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Electrostatic discharge, or ESD, is a single-event, rapid transfer of electrostatic charge between two objects, usually resulting when two objects at different potentials come into direct contact with each other. ESD can also occur when a high electrostatic field develops between two objects in close proximity. ESD is one of the major causes of device failures in the semiconductor industry.

Electrostatic charge build-up occurs as a result of electrons on the surface of a material. Such a charge build-up develops an electric field that has measurable effects on other objects at a distance. The process of electron transfer as a result of two objects coming into contact with each other and then separating is known as "triboelectric charging"

This charging process results in one object gaining electrons on its surface, and therefore becoming negatively charged, and another object losing electrons from its surface. A person can get triboelectrically charged in a number of ways, even by just walking across a room. The tendencies of various materials to charge up either positively or negatively are shown in a  Triboelectric Series.

There are three predominant ESD models for IC's: 1) the Human Body Model(HBM);2) the Charged Device Model (CDM); and 3) the Machine Model(MM). The HBM simulates the ESD even when a person charged either to positive or negative potential touches an IC that is another potential. The CDM simulates the ESD event wherein a device charges to a certain potential, and then gets into contact with a conductive surface at a different potential. The MM simulates the ESD event that occurs when a part of
an equipment or tool comes into contact with a device at a different potential. HBM and CDM are considered to be more "real world" models than the MM.

ESD-related failures manifest in a number of ways, exhibiting one or more of these attributes: junction leakage, short, or burn-out; dielectric rupture; resistor-metal interface rupture; resistor/metal fusing; and die surface charging.

 

ESD Controls

ESD controls come in a vast variety of forms. However, they may be classified into three major categories: 1) prevention of static charge build-up 2) safe dissipation of any charge build-up; 3) improvements in the ESD robustness of the product.

The first category works on the basic premise of "No Charge/No Discharge". Elimination of charge buildup would include the use of materials that have less tendency to generate static charges in the work area, i.e., antistatic and static dissipative materials. All equipment must be free of moving parts that may generate charges, e.g., rubber rollers, plastic stoppers, etc. Things that the devices may come in contact with or get transported on must also be antistatic or conductive. The use of ionizers to neutralize newly generated charges will also prevent charge build-up. The minimization of movements in the work area, as well as the use of ESD-safe apparel, will help in minimizing static charges generated by personnel.

The second category is very closely related to, and may sometimes even be indistinguishable from the first category, safe dissipation of accumulated charge involves the provision of a suitable electrical path that will allow charges to flow to ground. No ESD control program will be successful without a sound policy on grounding. As much as possible, there should only be one common ground for the entire factory.

Everything in the production line, from equipment to work tables to cabinets and racks, must be connected to this common ground. If the factory uses conductive flooring, then this should also be connected at regular intervals to this common ground. Having a single or common ground will ensure that everything in the production floor will remain at the same potential. Any charge build-up will immediately be dissipated by a good grounding system. The use of properly grounded wrist and foot straps or conductive shoes will also fall under this category, since these will bring any charge build-up on personnel to the common ground.

Control of RH is also important, since the moisture in the air acts as a conductive path that can bring static charges to the common ground. Thus, a very dry environment is inviting ESD. Care must be exercised though because excessive RH might trigger corrosion.

The third category does not actually control the ESD phenomenon per, but pertains to making devices more resistant to ESD damage. This involves incorporating ESD protection cells in the design of the IC, and the use of physically robust features that can withstand the high current brought about by an ESD event.

Proper training of personnel on ESD precautions is also a must. A good ESD control program therefore incorporates a training scheme that will ensure that everyone is aware of the company's ESD controls and SOP's. A regular audit of the manufacturing line for ESD control compliance is important.


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