Despite huge efforts put in by electronics industry in the past 25 years, electrostatic discharge (ESD) still affects production yields to a great extent and in turn reduces profitability and product quality.
Various studies around the world reveal the fact that the reason for damage in 60-90 per cent of defective devices is ESD. Seventy per cent of these failures are attributed to damage caused by ungrounded people. The cost of damaged devices, shipping, repairs and rework can be so high that it consumes the entire profit margin of a project. It is highly unlikely that any organisation which ignores ESD safety will be able to successfully manufacture and deliver undamaged electronic products.
ESD susceptibility symbol. The well-known ESD cautionary symbol comprises a yellow hand in the act of reaching, crossed by a bar, all within a black triangle. It intends to identify devices and assemblies that are susceptible to ESD and so the personnel should be grounded when un-packaging or handling them. The symbol should be used on assemblies and devices that have sensitivity towards ESD.
ESD protection symbol. This symbol appears on ESD protective products such as shielding bags and boxes. The asterisk sign is replaced with a letter indicating the primary function of the item such as:
‘L’ for low-charging materials. These materials are also known as astatic or antistatic materials. Packaging using such material minimises charge generation by separation from or rubbing with other materials.
‘S’ for shielding materials that act as a barrier or an enclosure that limits the passage of current and attenuates the energy resulting from an ESD.
‘D’ for dissipative materials with surface resistance between 1×106 ohm and 1×1012 ohm.
‘C’ for conductive materials with a surface resistance between 1×103 ohm and 1×106 ohm.
ESD common point ground symbol. This symbol intends to notify people of an ESD common point ground. This common grounding point grounds all components of the work area such as work surfaces, staff and equipment to one common point, thereby reducing the risk of ESD.
It is important to understand static electricity in order to prevent and control ESD. Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it is able to move away by means of an electric current or electrical discharge. ESD can change the electrical characteristics of a semiconductor device, degrade it or even destroy it completely. This static charge build up is majorly caused by tribocharging and electrostatic induction.
Tribocharging. Creation of electrostatic charge by contact and separation of materials is known as triboelectric charging. The atoms of a material without static charge have an equal number of protons in their nucleus and electrons orbiting the nucleus. The atom in this state is electrically neutral.
Fig. 1 shows two electrically neutral materials. When these materials are separated after contact, some negatively charged electrons get transferred from the surface of one material to the other. The material that loses electrons becomes positively charged, while the material that gains electrons becomes negatively charged.
Which material becomes negative and which becomes positive depends on the relative tendencies of the materials involved to gain or lose electrons. Some materials have a greater tendency to gain electrons than the others. Similarly, some materials tend to lose electrons easier than the others.
The triboelectric series (Table I) lists various materials according to their tendency to gain or lose electrons. It usually lists materials in order of decreasing tendency to charge positively (lose electrons), and increasing tendency to charge negatively (gain electrons).
Somewhere in the middle of the list are materials that do not show strong tendency to behave either way. Note that the tendency of a material to become positive or negative after triboelectric charging has nothing to do with the level of conductivity (or ability to discharge) of the material.
The charge created by triboelectric effect depends on various factors such as area of contact, the speed of separation, relative humidity and chemistry of the materials and surface work function. Table II shows the common charge generation situations and the resulting voltage levels generated due to static electricity.
The contribution of humidity in reducing charge accumulation is also known. It should be noted, however, that some static generation occurs even at high relative humidity. It is not recommended to have relative humidity that is too low, say, below 30 per cent. ESD control becomes especially challenging at low-RH levels. A relative humidity between 40 per cent and 60 per cent is recommended for a typical assembly area.
When an electrostatically-charged person or object touches an ESD-sensitive device, there is a possibility of the electrostatic charge being drained through sensitive circuitry in the device. If the ESD possesses sufficient energy, damage could occur to the device due to localised overheating. Generally, devices with finer geometries are more susceptible to damage from ESD. The common sources of static charge in a lab are mentioned in Table III.
Electrostatic induction. This is another phenomenon responsible for static charge build up. It occurs when an electrically charged object comes near a conductive object isolated from the 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 will be zero, but it will have surfaces with excess positive and negative charge and so an ESD event can happen from those surfaces.
Classification of ESD sensitivity
It is important to define ESD sensitivity level of various components and products. These are commonly characterised to three defined models:
1. Human body model (HBM)
2. Machine model (MM)
3. Charged device model (CDM)
ANSI/ESD S20.20 defines a control program for items that are sensitive to 100V human body model. The procedures in ANSI/ESD S20.20 may need to be tailored or expanded in specific situations. Based on the models used, the ESD-sensitive parts can be classified in accordance with Table IV. It should be noted that the HBM, MM and CDM voltage levels do not correlate with each other.
Here an important question arises: How do we determine the sensitivity of our parts and assemblies, or where can we get this information?
The first source would be the manufacturer or supplier of the component itself or the part data sheet. An additional source could be an organisation like System Reliability Center in Rome, which publishes ESD susceptibility data for 22,000 devices, including microcircuits. This data is very generic and may not specifically cover the part you are actually using, but it can give a very good idea about the approximate levels.
Now that we have discussed the cause of ESD, let us see how we can control it. To have good ESD control, we first need to identify the problem areas very carefully.
The work area should be marked as electrostatic protective area (EPA) and you should only be handling ESD-sensitive components within this area. This area needs to be protected by proper ESD grounding of the workers, equipment and work surfaces.
Problem area. Table V shows typical areas which need special attention for ESD control. Each electronic component goes through various levels during the manufacturing process, so it is important that ESD control is implemented everywhere. Otherwise, damage could be caused at any stage.
There are basically two categories of damage from ESD.
The first is catastrophic damage where the electronic device is inoperable immediately after the ESD event. A semiconductor junction or a connecting metallisation could have been damaged by the ESD.
The second is latent damage where the electronic device appears to be working fine following the ESD event but the sensitive circuitry has been damaged and could fail to operate properly at some time in the future.
The static charge generated due to tribocharging and electromagnetic induction flows from higher to lower potential causing ESD events. The only way to control ESD events is by grounding all the generated charge as quickly as possible with the ESD common ground. All conductors and dissipative materials in the environment, including workers, must be bonded or electrically connected to this ground.
Now the ESD common ground should be connected to electrical ground connection as recommended in ANSI/ESD S6.1-Grounding standard. This is a good practice as all electrical equipment at the workstation is already connected to this ground. Connecting the ESD control materials or equipment to the electrical ground brings all components of the workstation to the same electrical potential.
Personal grounding. Grounding works very efficiently in ESD control systems and reliably removes electrostatic charges to ground. For such a grounding system, it is important that electrical wiring system of your lab is correct.
Electrical outlets need to be checked to verify the correct wiring of live, neutral and ground. Wrist straps are the most common way of personal grounding. Clothes are proven to be a 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 straps as an assembled device comprising a wrist cuff and ground cord that provides electrical connection of a person’s skin to the 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 amongst all is the wrist strap continuity and resistance which should be one mega-ohm ±20 per cent, for acceptance. The same applies to shoe straps. Figs 3 and 4 show the wrist strap and shoe strap, respectively.
ESD garments. ESD garments such as apron, glove and shoe covers help in reducing the generation of static charge. The standard document ANSI/ESD STM2.1 defines the test methods for determining the electrical resistance from sleeve to sleeve and point to point of static control garment. The resistance range suggested by the document is 1×105 ohms to 1×1011 ohms, but most of the manufacturers do mention that the garments are tested as per ANSI/ESD STM2.1 standard, so do look for that. Fig. 5 shows an ESD-safe apron.
Work surface grounding. It is extremely important to ground the work surfaces, and the work surfaces that we will discuss here are:
Workstation surface. ESD mats should be used to cover the complete bench-top. The mat needs to be grounded and 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 the testing of previously installed work surfaces. The guidelines for work surface in the document are mentioned below:
1. Resistance-to-groundable point 1×106 to 1×109 ohms
2. 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 ESD and 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 be capable of discharging to other items. Fig. 6 shows an ESD mat.
Flooring. First decide on which 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 are lowest in cost 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 a system resistance of a person through the floor and to ground of less than 3.5×107 ohms (35 mega-ohms) when you use the floor 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 100V using the ANSI/ESD S97.2 test method.
There are various options available that you can choose from, such as mats and tiles amongst others, but ensure that whatever you chose it complies with the standards mentioned above. Fig. 7 shows some ESD tiles.
ESD-protective tools and work station
When you are working with sensitive electronic components, you should consider buying all the equipment for your workstation that is marked as anti-static or ESD-safe. The materials mentioned below are optional but can be used for better EPA:
1. ESD table, chairs and stools
2. ESD-safe toolkit (cutter, plier and desoldering pump)
3. ESD-safe equipment such as soldering iron
4. ESD-safe brush
5. ESD-safe tray, bins and cabinets
A bench-top ioniser can also 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 voltage (balance).
In addition to that, the discharge time to reduce +1000V to +100V and reduce -1000V to -100V should also be measured. Faster the static elimination time, the better. Do look for one that strictly complies with ANSI/ESD S20.20. Fig. 8 shows a bench-top ioniser.
Proper packaging. Place sensitive items in appropriate protective packaging when transporting or moving to storage. Static protective bags are really an integral part of a static control program. Selecting the appropriate bag can help reduce static damage and save money on costly repairs and rework. The cost of static protective packaging is insignificant as compared to the losses that may be there without them.
There are different types of bags available in the market, but just by looking at them you will not understand 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 and keeps charge from building up on the package. The material is also anti-static, suggesting that it will not charge up by 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 electronics inside.
Black conductive poly bags. Black poly bag is highly conductive as compared to other bags, usually about 103 to 104, and will dissipate the charge very quickly. But, unfortunately, this fast dissipation also creates the possibility of spark at its surface. The material is conductive, therefore it does provide some small measure of shielding. However, there is no plastic layer (dielectric) to isolate a device inside a bag. The charge may be transferred through the volume of the material to the device instead of around the material to ground.
Shield bags. These provide the 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 the various bags. Shield bags will generally stop 97 per cent of a 1000V static pulse applied to the outside of the bag from reaching the inside. Pink poly bags will stop only about 10 per cent and black poly bags about 30 per cent.
Moisture barrier bags. These 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 such as pink bubble bags, anti-static foam and anti-static boxes which are generally used in packaging for improving the reliability of the equipment in fields.
The wrist-strap/footwear tester is a highly recommended and widely used testing equipment that can be used to assure the compliance for proper grounding of the workers as required by ANSI/ESD S20.20. Consider buying one to complete your EPA. Fig. 13 shows a wrist-strap and footwear tester.
The author is a technical editor at EFY