How to Prevent Electrostatic Discharge (ESD)
To effectively combat and prevent ESD, the right equipment must be used correctly. Thanks to a range of powerful closed-loop ESD prevention, monitoring, and ionization devices, ESD can now be considered a process control issue.
Electrostatic discharge (ESD) is a familiar yet underestimated source of damage to circuit boards and components in electronic assembly. It affects every manufacturer, regardless of size. While many believe they are producing in an ESD-safe environment, the reality is that ESD-related damage continues to cost the world's electronics manufacturing industry billions of dollars annually.
What exactly is ESD? Electrostatic discharge (ESD) is defined as the discharge (electron flow) of a previously existing (fixed) static charge (insufficient or excessive electrons). Charge is stable under two conditions:
When it becomes "stuck" in a conductive but electrically insulating object, such as a metal screwdriver with a plastic handle.
When it resides on an insulating surface (such as plastic) and cannot flow across it.
However, if a conductor with a sufficiently high electrical charge (such as a screwdriver) approaches an integrated circuit (IC) with an opposite potential, the charge "bridging" occurs, causing electrostatic discharge (ESD).
ESD occurs very rapidly with extremely high intensity, typically generating enough heat to melt the internal circuitry of a semiconductor chip. Under an electron microscope, this appears as a small, blown-out hole, causing immediate and irreversible damage.
Even more serious is that only in one tenth of cases does this damage cause the entire component to fail during final testing. In the other 90%, ESD damage only causes partial degradation-meaning the damaged component passes final testing undetectably, only exhibiting premature field failure after shipment to the customer. This is the most reputational and costly area for a manufacturer to correct any manufacturing defects.
However, the main difficulty in controlling ESD is that it is invisible yet capable of damaging electronic components. A discharge that produces an audible "click" requires a relatively large charge of approximately 2000 volts, while 3000 volts can produce a small electric shock, and 5000 volts can produce a visible spark.
For example, common components such as complementary metal-oxide-semiconductors (CMOS) or electrically programmable read-only memory (EPROM) can be damaged by ESD potential differences of only 250 volts and 100 volts, respectively, while increasingly sensitive modern components, including Pentium processors, can be destroyed by as little as 5 volts.
This problem is compounded by the damaging activities that occur daily. For example, walking on a vinyl factory floor generates friction between the floor surface and shoes. The result is that a purely charged object can accumulate a charge of 3 to 2000 volts, depending on the relative humidity of the local air.
Even the friction generated by a worker's natural movement on a workbench can produce 400 to 6000 volts. If workers handle insulators while unpacking or repacking PCBs in foam boxes or bubble wrap, the net charge that can accumulate on a worker's body surface can reach approximately 26,000 volts.
Therefore, as a major source of ESD hazard, all personnel entering an Electrostatic Protected Area (EPA) must be grounded to prevent any charge buildup, and all surfaces should be grounded to maintain everything at the same potential, preventing ESD from occurring.
The primary products used to prevent ESD are wristbands, made of plush corduroy and dissipative surfaces or padding-both must be properly grounded. Additional aids such as dissipative footwear or heel straps and appropriate clothing are designed to prevent the accumulation and retention of net charge while personnel move within the EPA.
During and after assembly, PCBs should also be protected against ESD from internal and external transport. Many PCB packaging products are available for this purpose, including shielding bags, shipping boxes, and mobile trolleys. While the correct use of the above equipment will prevent 90% of ESD-related problems, another layer of protection is needed to reach the final 10%: ionization.
The most effective way to neutralize assembly equipment and surfaces that can generate electrostatic charges is to use an ionizer-a device that blows an ionized airflow through the work area to neutralize any charge accumulated on the insulation.
A common misconception is that because hand straps are worn at workstations, the charge on the insulation in that area, such as polystyrene cups or cardboard boxes, will safely dissipate. By definition, insulators do not conduct electricity and cannot be discharged except through ionization.
If a charged insulator remains in the EPA, it will radiate an electrostatic field, generating a net charge on any nearby objects, thus increasing the risk of ESD damage to the product. While many manufacturers attempt to ban insulation from their EPAs, this approach is difficult to implement. Insulation is so prevalent in everyday life-from the foam mats where operators sit comfortably to things in plastic covers.
Due to the use of ionizers, manufacturers can accept the presence of some insulation in their EPAs. Because ion generation systems continuously neutralize any charge buildup that may occur on insulators, they are a worthwhile investment for any ESD program.
There are two basic types of ion generators in standard electronic assemblies:
Desktop (single fan)
Overhead unit (a series of fans within a single overhead unit)
Indoor ion generators are also available, but are now primarily used in cleanroom environments.
The choice depends on the size of the area to be protected. A desktop ion generator will cover a single, uniform work surface, while an overhead ion generator will cover two or three. Another advantage is that ion generators also prevent static dust from adhering to products, which can degrade their appearance.
However, no protection program is complete without proper testing and monitoring of the effectiveness of ESD equipment. Leading ESD control and ionization experts have reported examples of manufacturers using ineffective (and therefore useless) ESD equipment without realizing its failure.
To prevent this, in addition to standard ESD equipment, ESD suppliers offer a variety of constant monitoring devices that automatically alarm if a performance exceeds specified limits. Monitors can be used as standalone units or connected in a network. Network software for automatic data acquisition is also available, providing real-time displays of system performance for operators and workstations.
Monitors can simplify ESD planning by eliminating many routine tasks, such as ensuring proper daily tape measurements, ion generator balancing and proper maintenance, and undamaged workbench grounding points.
Conclusion
The first step in preventing ESD is correctly assessing how seemingly minor details can cause irreparable damage if overlooked. An effective plan requires not only the use of effective ESD protection equipment but also rigorous operating procedures to ensure ESD safety for all personnel on the plant floor.
While many manufacturers use automatic tape testers, it is common to see operators either pass or fail the test due to a loose tape. Many operators attempt to pass the test by simply gripping the tester close to their wrist with their other hand.
Nevertheless, the good news is that ESD is preventable. The time and money invested in the right equipment and improved safety procedures will pay off with increased pass rates.