When components are first manufactured, they are typically well protected from electrostatic discharge (ESD) by the manufacturer, who are well aware of the potential for damage. During shipping and storage, these components are usually in high quality ESD containers to maintain protection. After assembly, the components are mounted into circuits which can provide some protection either from transient filtering provided, or due to the distribution of the energy across the circuit. Even then, proper handling and storage is important.
However, between storage and final assembly, the individual components can become exposed and unprotected to a variety of ESD sources. If the part is improperly handled or comes in contact with any conductive device or material, damage can occur. To avoid this, dissipative materials are used in the handling and storage of ESD sensitive components. Dissipative materials are those which have some conductivity, say around 1 M? between two points on the material, which can provide a path to a reference discharge point (ground) through a gentle path. These materials are different than conductive materials, which have much lower resistance and will conduct higher current and allow rapid discharge.
Common Causes of ESD
First, components are damaged by ESD when they touch or come near a component or conductive material which is at a different voltage with a capacity to transfer a significant charge. A rapid transfer of charge can cause the small current paths internal to the device to overheat and fuse causing the device to fail. If the path is partially fused, the current path for normal operation becomes restricted, and a future failure may happen, which is called a latent failure. Also, direct discharges, or stresses due to voltage field effects, can cause similar issues.
Let us look at a scenario: A tube of IC’s is stored in an ESD shielding bag, where they will remain protected. The bag is placed on a cart and wheeled to an assembly location. If the cart is conductive, and has rubber or synthetic wheels, a charge can build up on the cart, placing a voltage on the bag, the tube, and the person pushing the cart. Let us assume the cart and employee build up a charge of 10 kV. If that employee decides to avoid physically touching the devices because of this charge, they may open the package and dump the components onto the ESD bench. However, the way this packaging materials work is to provide a conductive path to bring all components slowly and safely to the same voltage. By dumping the components onto a grounded bench, the devices can experience a rapid discharge from 10 kV down to ground, and potentially be damaged. The better solution is to place the packaging onto the bench, which will then gently discharge the components so they can be handled.
Automation and ESD
Assembly of equipment is increasingly by automation. Automated equipment requires gears, wheels, motors, belts, and other devices which can be made of dissimilar materials. Tribocharging, or the Triboelectric Effect, is the act of charge build up from dissimilar materials. Friction or rubbing of these materials is not required, simply bringing them into contact can transfer charge. When performed repeatedly, as with a gear or two wheels in contact, a flow of charge may be transferred to induce a significant voltage. This becomes a significant issue for automated equipment where charge build up is an enemy of sensitive components.
Thus, automated equipment is carefully manufactured with parts that are static dissipative and/or minimize dissimilar materials. Metal in the equipment is referenced to safety ground. Dissipative materials are also referenced to grounded components. However, when the dissipative materials or the surface of the metal contacts become contaminated or are coated with non-conductive material, these discharge paths break down, and charge may begin to build up. For this reason, maintenance and cleaning of automated equipment, and testing of discharge paths with ESD test equipment, is important to perform on a regular basis.
Protecting Against ESD
Of course, assembly benches are primary areas for ESD protection and testing and must be subjected to cleaning and maintenance. The ESD bench or mat must maintain contact with a reference ground continuously, as well as assemblers who use wrist straps and ESD smocks. Hand tools and soldering irons must have the ability to discharge static buildup, either through the worker, or through electrical contact with safety ground. The containers used to handle and hold tools, materials, components, and all other materials required for work, must be either made of dissipative materials and reference to ground, or held in protective areas away from sensitive components. These may include drawing and assembly instructions, the computer, monitors, keyboard, computer mouse, as well as test equipment, digital meters, scopes and the like. Conductive floors are common. Air ionizers, which supply both positive and negative ions to the area, can be used to maintain a literal atmosphere of neutral charge.
A wide range of materials must be used around sensitive components. All of them must be carefully chosen, maintained, and tested to avoid ESD damage to your electronics.