The best protection from electrostatic discharge is a fully shielded enclosure with all I/O and power connectors well bonded to the enclosure. Many low-cost products, however, lack shielded enclosures because of cost. That creates a problem in protecting the circuitry from ESD.
Adding ESD protective devices to the I/O and power traces will help a great deal, but diverting the main current surge (as much as 30A or more), around your circuitry is best. See Figure 1.
A generalized ESD pulse, whose amplitude can easily reach 30A or more.
So, how do you divert 30A of current around your circuit board without the use of a shielded enclosure? This is the typical situation I've found during many client assignments.
The circuit board is generality mounted in one half of a plastic enclosure with the I/O and power connectors penetrating through the plastic (Figure 2). In this latest assignment, I was asked to determine why simply touching the I/O connectors or plugging in the cables seemed to reset the microprocessor.
This is a very typical situation for low-cost products. The ESD pulse has nowhere to go, but through the circuitry to earth through the power cable.
For most EMC issues, especially ESD, it's important to trace out the path of current. High-frequency currents tend to follow the path of least impedance, and in this case, that path was right through the ground plane to the power cord and back to earth. The current pulse traveling throughout the board coupled into the reset line of the processor and caused it to reboot.
One solution that keeps the cost minimal is to add a metal plane under the circuit board. This plate does not need to be thick metal, but a thin aluminum or steel shim stock will work. It's imperative that the I/O and power connector ground shells be well bonded to this plane, which will form a low-impedance path to earth. Very often, the plane may be attached using the same mounting hardware as the PC board (Figure 3).
Adding a metal plane under the existing circuit board provides an alternative low-impedance path to earth.
One question that may come to mind is: What about portable products without a connection to earth? This is the case for handheld calculators, music players, phones, and many other small, low-cost commercial products. In these cases, the ESD charge tends to bleed off through the capacitance between the plate and earth. For devices such as calculators, where the user presses buttons -- potentially introducing ESD pulses into the top of the PC board -- a gridded ESD shield is used between the buttons.
Any discharges tend to hit the shield, rather than the circuitry. Just be sure the top grid is well bonded to the bottom plate. Adding a low-cost metallic diversion plate is also an easy way to retrofit existing designs to achieve ESD protection.