Ten easy steps to ESD control

OK International,Semtronics, Chandler’s Ford, U.K.

Electrostatic discharge (ESD) remains one of the most familiar and underestimated sources of board and component damage in assembly, and affects almost every manufacturer, regardless of size. Yet, a surprisingly large number of ESD problems can be successfully eliminated through the application of a small number of simple process control measures.

1) Educate yourself about ESD

Education is the single biggest tool in combating ESD, not to mention evaluating the right products and suppliers to assist. It therefore pays to take time to learn what ESD actually is, and dispel many of the myths that surround the subject. This can be achieved by talking to informed work associates, contacting a trade organization, reading relevant press articles, or entrusting the services of a reputable vendor.(A free ESD tutorial and numerous links to ESD-related trade associations is featured on the website of Metcal’s ESD control and ionization division, Semtronics.) The main thing is that whichever the route, the destination is the same: a thorough grounding in the science behind ESD and how it is caused, leading to a genuine understanding of how it can be prevented. ESD is caused when an insulator (an ungrounded operator, for example) carrying a net electrical charge comes near a conductor (such as a component on a PCB) at lower electrostatic potential. As any object carrying a net electrical charge will seek to attract enough opposite (positive or negative) charge to neutralize itself, this can result in a small avalanche of electricity being conducted between the two via ions (atoms or molecules carrying a net positive or negative electrical charge) that are naturally present in the atmosphere.

This manifests itself as a short but tiny electric current in which a stream of negative charge snakes up from the conductor (PCB component) to the insulator (operator). This eliminates the positive electrostatic charge imbalance on it by earthing it to the same lower potential of the conductor and its neighboring environment (referred to as ground). This stream of charge, however, is very short and intense and can typically produce enough heat to literally melt the fragile interior circuits of a semiconductor chip (under an electron microscope this looks like small bullet holes blown outwards) to cause instant and irreversible damage.

The biggest problem is that in only one in ten cases is this damage catastrophic enough to cause a total component failure on a board that can be detected at final test. In the other 90% it will cause only a partial degradation of a device. This means it could pass the final test completely unnoticed, only to doom the PCB its sits on to an early field failure when shipped to the enduser.

2) Appreciate that ESD is ‚invisible‘

The main difficulty with controlling ESD is that it’s totally invisible at the level that can damage electronics componentry. It takes a relatively large charge build-up of around 2000V to produce a discharge that can be heard as a faint ‚hiss‘, 3000V that can be felt as a harmless small electric shock, and 5000V that can be seen as a spark. The accompanying currents are very small in the range of microampere. Devices such as a CMOS or EPROM chips, for example, can be damaged by an ESD potential difference of only 250 and 100V respectively, while an increasing number of sensitive sophisticated devices, which include processors, will be destroyed by just 5V.

3) People are the main cause of ESDdamage

Due to their continuous movement into, within and out of designated ESD protected areas (EPA), factory floor staff are the greatest hazard to ESD sensitive electronics componentry. The problem is compounded by the fact that everyday activities cause the damage. When personnel walk across an isolating vinyl factory floor, for example, they create friction between the surface of the floor and their shoes. This can result in a net electrical charge build-up on their body of somewhere between 3 to 12kV depending upon the relative humidity of the local atmosphere. Even the friction of the natural motions of a worker at a bench can create between 400 to 6000V. And if they have been handling insulators during the unpacking or packing of PCBs in foam-lined boxes or bubble packs, the net charge build-up on the surface of their bodies can reach around 26kV.

4) Select a good equipment vendor

Historically, suppliers of ESD products to the electronics assembly industry have often attempted to wrap the subject up in a certain amount of mystery for their own benefit. A trustworthy supplier, however, should totally take the opposite approach and actively strive to clarify ESD in their customers‘ minds. This will help users to identify and implement a genuinely effective program that directly addresses their individual process needs and ESD problems. As a result, the ESD vendor will then be able to supply the customer with the equipment they actually need.

5) Range of equipment alongsidethe process

Because factory floor staff are the prime cause of ESD damage, they need to be protected with suitable clothing and accessories such as safe footwear, overalls and/or heel-straps which are all designed to prevent them from retaining a net electrical charge as they move around a facility. Another prime source of damage is insufficiently grounded workstations and tools. Grounding equipment is therefore required to maintain everything within an ESD protected area at the same potential. Products available for this purpose include earth connecting points and chairs, and dissipative mats.

But the primary method of protecting PCBs against ESD damage is without doubt the operator wrist band and curly cord, which allows voltages to drain away from operators slowly and to ground safely through a large (typically 1Mh resistor. During and after assembly, PCBs should also be protected from ESD during both internal and external transportation. A range of board packaging products is available for this task and includes shielding bags, tote boxes and mobile trolleys.

6) Consider ionization

Although the correct use of the above equipment will guard against 90% of ESD-related problems, to achieve the final 10% demands going up to the next tier of protection: ionization. The most effective way to neutralize assembly equipment and surfaces that can generate static charges is by using ionizers. These essentially blow a stream of ionized air (negative and positive ions) over a work area to neutralize any electrical charge build-up on isolative materials present.

It is a common fallacy to believe that because someone is wearing a wristband at a workstation, any charge carried by an insulator in the area, such as a polystyrene cup or cardboard box, will safely dissipate away. But insulators, by definition, do not conduct electricity and it is impossible to discharge them except by using an ionizer. The problem is that if a charged insulator remains within an EPA, it will radiate an electrostatic field that will induce a net charge onto any insulator nearby and therefore create the risk of ESD damage to the product. Although many manufacturers attempt to ban isolative materials from their EPAs, this is extremely hard to enforce. Isolative materials are too much a part of everything: from the foam cushion an operator sits on to the assemble information they are referencing in a plastic cover.

Ionizers therefore allow manufacturers to accept that some insulators will appear within their EPAs, and to continuously neutralize any charge build up that may occur on them. Ionization equipment comes in one of two basic formats: benchtop (single fan) and overhead equipment (which is essentially a series of fans together within a single overhead unit). Ionizers are also available for cleanroom environments. The choice depends on the size of the area that needs to be covered. A benchtop ionizer will cover a single work surface, while an overhead model will cover two or three. Another benefit is that ionizers also prevent dust from elec-trostatically adhering to a productwhich can degrade its cosmetic appearance, or, even worse, lead to functional problems.

7) Continually monitoring equipmenteffectiveness

No ESD protection program is complete without regular testing and monitoring of the effectiveness of existing procedures and equipment. Yet, there are many manufacturers using completely failed (and therefore useless) ESD equipment because they are unaware of it. To combat this, in addition to standard ESD equipment, a range of constant monitors is available (either built-in or in stand-alone format) that will automatically alarm if the performance of an item falls out of specification (its ’safe‘ ESD-protecting performance level). This can dramatically simplify an ESD program by automating many routine tasks: For example, ensuring that wristbands and curly cords are properly tested on a daily basis, and that heel straps and footwear, ionizers and grounding points on benches are undamaged.

8) Training and monitoring all factory floor staff

Even with the best of intentions, it is human nature to develop bad habits or forget ESD safty procedures that were taught a long time ago. As the prime source of ESD damage, it is vital to ensure that the subject is kept fresh in the minds of factory floor staff by maintaining a constant feedback process with them. That should ideally be built into an ongoing training and development program.

As such, many problems are blamed on faulty equipment, when in reality the equipment itself is working fine but is being used improperly. Many manufacturers use wristband testers, for example, which operators are meant to plug their wristbands into, do a test, and then initial a piece of paper to say they’ve passed. It is very common, however, for operators to walk up, sign their name and simply walk off without doing the test. And often people will fail the test altogether, but then force themselves to pass in an unacceptable way. A classic example is where an operator’s wristband is too loose, but instead of tightening it, they just grip it with their free hand to force a pass and then sign their name. Because these kinds of procedures are open to abuse, it is vital that they are monitored closely.

9) Consider closed-loop full processcontrol

To further guard against all the problems, the final step in minimizing ESD is to install full closed-loop process control. Although most manufacturers closely monitor the performance of every stage of their production process, few companies monitor their ESD protection program to anywhere near the same degree. The equipment required to treat ESD as a process(rather than quality) control issue, however, does now exist, and comes in the form of powerful data acquisition and network control equipment.

The Semtronics static control communications network (SCCN), for example, exploits a series of networking hubs (to which 16 peripheral devices can be connected), routed to a PC via the RS-485 communications protocol. This allows users to automatically acquire and analyze ESD data in real time to achieve total process control, and completely removing the burden of manually maintaining and monitoring an ESD program.

When this is allied with Semtronics‘ Audi-Mate (SAM) software, every item of ESD equipment on a factory floor can be displayed live in a simple color-coded floor plan schematic where any danger areas will be immediately alarmed and change color on screen. SAM also offers a host of powerful SPC and other statistical analysis functions that can be customized to meet the needs of users, and eliminate the huge amount of tedium and paper trail cost associated with maintaining traditional ESD programs.

10) Sensitivity of the products

Ultimately, a manufacturer’s choice of ESD equipment should be based upon the ESD sensitivity of the products being made. A very important step to combating ESD is to appreciate how the apparently little things can cause irreparable damage if overlooked. This demands not only the use of effective ESD-protective equipment, but also tight operational procedures to ensure that the behavior of all factory floor staff is ESD-safe. That said, ESD is avoidable. Time and money invested in both the right equipment and tightening up on procedures will be amply rewarded by a corresponding increase in yield.

EPP 155

Man sieht sie nicht und man hört sie kam: Selbst minimale elektrostische Enladungen in der Fertigung können unbemerkt erhebliche Schäden an empfindlichen Halbleitern verursachen. Fortlaufende Kontrollen, um den Aufbau von elektrostatischen Ladungen zu überwachen und zu verhinden, sind mithin an den Arbeitsplätzen nötig.

Elles sont invisibles et pratiquement inaudibles: même les décharges électrostatiques minimes dans la fabrication peuvent occasionner imperceptiblement aux semi-conducteurs sensibles des dommages considérables. Des contrôles suivis visant à surveiller et à empêcher la formation de charges électrostatiques sont par conséquent nécessaires aux emplacements de travail.

Non si vedono e non si sentono: anche le scariche elettrostatiche più deboli possono causare notevoli danni ai sensibili semiconduttori. Sul posto di lavoro si rendono dunque necessari costanti controlli finalizzati a sorvegliare e ad impedire la formazione di cariche elettrostatiche.