Prevent Discharges from Entering or Exiting the Housing

10.6.1.1 Avoid Holes in the Housing.

Eliminating housing holes is the first rule in the book. In fact, a waterproof network node would be ideal. It may seem strange to describe ESD as a mechanical engineering problem, but in this regard, it truly is so. Network node development programs that start from the first day worrying about openings in the housing, and how to keep conductors away from them, are far ahead of those who do not think of ESD until the first customer shipment. Holes in plastic housings of wireless sensor network nodes can be greatly reduced or eliminated by the use of a few design techniques:

• Use tang-and-clevis joints. As noted earlier, an air gap of 15 mm is needed to prevent a 15-kV discharge. This is obviously impractical for miniature wireless sensor network nodes that may be only 25 mm wide. The solution is to make the discharge travel an extended, nonlinear path. Where housing openings cannot be eliminated, create hairpin curves so that the path of the discharge through the opening doubles back on itself. This problem occurs often where two sides of a housing meet. The first impulse of most designers is to employ a butt joint (i.e., a joint in which the housing sections are simply abutted together). The butt joint inevitably leaves a gap, however, through which a discharge may travel to reach the sensitive circuits inside. The solution is to use a tang-and-clevis joint. See Exhibit 3. In the tang-and-clevis joint, the path of the discharge into the housing is serpentine and greatly extended, protecting the circuit board components inside.

  Exhibit 3: The Tang-and-Clevis Joint

  

• Use elastomeric buttons and switches. Because they represent holes in the housing, the number of buttons (and other user interface components) should be minimized. When buttons are required, however, elastomeric (e.g., silicone rubber) buttons and switches have many benefits for ESD protection. They are nonconducting, and they seal the housing switch opening with a layer of elastomer that can stay flat against the housing for a considerable distance, offering excellent ESD protection.

• Avoid metallic external connections. Wireless sensor network nodes typically connect to external sensors and actuators, often by cables. When the cables are in place, they can be significant ESD problems because they are large, conductive, and typically travel directly to the most sensitive circuits of the node (e.g., the microcomputer). When the cables are not in place, their external connectors represent metallic contacts leading directly from the outside to the sensitive circuits. The node must be protected in both conditions (i.e., with and without the cable present); however, it is very difficult to provide good ESD protection for these external metallic contacts. As an alternative, consider infrared (IR) or other noncontact communications. Using IR eliminates this problem, as long as there is an ESD-proof seal (i.e., a seal without any gaps or other openings in the housing) around the lens covering the IR LED and the phototransistor.

10.6.1.2 Locate Circuit Boards and the Metal on Them Away from Housing Holes.

Due to product requirements other than ESD protection (e.g., customer requests, need for compatibility with other systems, etc.), almost every housing design is a compromise and has at least one hole. When holes are inevitable, locate circuit boards as far as possible from them. If moving the circuit board is not possible, at least move all metal runners (ground included) away from the holes. There is no need to invite a discharge, and discharges will not land on bare (i.e., etched) circuit board material. Do not forget to move internal metal on multilayer circuit boards back away from the edge of the board, as well — discharges can travel from the edge into the board, along the layer laminations.

10.6.1.3 Eliminate Metal Points and Burrs.

Electric fields are particularly high near the points of sharp conductive objects — that is why lightning rods look like they do. The same effect happens anywhere in a wireless sensor network node where metal burrs, the cut ends of wire battery contacts, or edges of chrome connectors can get exposed to an electrostatic discharge. The resistance of a network node to a discharge can be improved to a remarkable degree by simple mechanical changes to metallic components. For example, turn the ends of a wire loop antenna inward, away from the edge of a circuit board. This makes it less likely that any metal burrs on the end of the antenna, produced when it was cut and formed in the manufacturing process, will be the destination of a discharge from outside the node. Continuing a wire battery contact spring for an additional one-half turn, so that the end of the spring is near the inside of the node, instead of the outside, is another example. Use round wire and large radii wherever possible, instead of stamped metal, due to the sharp points that can exist along stamped metal edges.