Electrostatic discharge (ESD) is the rapid transfer of electrostatic charge between two objects. This normally happens when two objects at different potentials come into direct contact. This static charge buildup is majorly caused by tribocharging and electrostatic induction.
Tribocharging is the generation of static electricity due to separation of electric charges that occurs when two materials are brought into contact and then separated. For example, when you walk on a carpet, the friction between your shoes and carpet results in tribocharging, thus creating a difference of electrical potential that can lead to an ESD event.
Electrostatic induction occurs when an electrically charged object comes near a conductive object isolated from ground. The charged object produces an electrostatic field that causes electrical charges on the surface of the other object to redistribute as shown in Fig. 2. As such, the net charge on the object is zero but it has surfaces with excess positive and negative charge, so an ESD event can happen from these surfaces.
How does ESD affect electronics
Electronic products are becoming smaller and faster but also becoming more sensitive to ESD. ESD can damage sensitive electronic components, resulting in failures, reduced reliability and increased rework costs. The worst part is that most often the chips get damaged only partially and start malfunctioning later. The cost of a single failure can be astonishing—in defence, such failures can cost lives.
What standards do we have
ESD standards are highly necessary because to objectively evaluate and compare competitive ESD control products. These standards are followed by the manufacturers and users of ESD-sensitive devices and control products, and certification organisations that help them in developing, implementing, auditing and certifying ESD control programmes.
Due to high importance of ESD control in assuring reliability of electronic products, standards for ESD control have evolved pretty quickly. The standards that we have today can be separated in three groups.
The first group consists of those that provide ESD control programme guidance or requirements. These include documents such as ANSI ESD S20.20-2007 Standard for the Development of an ESD Control Program, ANSI/ESD S8.1-Symbols-ESD Awareness or ESD TR20.20-ESD Handbook. Such documents give a broad overview of the requirements of ESD control programme. For more specific understanding of each process you will have to go through the second group of standard documents.
The second group covers requirements for specific products or procedures such as packaging and grounding. For example, ANSI/ESD S6.1 for grounding and ANSI/ESD S541 for packaging materials for ESD sensitive items.
The third group of documents covers the standardised test methods used to evaluate ESD products and materials. These include standards such as ANSI/ESDA-JEDEC JS-001-2010 for device testing, Human Body Model and ANSI/ESD STM7.1 for resistive characterisation of floor materials to name a few.
Basic ESD control system
Fig. 4 shows a beautiful example of the basic ESD control system where the most crucial elements are indicated. Just by following these five steps a very efficient electrostatic-protected area can be developed to increase the reliability multifolds.
1. Personal grounding and isolation. A basic ESD control rule is to ground all conductors including people at the ESD workstation. Grounding works very efficiently in ESD control systems and reliably removes electrostatic charges to ground. For such a grounding system it is important that the electrical wiring system of your lab is correct. Electrical outlets need to be checked to verify the correct wiring of live, neutral and ground. Fig. 4 shows only a wrist strap for grounding the worker, but there are other ways like use of shoe straps.
Also, some clothes are a good conductor of ESD charges. Therefore ESD garments are a good idea for added protection. The basic ESD garments used in the electronics industry are ESD aprons, gloves and shoe-covers.
Wrist strap. Standard ANSI/ESD S1.1-2006 defines wrist strap as an assembled device consisting of a wrist cuff and ground cord that provides electrical connection of a person’s skin to ground. The standard document completely describes the parameters for evaluation, acceptance and testing.
Though the document describes the whole set of mechanical and electrical parameters over which a wrist strap needs to be evaluated and accepted, the most important parameter among all is the wrist strap continuity and resistance, which should be one megohm ± 20 per cent for acceptance. The document also suggests the testing procedure for the same. While buying these grounding material, do check if they comply with above-mentioned standard and specifications.
ESD garments. The standard document ANSI/ESD STM2.1 defines test methods for determining the electrical resistance from sleeve to sleeve and point to point of static control garment.
As described in the document, the sleeve-to-sleeve method is intended to test the integrity of the electrical resistance across the seams of the garment, whereas point-to-point test method is intended to test the electrical resistance between two points on the garment, which may include the electrical resistance across the seams of the garment. The resistance range suggested by the document is 1×105 ohms to 1×1011 ohms. Most of the manufacturers do not mention such values in the datasheet, but they do mention that the garments are tested as per ANSI/ESD STM2.1 standard. So look out for that.
2. Grounding of work surface. It is extremely important to ground the work surfaces. The work surfaces that we discuss here are workstation surface and floor.
Workstation surface. ESD mats should be used to cover the complete bench top. The mat needs to be grounded. The best practice is to use metal grounding hardware snaps and ground cords connecting the work-surface mat to the common-point ground.
ANSI/ESD S4.1 standard provides test methods for evaluating and selecting work surface materials, testing of new work surface installations, and testing of previously installed work surfaces.
In the document, the guidelines for work surface are:
• Resistance-to-groundable point: 1×106 to 1×109 ohms
• Resistance from point to point: ≥1 mega-ohm
These guidelines represent a range of resistance that has generally been proven to provide protection in the manufacturing environment. If the mat’s resistance is too low, static transfers to the mat will be so fast that a spark is created. This spark is an electrostatic discharge that will damage electronic devices. If the mat’s resistance is too high, static transfers so slowly that items placed on the mat will not lose their charge. When the item is removed from the mat, it will still have a static charge and will be capable of discharging to other items.
A mat’s performance is based on the material used to make it. Performance includes electrical values, tolerance to heat, chemicals and mechanical abrasion.
Vinyl and rubber are widely used in making the mats. Vinyl is used in most cases. Rubber is used when the required resistance to heat and chemicals is high.
Flooring. First, decide the type of flooring you will use: conductive (electrical resistance to ground range: 2.5×104 to 1.0×106) or dissipative (electrical resistance to ground range: 1.0×106 to 1.0×109). Dissipative floors cost the least, but if the floor is used as a primary ground for people and carts, a conductive floor should be chosen.
ANSI/ESD S20.20 requires the system resistance of a person through the floor and to ground of less than 3.5×107 ohms (35 mega-ohms) when floor is used as the primary ground. S20.20 allows static dissipative floors as long as the resistance to ground from anywhere on the floor is less than 1×109 ohms and personnel do not generate greater than 100 volts using the ANSI/ESD S97.2 test method.
There are various options available (mats, tiles, etc). Make sure that the one you choose complies with the standards mentioned above.
3. ESD protective tools and workstation. When working with sensitive electronic components, you should consider buying anti-static or ESD-safe equipment for your workstation.
The materials mentioned below are optional but can be used for better electrostatic-protected area:
A bench-top ioniser can be used to neutralise electrostatic charges at the workstation. This is the only ESD control method available to neutralise electrostatic charges on essential insulators or isolated conductors that may be at the workstation.
The required limit according to ANSI/ESD S20.20 is less than ±50V offset (balance). In addition to that, the discharge time to reduce +1000 volts to +100 volts and to reduce -1000 volts to -100 volts should be measured. The faster the static elimination time, the better. Do look for one that strictly complies with ANSI/ESD S20.20.
5. Proper packaging. Place sensitive items in an appropriate protective packaging when transporting or moving to storage. The cost of static-protective packaging is insignificant compared to the losses incurred without them.
One of the most common static-preventing elements is a bag. Static-protective bags are an integral part of a static control programme. Selecting an appropriate bag can help reduce static damage and save money on costly repairs and rework.
There are several types of anti-static bags available in the market and just by looking at them you will not be able to gauge their capabilities. Here is how to distinguish them.
Pink poly bags (dissipative poly bags). These bags have the ability to dissipate a static charge to ground, which keeps charge from building up on the package. The material is also anti-static, so it will not charge up through tribocharge. The material’s resistivity is in the dissipative range and is usually 109 to 1011. Unfortunately, these bags have no shielding ability. A static field or discharge occurring outside the bag will easily penetrate the bag and damage the electronics inside.
Black conductive poly bags. Black poly bags are highly conductive (with a resistivity of 103 to 104) compared to the other bags. These dissipate the charge very quickly but unfortunately this fast dissipation also creates the possibility of sparking at their surface. As the material is conductive, it does provide some small amount of shielding. However, there is no plastic layer (dielectric) to isolate a device inside the bag. The charge may be transferred through the volume of the material to the device instead of around the material to ground.
Shielding bags. Shield bags provide dissipative and anti-static attributes as well as add a metal shield and polyester dielectric to stop static from entering the bag. The test for shielding demonstrates the difference between various bags. Shield bags generally stop 97 per cent of a 1000V static pulse applied to the outside of the bag from reaching the inside. Pink poly bags stop only about 10 per cent and black bags about 30 per cent.
Moisture barrier bags. Moisture barrier bags provide dissipation, anti-static and static shielding properties together with moisture vapour barrier. The moisture barrier protects moisture-sensitive items and improves long-term storage.
Then there are other products like pink bubble bags, antistatic foam and anti-static boxes, which are generally used in packaging for improving the reliability of the equipment in fields.
Once all the above-mentioned five points are implemented, you can be rest assured that you have beaten up one of the biggest devils of the electronic industry. However, it is also important to keep a regular check on all ESD-safe materials for their performance.
Wrist strap/footwear tester
As mentioned in ANSI/ESD S20.20, test equipment are required to ensure compliance and they should be selected to make measurements of appropriate properties of the technical requirements that are incorporated into the ESD programme plan.
Wrist strap/footwear tester is a highly recommended and widely used testing device that can be used to ensure the compliance for proper grounding of the workers. Consider buying one to complete your electrostatic-protected area.
The author is a technical editor at EFY