Electrostatic Discharge (ESD) is a subject that can inspire a lot of discussion, head shaking and some misunderstanding. ESD damaging semiconductor devices is real. For Makers and Hackers, a couple simple steps will protect your semiconductor devices in most cases. In this blog I will explain what is ESD, how it is generated, how it can affect semiconductor devices, and what Makers and Hackers should do about ESD. This is from my experience as a semiconductor production quality and reliability engineer that has implemented and audited ESD systems in production environments and helped design ESD protection networks in integrated circuits (IC).
What is ESD?
ESD is basically the flow of electrical charge between two items with different static potentials. The larger the difference in potentials the larger the amount of charge and energy that flows. In many cases, the potentials are generated by rubbing of materials together leaving an excess of electrons on one material and a deficit of electrons on the other material. This is referred to as triboelectric generation. This is how large potentials are generated by a person walking across a carpet or opening a plastic bag. This is the model many are concerned with in handling semiconductor devices.
The potential difference may in some cases be formed by induction when a item with a large potential is able to separate the charge on an adjacent item causing a potential difference across the second item. This is may induce a large charge in a cable, such as a long ethernet cable, or antenna that can damage ethernet switches and ports, or instruments attached to antennas.
The key to remember is that the larger the generated potential and energy, the greater probability of damage during an ESD event.
What Affects the Magnitude of Potential and Energy
When rubbing two items together and then separating them the potential generated is dependent on several factors. The most important are the material that the item is made of and the ambient environment. In general insulators will generate higher potentials because the charge is not able to be conducted away. The triboelectric characteristic of the material is also important, for example, the famous school experiment of rubbing a glass rod with a fur will generate sparks.
A very important factor is the ambient relative humidity. The lower the humidity the more likely that charge will accumulate and build up a larger potential. As shown in the table below, relative humidity can make the difference between feeling or not feeling an ESD event and is a key component of ESD control.
The amount of energy that will be transferred during the ESD event is dependent on the potential and the capacitance by the equation:
1/2 * C * V^^2where C is the capacitance and V is the potential. The human body is often modeled as having only 100 pF of capacitance, so for semiconductor handling the potential is the dominant term. The model used widely is the Human Body Model (HBM), which attempts to replicate human handling of semiconductor devices. Because cables and antennas generally have higher capacitance, the energy generated will be greater for equivalent potential; these events are modeled differently.
ESD Damage of Semiconductors
Semiconductors may be damaged by low potentials of ESD as shown by the table below. The damage can occur in the extremely thin oxide dielectrics used for the gates of MOS transistors, in the minutely small metal interconnect contact to individual transistors or from ESD induced latchup. ESD damage can be difficult to detect initially, often inducing some damage that will become a failure at later time after additional ESD events or even normal device operation. This is why it is very important to prevent ESD damage. Semiconductor device manufacturers spend a lot of money and much engineering effort on ESD protection on the IC itself as well as during production testing, handling and shipping.
As you can see the damage threshold can be very small and an ESD event not humanly perceived will damage a device. Fortunately integrated circuit manufacturers build in protection networks that will protect most devices to 2 kV HBM. A higher level but still not perceivable by a human. Some ICs with sensitive or high speed inputs may have a less robust ESD rating. As device technology scales to ever smaller dimensions the ability to build in 2 kV HBM device protection has become much more difficult. This is driving an industry effort to reduce the ESD HBM robustness specification from 2 kV to 1 kV for advanced technologies. Be sure to check the datasheet for the devices you use for ESD susceptibility. The same holds true for discreet devices that do not have additional protection built in.
What Precautions Should a Maker or Hacker Take?
As a maker, I use two steps to protect the devices and boards that I am working on: limiting generation and accumulation of charge, and dissipate any accumulated charge.
As we learned earlier, a reasonable amount of humidity in the air will limit charge build up. Working in a comfortable environment with ~50% relative humidity will be enough in most cases. Removal of any charge generating items from the workbench is also important to limit charge build up.
In addition, I use a grounded static dissipative mat on my workbench. A grounded static dissipative mat will drain accumulated charge in a safe manner and equalize potentials. I have developed a habit of touching my hands on the dissipative mat and then placing any devices and boards I am working on on the dissipative mat. This way the potential differences will be minimized. This is sufficient in most cases. And, as it turns out, I prefer working on a static dissipative mat over a hard surface.
There are times that merit wearing a grounded wrist strap in addition.
In a heated space where humidity is not added. During the winter when I use a space heater in my workshop I wear a grounded wrist strap. I am also careful that any wool or synthetic sweater or other clothing don't come close to my work area.
When I work on printed circuit boards with sensitive or hard to repair devices I also wear a grounded wrist strap. I recently assembled a kit that involved adding through hole devices to a board with a number of analog surface mount devices. Not only are the analog devices more sensitive to ESD but they would be a pain to replace. So wearing a grounded wrist strap was a simple solution.
When handling sensitive discreet devices I wear a grounded wrist strap.
The above addresses human handling of devices and boards. In addition you need to be careful with long ethernet cables, antennas or other cables that have the capacity to store charge. I recommend grounding all the cable conductors for a few seconds prior to attaching to equipment and instrumentation. Most ethernet equipment have built in ESD protection on their ethernet ports but older equipment may not. I have heard reports of nanoVNAs being damaged by charge that has been accumulated on antennas and feedlines. Beware and be careful.
Production Assembly and Testing of Boards
As in commercial production environments there are extra precautions to take if you are assembling and/or testing boards for sale. The contractor that is assembling and testing the boards should have an ESD control program in place that includes relative humidity control; dissipative mats at workstations; operators using wrist straps; ESD control at assembly and test equipment; proper transport and handling of ESD sensitive devices; training on proper handling, transport and handling of ESD sensitive devices and periodic auditing of the program to ensure compliance. Production assembly and testing typically handle millions of devices and boards per month; customer expectations of single digit ppm quality levels require a more rigorous ESD control program. You should only use a manufacturing contractor that takes ESD control, as well as the rest of their quality program, seriously.
If you are doing the assembly and/or testing of the boards yourself, then I recommend that you ensure that the relative humidity is high enough, use a grounded dissipative mat and a grounded wrist strap. The wrist strap should be tested daily for ESD and safety reasons (1 MOhm < resistance to ground< 10 MOhm), and the dissipative mat ground visually inspected. Assembled boards should be shipped in a static shield bag.
Recent trends in using advanced wafer process technologies with increasingly small features and devices with substantially more pins has made it difficult for semiconductor manufacturers to meet acceptable Charge Device Model (CDM) thresholds. CDM occurs when a package accumulates charge as it moves within test or assembly equipment and the accumulated charge is dissipated quickly when the package touches a grounded surface. Production areas, whether at a contract manufacturer or the seller's own assembly, has to protect devices from CDM damage.
Nomenclature, Terms and Safety
Unfortunately many ESD related terms have been misused by vendors, engineers and technicians, and makers and hackers. The term 'antistatic' is misused quite a bit, often used for anything associated with the control of static potentials and ESD events. Here are definitions for ESD related terms and nomenclature. Be sure to read the important personal safety notes on the use of conductive materials and wrist straps.
Antistatic - this refers to the property of a material or item so it will accumulate little to no charge, preventing static potential buildup. Antistatic materials by themselves will not dissipate charge or shield sensitive devices from damage by an external field. Antistatic materials are often used in conjunction with other materials to form a component of an ESD control system.
Static Dissipative - static dissipative materials are used to ensure that any accumulation of charge is dissipated in a controlled manner to eliminate damage during the discharge. Static dissipative materials have a surface resistivity in the range of 1 MOhm to several GOhm. Static dissipative mats are used at ESD controlled work benches providing a path to dissipate any accumulated charge on a device, board or person. A good quality static dissipative mat will have at least two layers: a static dissipative top layer with a conductive bottom (or middle) layer. The top static dissipative layer is the work surface that will safely dissipate any charge while the conductive material ensures the entire dissipative surface will equally dissipate charge, not just around the grounding point. Static dissipative bags, usually pink in color, may be used to store or transport devices or assembled boards that have a relatively high ESD tolerance. Static dissipative materials are normally also antistatic so that they don't generate charge as well.
Static Shielding - these materials form a Faraday shield around a board, device or IC tube to protect from external fields that could induce damaging potentials. The dark bags often used to transport semiconductor devices and assembled boards are static shield bags. The outside and inside surface is an antistatic plastic that sandwiches a metalized conductive material. This conductive material provides the Faraday shielding. In some cases black or blue cardboard boxes are used to transport devices or assembled boards. The material that makes the box black is a conductive material that provides shielding and cardboard itself is antistatic in nature.
Wrist Straps - wrist straps are used to bleed any accumulated charge from the person to ground. It is very important that there is at least 1 MOhm resistance from the wrist contact to ground to protect the wearer from electrical shock. Wrist straps should be tested often to ensure there is this >= 1MOhm shock protection as well as low enough resistivity to dissipate charge (<=10 MOhm). A simple DMM may be used.
Conductive - a conductive material has resistivity of less than 1 MOhm and should be used with caution. Conductive materials used for static shielding should either be sandwiched in antistatic plastic (static shield bag) or high enough resistance so as not to conduct much current should it touch power mains or other high voltages (black cardboard box). Do not use a grounded metal surface as a work place surface, the user could be shocked and seriously injured or killed should a short to power mains or other high voltages occur. Not only are conductive surfaces dangerous to the user, it will also damage any device or board with an accumulated charge that is set on the grounded conductive surface. Static dissipative surfaces should be used instead at a workstation.
Static Shield Boxes
Static Shield Bags
Buying and Installing ESD Protection
Most electronic distributors will have static dissipative mats, wrist straps, and etc. but often in large quantities or sizes. If you are careful, you can source what you need from Amazon. When buying the static dissipative mat be sure that the mat has at least two layers: a static dissipative top layer and conductive middle or bottom layer. If the description does not designate this and just calls it antistatic, don't buy it. The mats will usually come with a snap or grommet for grounding. Wrist straps and grounding kits are also available on Amazon.
Grounding is simply connected to the AC safety ground of an outlet. Just be sure that the outlet third wire ground is actually connected to ground. Connect your static dissipative mat to this ground using the installed metal snap or grommet. Also connect your wrist strap directly to ground (after verifying the >= 1 MOhm safety resistor). Clipping the wrist strap alligator clip to the mat rather than ground is not correct and will not provide adequate draining of charge.
Here are a few examples from Amazon. I have not actually used these products nor am I endorsing them. But they appear to be good from the description and reviews.