An MD&DI May 1998 Column
EMI FIELD NOTES
Electrostatic discharge can be prevented by designing the enclosure to withstand necessary voltage and by closing potentially harmful gaps.
Electrostatic discharge is one of the most troublesome electromagnetic compatibility (EMC) problems in modern electronics. Nowhere is this more true than with portable electronics, an area in which medical electronics is proliferating.
ESD upsets can be caused by either direct or indirect discharge. Direct discharge occurs only to metallic members of the equipment, while indirect discharge can occur to a nearby metallic member, with electric or magnetic field coupling completing the circuit. Of the two, direct discharge is the most devastating; thus, preventing direct discharge is the primary focus of this article.
Shielded enclosures are designed to protect the device electronics from both direct and indirect discharge. Good shielding is fairly easy to accomplish when metal enclosures are used. However, most modern electronic equipment has plastic enclosures, which are much more difficult to shield adequately. So the best defense is to prevent discharge from occurring in the first place. Unfortunately, this approach creates two problems: First, shielding may still be needed to cope with radio-frequency interference (RFI), emissions, or indirect discharge; second, it is difficult to block discharge.
It is important to consider ESD when designing enclosures, because that is when steps can be taken to minimize discharge. ESD-proofing an enclosure after the design stage can be difficult. Regardless of any other EMC issues dealt with in the equipment's design, ESD considerations must be specifically addressed. For example, to work with a plastic enclosure that has a membrane keypad, an LCD, several data cables, and a power cord, the designer should identify all discharge points, even metallic surfaces that are not visible. Preventing discharge with these parameters can be accomplished in two ways: by increasing distance in the discharge path or by interposing dielectric material in the discharge path. Some general rules help avoid inadvertent discharge paths:
- Use materials with sufficient dielectric strength to avoid direct discharge. Perform a 15-kV breakdown test on the material, even if the equipment being tested only needs to achieve 8 kV.
- Recess metal members enough to avoid discharge through an opening (no matter how narrow). Allow an air path of 2 cm long minimum.
- Close air gaps with bonding or gasketing at least 0.5 cm wide to ensure adequate path blockage.
Discharge through materials should also be considered. Dielectric breakdown voltage varies with material and thickness. Most plastics used for electronic enclosures withstand static voltages without arcing through. This is not necessarily true, however, for thin plastic membranes. Keypad membranes may break down, depending on dielectric and thickness. It is crucial to ensure that the plastic will withstand the desired voltage.
CHOOSING ESD TEST LEVELS
Regulatory requirements for air discharge are typically 8 kV. A problem with this test level is that higher ESD levels can occur, especially in the Snow Belt of the United States. Therefore, even if equipment passes the mandated 8-kV test, it may still fail to operate in the field. To ensure customer satisfaction, equipment should withstand test levels of at least 15 kV. In addition, if the static voltage rises high enough to cause organic material to break down, a permanent carbonized path is left behind. Subsequently, this path will break down at much lower voltages.
CLOSING THE GAP
After establishing that materials sufficiently withstand the necessary voltage, designers must minimize the air discharge path. A gap in the material, no matter how small, can become a convoluted discharge path (see Figure 1). ESD of 15 kV will normally arc only about 1 cm in open air, but this can be misleading. When conditions contrive to produce very high electric field levels, such as from sharp tips of metallic members, the arc will reach several centimeters. The internal discharge path can be difficult to locate unless clear plastic is used so that it can be seen. Often the path will involve several metallic members, causing several arcs in the path. This requires the metal members to be recessed significantly farther than might be expected.
Figure 1. Circuitous ESD path.
The solution is to avoid direct air paths, no matter how convoluted. Unavoidable paths should be made as long as possible. Some air paths are obvious; others less so. A gap is obvious in a ventilation hole. A gap between the two halves of an enclosure is fairly obvious as well. Other mating surfaces, such as an environmental seal, are more difficult to locate and correct. If the seal is fully closed along the path, ESD is probably not an issue. However, if the seal is not fully closed, or if it is not wide enough along the perimeter, arc paths could occur.
Plastic welds or filler bonding can close a path as long as the bond is continuous and wide enough to provide the necessary breakdown voltage. Because of the variability of materials and consistency of the bead, the appropriate width is difficult to assess, but a minimum of 0.5 cm wide is generally adequate. Testing early in the design process can determine the required bond width. Contact adhesives provide relatively little closure, leaving a lot of air between the surfaces, so these adhesives generally need greater spacing to block ESD.
An electronic device may need shielding for several reasons, including indirect ESD, RFI immunity, and electromagnetic emissions. Although good internal EMI design techniques can minimize these problems, some level of shielding, which can include internal shielding, laminates, or coated plastic, may still be necessary.
Figure 2. Discharge to metallization.
When coated plastic or laminates are incorporated into a design, they create an additional ESD problem: a new set of direct discharge points. Figure 2 shows a typical conductively coated plastic case with clamshell halves. For shielding to be effective, the coated surfaces of each half must mate conductively. Since the coating must be brought right to the edges, discharge can occur. This problem is prevalent in membrane keypads, which have conductive laminations that typically come right to the edge of the membrane.
The first line of defense against ESD problems is to prevent discharge from occurring directly to the equipment. This requires a dielectric material that withstands the electrostatic potential and provides adequate clearance where gaps occur in the dielectric. If discharge cannot be prevented, then shielding problems increase significantly. It is important to remember that ESD can still enter the device via wires and connections, and shielding does not block these paths. If discharge cannot be blocked by distance or dielectric, then it will occur. Such discharge currents must be diverted by other techniques, which will be explored in a future column.
William D. Kimmel and Daryl D. Gerke are principals in the EMI consulting firm Kimmel Gerke Associates, Ltd., based in St. Paul, MN.