatic charges in a production process. Further critical steps are, for example, the printing of PWBs, the labeling of PWBs and assemblies, as well as test constructions.

Manual handling of individual components is no longer common. PWB assemblies are handled mainly by equipment and the final phases of mechanical assembly are done by both humans and robots. In consequence of this, HBM is no longer a valid ESD simulation model. The main electrostatic risk during automated manufacturing is with CDM-type of electrostatic discharges. The additional model, but not standardized yet, is CBM.

In the CBM-type of ESD, the assembled PWB or some of the mechanics parts can be charged during handling and the discharge to ground or between the objects can happen. A CBM-type of discharge is typically more severe than other models for components due to high capacitance and high stored charge of PWB assemblies or mechanics. There are some main ESD control principles which are important in ESD Protected Area (EPA), as well as in automated process equipment:

1. All conductive and dissipative items are grounded.
2. Materials or parts to be contacted with ESDS are made of electrostatic dissipative material.
3. Non-essential insulating materials are excluded.
4. Where insulating materials or parts are needed, the possible charges shall be minimized by special measures, like ionization, shielding or coating.

Table 2: Requirements for AHE (automated handling equipments) [4]. 

There are many materials which can be in contact with ESDS items. Components to be placed are stored in reels with plastic tapes covered and a nozzle picks the component from reel. Components are placed on the PWB and the PWB is contacted with conveyor belts and possible support pins, gripper, clamps, etc. All these materials should be made of electrostatic dissipative material at least in contact area and a resistance to ground value shall be between 10⁶ and 10⁹ Ω.

Components and PWB materials have plastic, insulating material and they can become charged by tribocharging, e.g. by rubbing against conveyor belt, touching other product parts or in routing process. The charged ESDS item can subject to a CDM or CBM risk. All rotating and sliding elements form an ESD risk. The tribocharging during automated manufacturing shall be minimized and metal contact to ESDS shall be prevented. Normally it is not enough, an ionizer shell be installed in the area of rotating material.

Ionizers are applied sometimes to remove electrostatic charges from machines. Electronic components and PWBs cannot be grounded. Thus, ionization is the only method minimizing electrostatic charges at the moment.

Ionization is just one opportunity decreasing electrostatic charges. Intelligent ionizers are able to detect electrostatic charges in machines and to generate equivalent charges for their decrease either. The limits are shown in Table 2.

The mentioned ESD control steps are in common with the today's knowledge.

Measurement Methods

Typically, resistance measurements are realized. At such measurements, the grounding resistance from a person to a metal plate or an existing floor is measured. The first test is an entrance test before entering an EPA. Another measurement is realized at the inspection of the EPA's system resistance. Additionally, all ESD equipments are tested through a resistance measurement.

Clothing is also important for the personnel equipment. The measurement of the clothing's surface resistance is the only method to confirm the clothing's ESD characteristics. Some special clothing permits a measurement to an earth grounding point. Such measurements are often not sufficient. An employee can be electrostatic charged. Independent of wearing ESD clothing, electrostatic charge can be stored by the employee's "normal clothing." A part of this electrostatic charge is grounded by the ESD clothing, but nevertheless, the person is still electrostatic charged.

The amount of the electrostatic charge can be measured typically with an electrostatic voltmeter or a measurement construction of a Charge Plate Monitor. Figures 7, 8 and 9 show the electrostatic charge of a person, who walks on a carpet (7), who sits on an ESD chair (8) and who is grounded by a wristband (9).



Figure 7: Electrostatic Voltage - ESD chair - person without ESD floor and ESD shoes.



Figure 8: Electrostatic Voltage - ESD chair - person - ESD floor and ESD shoes.

 

Figure 9: Electrostatic Voltage - ESD chair - person - ESD floor and ESD shoes - Wrist strap.

Attempts show that the person grounded by a wristband is the best opportunity.

The following measurements are recommended to perform in order to evaluate the capability of automated equipment:

1. Resistance to ground;
2. Point to point resistance;
3. Electrostatic potential;
4. Electrostatic field;
5. Accumulated charge; and
6. Electromagnetic Interference (EMI).

Electrostatic potential is measured from ESDS and PWB assemblies by a qualified meter. Electrostatic field is measured from insulating materials according to the meter manufacturer's instructions. The large conductive and grounded area can affect to the measurements of potential and field.

For evaluation of real ESD risk in automated processes, the handled ESD sensitive devices or products have to be analyzed while production is on-going. The methods are to measure the potential and charge of the devices. The charge can be measured by an individual charge meter or by measuring the discharge curve from the charged device. From the discharge curve the discharged current, energy and charge can be calculated. There are no exact acceptance levels; they must be analyzed according to ESD sensitivity of the device in case. Some requirements are shown in Table 1.

So-called contact voltmeters (CVM) offer another opportunity to detect electrostatic charges. These CVMs are electrostatic voltmeters with a high input impedance (> 1 ∗ 10¹⁴ Ω, better 10¹⁵ Ω) and a low input capacity. Thus, electrostatic charges can be measured directly on the ESDS of the PWB without any damage. Those CVMs are new, so just a few tests have been realized. Electrostatic charges of about 200 V were measured on ESDS in the SMT process. Further practical tests will be realized in the next time to detect possible electrostatic charges.

Summary

Electrostatic charges of people are almost controllable. Many sufficient ESD control steps exist today, which ensure the permanent discharging of employees in an EPA. Much bigger risk sources for electrostatic charges are automated handling equipments (AHE) or machines generally. An electronic component (ESDS) charges itself mostly, because it is equipped with an isolating chassis. However, PWBs are also made of isolating material, which charges itself very highly.

ESDS and PWBs mostly have a different potential, which always cause a potential balance, as well as a generation of electrostatic charge. The most existing ESD control steps are not as sufficient as they should be, because this process of a potential balance happens very quickly. It is also really difficult to record such processes with a measurement construction. The today's measurement methods and instruments are still slow. Furthermore, the potential differences are very low, but energy-loaded.

The next step will be a better adaption of the existing measurement instruments and methods to the requirements and, of course, the development of new measurement instruments.

References

1. IEC 61340-5-1 Electrostatics - 08.2007: Part 5: Specification for the protection of electronic devices from electrostatic phenomena, Section 1: General requirements.

2. IEC 61340-5-2 Electrostatics - 08.2007: Part 5: Specification for the protection of electronic devices from electrostatic phenomena, Section 2: User guide.

3. ANSI/ESD S20.20-2007 ESD Association standards for the Development of an Electrostatic Discharge Control Program for - Protection of Electrical and Electronic Parts, Assemblies and Equipments.

4. ESD SP10.1-2000 ESD Association standard practice for Protection of Electrostatic Discharge Susceptible Items - Automated Handling Equipment (AHE).

5. J. Paasi, P. Tamminen, H. Salmela, J-P. Leskinen, T. Viheriäkoski, "ESD Control in Automated Placement Process," Proc, EOS/ESD Symposium EOS-27 (2005).

6.Wunsch, D.; Bell, R. R.: Determination of threshold failure levels of semiconductor diodes and transistors due to pulse voltages. IEEE Transactions on Nuclear Science, NS-15, 1.

Chassis of the machines is normally made of conductive material. The conductive chassis should have a straight and reliable connection to ground and the distance of the insulating parts should be long enough not to create high electrostatic fields close to ESDS. Special attention should be paid for grounding of parts which are separated from the chassis or are movable, like adjustable conveyor.