One of the most commonly overlooked quality precautions in a production assembly area is electrostatic discharge. Excessive static discharge can cause permanent damage to sensitive electronic components which can appear immediately or at a later date causing expensive and potentially life threatening defects. Examples of these defects range from a computer printer failing causing a loss of productivity in the office to a pacemaker failing shortly after it was implanted in a patient’s chest. Many times when a device fails, a component is blamed, and it is assumed that the failure was caused in the component’s manufacturing process, when it may actually be due to an electrostatic discharge in the assembly or handling stages of the board.
What is Electrostatic Discharge (ESD)?
By definition, static electricity is an electrical charge caused by an imbalance of electrons on the surface of a material. This happens with the separation of two similar or dissimilar materials. When the materials are separated, electrons are drawn off of one material to the other, leaving loose electrons on the other material. The triboelectric charge is the transfer of electrons from one material to another where one material takes on a positive charge and the other will have a negative charge. The amount of static electricity (or loose electrons) produced are determined by the material, area of contact, speed of separation, and the relative humidity. Static electricity is scientifically measured in coulombs but in our industry it is measured in volts. It can typically produce very high voltages with little current.
There are many common ways to produce static electricity in our workplace. Walking across carpet can produce up to 35,000 volts while walking across a tile floor can produce up to 12,000 volts. Simply picking up a plastic zip-lock bag from a work bench can produce 20,000 volts. Sitting in a chair with urethane foam can produce 18,000 volts. 1,000 volts or less is enough to damage many sensitive circuits common in the electronic industry.
Before controlling ESD in the workplace, it is important to understand how different types of materials effect buildup and discharge of static electrons. Simply put, there are three different types of materials to consider in ESD protection.
The first material type is an insulator. An insulator allows electrons to flow easily along the surface of the material but not throughout the material. As a result, a large static charge can build up on the outside with a potential quick discharge that may damage sensitive electronics. The use of insulation materials around electronics is the most common cause of ESD damage. Examples of insulators include non ESD safe rubber bench mats, paper, cardboard, clothing, and wood.
Conductors are another type of material used in ESD protection. A conductor has low electrical resistance allowing static electrons to quickly and easily discharge. Electrons will also easily distribute throughout the conductive material avoiding a buildup on the outside. Conductors are a better material to use around static sensitive electronics than insulators but may still allow a quick discharge. Some examples of conductive materials are copper, steel, carbon, and aluminum.
The third and final material to consider when implementing ESD protection is static dissipative materials. A static dissipative material will allow the transfer of electrons to ground but at a much slower rate than a conductor and a faster rate than an insulator. This avoids a static buildup leading to an electrostatic discharge. Static dissipative materials include pink and static dissipative plastic bags, pink plastic static dissipative bubble wrap, and rubber static dissipative bench mats.
ESD Damage – How devices fail
An electronic device can have three different causes for an electrostatic discharge. Each is unique to the situation and environment with which the device is subjected to.
One common cause is a discharge to a device or circuit. This can happen when any charged object, including the human body, comes into contact with a static sensitive device allowing the object to suddenly discharge. This is the most common electrostatic discharge in an assembly production area. For example: a person has been walking across a vinyl tile floor, sits at a workbench on a polyester cushioned chair and touches a static sensitive circuit board. Other common tools on the workbench that may buildup static and discharge on a circuit are wooden cotton swabs, plastic handled tools, or paper coverings of a work bench.
The second cause of a static discharge is discharge from a device. This can occur when static sensitive components are passing through a conveyer system and builds up a static charge from that system. When the components are near a ground they may discharge potentially damaging the parts.
The final way that a component can be damaged is though a field induced discharge. This happens when a static sensitive device passes through an electrostatic field caused by a motor or other machinery. When the device passes through the electrostatic field it may buildup an electrostatic charge potentially large enough to damage a sensitive device. Once the sensitive component is damaged, it can be categorized into two different types of defects. The most obvious defect is a catastrophic failure. The electrostatic discharge may have caused a metal melt, junction breakdown, or oxide failure. In this case, the component is damaged and ceases to function. Most of the time it is detected when the unit is tested prior to shipment causing costly rework but no failure in the field.
The second type of defect is a latent defect. Here the unit is partially degraded but still functions; however, the unit will likely experience a premature failure at a later time in the field after it has been put into service. Latent defects are much more difficult and costly to detect and troubleshoot than a catastrophic defect.
The amount of damage that can be inflicted on a component can be determined by the sensitivity of it. Generally, static sensitive components are placed into three classes. Class 1 sensitivity means that between 0-1000 volts is enough to damage the component. These may include MOS devices commonly found in timing circuits. The other classes are class 2 which can be damaged from 1000-4000 volts and class 3 which require 4000 to 16,000 volts to damage them. If the sensitivity classification is unknown, then it is safe to assume that the components are class 1.
ESD Protective Measures
Once the causes and effects of electrostatic discharge are understood the next question is which protective measures are necessary to have an ESD safe working environment. The most important measure is to have an established and written ESD policy understood and enforced throughout the organization. The following are six examples of how to protect the components from static discharge. Many more exist and each organization should tailor their policy to meet individual needs.
Electrostatic sensitive parts should only be stored in ESD approved bags and containers. ESD safe containers and materials will have a coating which prevents a static charge buildup. Pink is the established color for ESD approved materials, but pink is not the only indication. Some materials may not be pink but will have the ESD symbol on it indicating that it is ESD safe.
Always wear ESD safe gloves or finger cots. As with the ESD approved bags and containers, the approved gloves and finger cots will prevent a static buildup potentially damaging sensitive components. Latex or other rubber gloves and finger cots will buildup a static charge that may damage sensitive electronics.
Always work with sensitive components on an ESD safe rubber mat. The mat must be earth grounded with the ground going at least 8 feet into the earth. This mat will draw off any static making a safe working environment for the electronics.
While working with sensitive components always wear a wrist strap with an earth ground. The strap must be in contact with the operator’s skin and like the mat will draw static off of the operator’s body. Other similar protective measures exist such as heel straps and floor mats. Daily testing of the wrist strap using a commercially available testing device will help avoid a costly ESD defect.
Always use an approved soldering iron when soldering static sensitive devices. It is important that the iron has a grounded plug (minimum tip to ground resistance is less than 5 ohms and transient voltages are not to exceed 2 millivolts). The use of soldering guns or irons with a transformer incorporated into the hand piece should not be used since it can form an electrostatic field, building a static charge on the electronic component.
Heat guns and solder pots are commonly used in production areas. These devices must be grounded to prevent a static field which can cause an electrostatic buildup leading to a failure with a sensitive component.
Understanding ESD, implementing a policy, and using safety measures tailored to your organization will ensure that safe and reliable electronics come out of your production facility.
J-STD-001E Soldering Electrical and Electronic Components.
ESD-S-20.20 ESD requirements per J-STD-001E
MIL-STD-1686 Military standards for ESD implementation
MIL-HDBK-263B Guide to implementing an ESD program
An Introduction to ESD – ESD Association, Rome, New York