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ESD (ELECTRO-STATIC DESTRUCTION) FOR SENSITIVE ELECTRONICS!
All you have to do is touch a sensitive device, and Z-Z-Z-ZAP! it's dead . . . and you have no idea when it happened. You can't see it and you can't feel it! It only takes 50 - 250 volts to fry FET's, CMOS, SAW, and other low-noise wide-bandwidth static sensitive devices, but it takes 1500 volts or so before we can feel the pain. ESD comes in all amplitudes, but a simple worst-case model for a human body discharge is a single 35 kV, 75 amp sawtooth with a 2 ns rise time and an 8 ns fall time. A great big ouch . . . nearly impossible to achieve with human body charging! But it does help us to acquire a feeling for the magnitude of the problem. Even so, this transient is not nearly as big as can be developed from carts, furniture, and moving devices. ESD is a formidable problem that usually ends with the destruction of sensitive semiconductor devices, unless the system is carefully designed to divert the ESD's short duration 2-3 Megawatt surge away from them. Even then without additional protection, we can still expect some systems response to the transient radiated electric and magnetic fields. Probably the only good thing about ESD is that it makes a great way to evaluate the electromagnetic susceptibility/immunity design of a system. If the system continues to operate during and after ESD testing then it will probably withstand most anything.
Charge development is very interesting. Many of us observed the phenomena in physics class. I'm sure a lot of you still remember rubbing the glass rod with cat fur and the hard rubber rod with wool to create positive and negative charges. Others witness the phenomena during the dry winter months when they walk across the nylon carpet (worst type for ESD), reach for the door handle, and are shocked. Charge (i.e. electrons) transfer and accumulation occurs between non-conductive materials (>10e9 ohms/sq) when they are brought into intimate contact and then separated. Hence ESD problems occur principally through rubbing, friction, rolling, etc. of rubber, paper, textiles, plastics, leather, people, and dry powders and gases. Since electrons have a negative charge, an accumulation of electrons results in a negatively charged body, while the loss of electrons results in a positively charged body. Polarity is determined by the relationship of the contact materials and magnitude is limited principally by the surrounding relative humidity. Because charge accumulation is so closely linked to humidity, maintaining a relative humidity over 50% will minimize ESD problems even under the worst of conditions. When a charged body is brought close to another charged or neutral body, especially if the neutral body is grounded, recombination of charges serves to neutralize or equalize the charge, and results in very high rates of current flow (di/dt). Keep in mind however that when equalization of charge occurs, both bodies are still charged and discharge can occur between these bodies and other bodies with different or neutral charge potential.
ESD coupled failures range from temporary to major disaster. The major disasters are caused by direct current injection into semiconductor materials. These hard permanent failures are the result of junction burnout or shorts, dielectric breakdown, and metallization melt. The failures depend on how the devices are constructed and if these devices have internal protection diodes. For example 90% of BiPolar failures are due to junction burnout/shorts and 10% from metallization melt. Where as 63% of the MOS failures are from metallization melt and 27% from dielectric breakdown. These failures are permanent! So it is extremely important that devices be protected from direct current injection! The soft failures are usually recoverable and are caused by the radiated electric and magnetic fields or by the electrostatic field. A nearby electrostatic field causes a small internal charging current flow, polarizes the material, and maintains an EF gradient while it is present. This is not typically a problem. On the other hand, the radiated transient electric and magnetic fields created by the ESD current are most definitely a problem. The electric field strength can reach values of 200V/m with all frequencies present. There are no harmonics! Consequently it does not matter if the system has resonant circuits because (regardless of the tuned frequency of the circuit) high level ESD generated electric fields will exist at the tuned frequency of the circuits. These are typically coupled as common mode. Even though ESD problems are largely magnetic, the radiated magnetic field is somewhat less of a problem because its intensity decreases rapidly with separation distance from the ESD current path. Since magnetic fields are coupled into loops, the magnetic field coupling is more likely to be differential mode. In both cases the energy coupled into resonance circuits increases with increasing bandwidth.
Systems must be designed to withstand ESD events. The ESD environment is so severe that we can't trust to luck. Plus, equipment being shipped to Europe and other countries is required to meet ESD limits based upon operational requirements. Even if there is no legal requirement, there is a design obligation to meet some minimum ESD limit to facilitate reliable operation of the system. It's a lot easier to sell a system that works everywhere and all throughout the year! Fortunately some of the most important protection methods are free if they are designed in from the beginning and not added later as a retrofit to a non-functional design. The following discussion summarizes the five most important design categories for ESD protection: segregation/isolation, PCB/electronics design, cable design, filtering, and shielding.
- Segregation/Isolation. All metallic areas/devices that the operator can touch should be grounded and the ground should be routed away from the electronics. Spacing is important! In order to protect semiconductor materials from direct current injection and reduce the coupled magnetic fields, provide at least 2.2mm separation for uninsulated ground traces or wires, and 20mm (20 kV) for uninsulated electronics.
- PCB/Electronics Design. Because the voltage induced into a coupling loop is a function of the frequency, loop area, and circuit bandwidth, keep wide bandwidth loop areas small. Amplitude and/or bandwidth protect sensitive inputs with transient protectors, filters, ferrites, capacitors, etc. Do not have floating inputs!
- Cable Design. Shield cables to sensitive circuits. Ground cable shields using high frequency techniques. Use high quality shielded connectors with the shield terminated on the outside of the equipment enclosure. Do not use pigtails! Running a cable shield through a connector pin and attaching the shield to ground inside the enclosure is a pigtail! And do not route cable shield grounds to the PCB/Electronics.
- Filtering. Critical leads should have transient protection/filtering, and the filters should be placed at the end closest to the sensitive device. If filter capacitors are used, they should have wide bandwidth and be capable of withstanding the ESD transient amplitude. Bandwidth is a function of both the dielectric material and lead inductance. 1 kV ceramic capacitors are generally a good choice. Do not filter the ESD current path.
- Enclosures/Shielding. All plastic enclosures with no exposed metallic areas/devices were thought to be the solution to the ESD problem, and they do a great job of isolating the electronics from direct current injection and preventing direct EF/HF radiation. After all, no current no radiation. Unfortunately they provide no isolation from indirect radiated EF/HF. Circuits/equipment sensitive to indirect radiation should be shielded. Seams should overlap. Apertures should be smaller than 20mm and spaced more than 20 mm. Exposed metallic panels/devices should be grounded or the segregation/isolation rules followed. Bonding resistance should be less than 2.5 mohms.
ESD transients are very severe with high energy levels at frequencies up to about 300 MHz. Consequently, ESD testing can be used as an engineering susceptibility (immunity) test to verify PCB layout (loop areas, decoupling capacitors, and grounding), I/O port/cable pickup, filter installation, and shield integrity. Testing is performed by dividing the equipment into zones and applying lower level ESD pulses to metal panels, screws, devices, cables, seams, etc. within each zone and recording any operational disruptions. If the enclosure is plastic an indirect test is performed on the system by placing a grounded metal panel adjacent to its plastic enclosure and applying the ESD to it. After all zones have been checked, the ESD level is increased by 1 kV and the test is repeated. The failure data is then analyzed and fixes applied to the system starting with the weakest area. The test and analysis cycle is repeated until the required ESD objective has been met. The objective is probably to meet the EU ESD test criteria called out in EN61000-4-2. This ESD standard has become the de facto international standard. Incidentally, ESD compliance testing is often performed only once with the ESD levels set to the required pass/fail level. If the test is performed this way and the device passes great! If it fails, then the testing is usually finished until repairs can be made. Just as a suggestion, perform the compliance test in much the same way as an engineering test so that when or if it fails there will be some data to aid in the repair process.