How To Control Esd

Transcription

1 DVD-77C ESD Control for Material Handlers Below is a copy of the narration for DVD-77C. The contents for this script were developed by a review group of industry experts and were based on the best available knowledge at the time of development. The narration may be helpful for translation and technical reference. Copyright IPC Association Connecting Electronics Industries. All Rights Reserved. Do you see anything wrong with these pictures? Some of these jobs are being done with unsafe ESD practices. The operators can t see or feel what s going on, but damage may be occurring inside some of the electronic components. We ll come back to these scenes later in the program and identify and discuss the unsafe practices. This video is about controlling ESD. You ll be introduced to ESD and see how it affects the world of electronics and electronics assembly. Then we ll show how ESD control applies to material handlers. We ll discuss personal grounding and ESD safe workstations, and follow the flow of devices through receiving, incoming inspection, storing, kitting, transporting to and from the production floor, and shipping. The video will conclude with an identification and explanation of the poor ESD practices you saw in the first scenes. ESD is the sudden transfer, or discharge of electricity from one object to another volts is the lowest level of ESD that most people feel. But static zaps under twenty volts are capable of damaging, or destroying sensitive electronic components. Let s begin by looking at how electrostatic charges are generated. Every object has the ability to take on an electrostatic charge. The charge may be positive, negative or neutral. A positive charge means some atoms or molecules on the surface of a material are missing electrons. A negative net charge means that some atoms or molecules on the surface of a material have acquired extra electrons. A charge is neutral on the surface of a material when the atoms and molecules have a balanced number of protons and electrons. Positive or negative charges are usually developed when two objects are placed into contact, then moved against each other and separated. One material attracts electrons from the other material. Therefore one material has a positively charged surface. The other material is negative in net charge. When an item has a charge, this charge is called static electricity. The word static means at rest so we can say that the charge is just sitting on the item waiting for an opportunity to move.

2 When two conductive objects with different levels or polarity of charge come close together, or in contact with each other, the charge rapidly moves from one object to the other. This rapid movement of the electrical charge changes it from static electricity to ESD. It is important to understand how certain materials behave with these electrostatic charges. We ll separate them into three categories. The first material we ll talk about is a conductor. Conductors are typically some type of metal, like a piece of wire. The term conductor means that this material will conduct electricity. It will freely allow the movement of electrons through it. This will enable us to use grounding techniques to eliminate charges. Grounding means providing a path for the extra charges to flow into the earth, leaving the material with a neutral charge. The second material is called an insulator. Insulators prevent the flow of electricity. As with conductors, these materials can also become charged, but grounding techniques will have little to no effect of charge neutralization. The third material is called dissipative. These are materials that are partially conductive, and their resistive properties fall somewhere in between conductors and insulators. They are conductors of electricity just not very fast conductors. The human body, most electronic components and static control materials are classed as dissipative. They will allow grounding techniques to be used, but electrical charges are drained slowly. This is actually a desired feature in many cases since draining charges slowly minimizes the chance of damaging electronic components. People are the biggest cause of ESD. That s because our bodies easily pick up static electricity. In fact, our skin can store relatively large amounts of this electric charge. Door knobs probably don t mind the spark. But when ESD passes through an electronic device such as an integrated circuit, it can cause serious internal damage to that component resulting in a scrapped component. The tiny circuit paths in these sensitive devices can be burned up, or severed by an ESD voltage so small that our senses can t even detect it. Remember, we can only feel discharges above about 2,000 volts, but some ESD sensitive components can be damaged by voltages less than twenty volts. And you won t even know the damage has occurred but the destruction will exist inside the component. Even when the device is soldered onto a printed circuit board, it s still not safe. It can be damaged by a discharge that passes through the board s conductive pattern to the ESD sensitive component. The amount of voltage needed to damage or destroy varies from component to component, but as these devices become smaller and more complex, their sensitivity to ESD generally increases. Sometimes the damage caused by ESD doesn t mean the component will immediately fail. Instead, the assembly passes inspection, gets shipped to a customer and then fails later on, creating a very unhappy customer. ESD is one of the most serious problems facing the electronics industry today. If it isn t controlled, the results are higher costs, lower product quality and angry customers especially if a product fails at a critical time. On the other hand, controlling ESD means product quality can be improved and costs can be kept down. This usually leads to increased customer satisfaction and sales which is a win-win situation for everyone. Let s stop for a moment to review the information we just covered.

3 The most important thing to learn about controlling ESD is that we are the most important part of ESD control. In order to reduce the risk of ESD damage to the devices, it s necessary to wear personal grounding equipment. The idea is to drain off charges in a controlled manner before they reach the devices. To do that, all conductive surfaces and our bodies need to be connected to a common grounding point. If a static charge comes into contact with any partially conductive surface, this contact safely channels the charge away from ESD sensitive devices and neutralizes them into the earth, or ground. There are several methods used to drain off charges that we might carry into an ESD safe area, or that we may generate while doing our jobs. The most common are the wrist strap and heel strap also known as the shoe grounder. We ll look at the wrist strap first. Wrist straps are very effective for any operation where people are seated at their workstations. Shoe grounders are not as effective when seated because people may raise their feet when seated and lose contact with the conductive floor. The wrist strap needs to be worn snugly against bare skin. It should be attached securely to a ground wire with a current limiting resistor and some form of quick connect/disconnect. The current limiting resistor provides a slow controlled drain of any charges to limit the current level and prevent damage when the charge is drained to ground. Shoe grounders are designed to connect your body through your sock to a conductive floor or mat. Shoe grounders are very effective for stand up operations. This eliminates the problem of a long cord from the wrist strap. Most companies, and all industry standards, require the use of two shoe grounders for standing activities and a wrist strap for seated operations. The strap is adjusted so that one end is in contact with your sock and the other end is in contact with the floor. There is enough moisture in your sock to provide a complete electrical connection from your body to ground. Shoe grounders and wrist straps should be tested at regular intervals to make sure they are working properly. Your company will let you know where and how often you should do this. When testing shoe grounders, place one foot on the metal plate and press the test button. The pass light indicates that the device is functioning properly. Remove that foot and test the other foot. Testing both feet at the same time will not indicate a failure unless both straps fail. When testing the wrist strap, remove both feet from the metal plate and use the opposite hand to press the test button. Shake your wrist to check for intermittent failures. If you experience any problem with personal grounding devices, let your supervisor know immediately. Your company may also require you to wear a conductive smock. Conductive smocks are used to cover static generating clothing and drain any charges to ground. It s important to completely button the smock so no part of your clothing is exposed. The sleeves should never be rolled up to the point where underlying clothing is exposed. When conductive smocks are not provided, you ll need to be aware of what you re wearing. Clothing made of cotton, rather than wool or synthetic materials, will help control ESD. Also, you should avoid wearing loose clothing that drapes down and may come into contact with ESD sensitive devices. Let s stop and review the material we just covered.

4 Now, let s see how ESD control applies to handling and transporting electronic components and assemblies. The three biggest causes of ESD damage in material handling are lack of personal grounding, improper packaging and the formation of charge fields around components and assemblies. Now, let s follow the flow of electronic components from the time they arrive at the company until they are shipped out as circuit board assemblies to the customer. The material handling jobs we ll be looking at are receiving, incoming inspection, stockroom storage, kitting, transport to and from production, and shipping. When you take delivery of components, the boxes should not be opened until you are at an ESD safe workstation and are properly grounded. Although the majority of ESD sensitive components are in protective packaging, some may not be. The ESD safe workstation basically consists of a static dissipative work surface and static dissipative flooring, or floor mats. One way of controlling ESD at your workstation is to eliminate or reduce the number of static generating materials. What we re talking about here are common plastics such as hairbrushes and tape dispensers and paper products. These types of non-conductive materials tend to generate and hold static charges thereby creating electric fields. When a charged object such as these plastic safety glasses is brought close to an ESD sensitive device, the electric field causes charge separation in the ESD sensitive device. If the ESD sensitive device then comes in contact with a conductive item while exposed to the field, the device can be damaged. Here are some examples of non-conductive materials that should be eliminated from your work area whenever possible. When it is not possible to remove a non-conductive item, an air ionizer may be used to neutralize charges. This machine blows ionized air into the work area to help neutralize any positive or negative charges that accumulate on non-conductive items, or items that are not connected to ground. In spite of all precautions, ESD sensitive devices will sometimes become charged. Therefore, it s important that they are not allowed to discharge rapidly since the rapid transfer of a charge to a conductive surface may cause damage. That s why we use the static dissipative work surface. These dissipative materials conduct electricity but they do it slowly enough to leave the ESD sensitive device undamaged. Now that you ve seen what makes an ESD safe workstation, let s get back to the components that have been received. ESD sensitive components generally come from the supplier in ESD protective packaging. The components will be on tape and reel, in tubes or in trays. There are two categories of packaging available. One type is known as low charging. This may include dissipative and anti-static. The other type is static shielding. The packaging is often a combination of these two types. For example, these ICs are packaged in low charging tubes that are placed in static shielding bags.

5 In many cases, these components will need to be inspected for count, correct part number and for damage. This is called incoming inspection. Again, incoming inspection should be done at an ESD safe workstation with proper grounding. Although it is best not to unseal the packaging, the inspection policy of your company may require that you open packaging and even pick up a component. If you need to handle a component, pick it up by the non-conductive portion of the body, rather than the leads. The leads are the most conductive pathway for an ESD event. If you need to set the component down, place the device with the leads down on a static dissipative surface. Don t slide the device over any surface since sliding can generate static charges. After inspection, it s important to reseal the components in their original packaging. If you ve opened low moisture packaging, it may be necessary to reseal the packaging with a humidity indicator and a dessicant. Your company will explain its policy for dealing with environmental packaging. The next step is to store the components that have been inspected. The location of the stockroom will determine how the components are transported for storage. For example, if the stockroom is in the same ESD safe area as the receiving department, then containers and carts simply need to be dissipative. However, if you need to go between ESD protected areas, it s important to also utilize static shielding bags or containers. The racks or push carts should be designed for ESD sensitive devices. When storing the components in the stockroom, you need to wear shoe grounders and be standing on a conductive or dissipative floor or mat. The next step in the material handling process occurs when the components are required for production. The gathering of components for a specific job is called kitting. Proper quantities of components are pulled from various locations in the stockroom and are put on a kit cart. Again, you need to be wearing your personal grounding equipment and use ESD totes when required. The components will usually be sealed in their original packaging. But sometimes you ll need to remove a specific quantity from the static shielding packaging. In this case, if you re transporting the kit cart from one ESD safe area to another, it s important to store the open components in an ESD tote with the cover on. After the devices arrive on the manufacturing floor, production begins. While on the manufacturing floor, partial assemblies may be transported from one production area to another. For example, surface mount components may be placed on circuit boards and reflow soldered in this area. The partial assemblies are then transported on an ESD safe rack to the through-hole production area for component insertion and wave soldering. As long as these assemblies remain in ESD safe areas, they can be transported in open totes. The finished assemblies are then tested. Assemblies that pass electrical test are ready to be shipped to the customer. These assemblies should be loaded into the partitions of ESD protective totes. The totes need to be clean. And they need to be the right size. It s important to handle the assemblies only by the edges minimizing contact with any conductive surface. Since the components are connected to the conductors on the circuit board, ESD damage to a component on the assembly can occur without direct contact.

6 The last material handling operation is shipping. Again, handling of assemblies needs to be done at an ESD safe workstation with personal grounding. There are a number of methods of properly packaging ESD sensitive assemblies for shipping. Some companies use static shielding boxes with partitions for the assemblies. Other companies pack each assembly in a low charging and dissipative bag. This bag is then sealed inside a static shielding bag. Sticking ESD labels on the packaging will inform the customer that these are ESD sensitive assemblies. We ve just looked at a number of ways to control ESD during material handling and transporting. ESD control works most effectively when you assume that all the components and assemblies that you handle are ESD sensitive even when they re not. ESD sensitive devices should be properly labeled, but sometimes someone forgets. That s why it s important to get in the habit of performing all activities with ESD safe practices. Let s stop again to review the material we just covered. Our last section deals with recognizing unsafe ESD practices. Let s take a look at the scenes we watched at the beginning of this video. Do you see anything wrong with these pictures? This person isn t wearing any personal grounding equipment. In this case, the ESD sensitive items being transported aren't protected. At this workstation, there are non-conductive items near to the ESD sensitive components. These finished assemblies are being packaged in low charging and dissipative bags, but are not protected by static shielding packaging. This program has presented the details of ESD control for material handlers. First, we discussed static electricity and took a look at how poor ESD practices can affect electronic components. Then we discussed personal grounding and followed the flow of ESD sensitive components from when they arrived from the supplier until they were shipped out as part of a circuit board assembly. We concluded with an exercise to recognize unsafe ESD practices during material handling. ESD control is your responsibility. It s important to wear the appropriate equipment and to follow the proper procedures for handling and transporting components and assemblies. And you need to report any problems or ESD hazards promptly. In this way, you can make a difference.