AN3960

Protecting against ESD

Doc ID 022082 Rev 1

11/21

Use of the dielectric overlay is effective and is almost mandatory for many applications

however, there are some drawbacks, namely that the overlay does not surround the whole

application and that an ESD event can bypass the overlay. If the user can avoid accessing

the application through the front panel (example, by accessing it from the back or

elsewhere) or if the ESD event can bypass the front panel, one of the other methods which

protect the device against an ESD event should be considered.

3.2

Spark gaps

Physical techniques, such as the addition of spark gaps, can give supplementary protection

to the input/output lines of a circuit board which are susceptible to extraneous voltage such

as ESD. For example, printed circuit board (PCB) spark gaps can be used to route ESD to

earth in products using capacitive sensing electrodes (see Figure 3: PCB with spark gap).

A spark gap consists of an arrangement of two conducting electrodes separated by a gap

usually filled with a gas such as air which is designed to allow an electric spark to pass

between the conductors. When the voltage difference between the conductors exceeds the

gap's breakdown voltage, a spark forms, ionizing the gas and drastically reducing its

electrical resistance.

The spark gap shown in Figure 3 is an 8 mm gap which is a common PCB tolerance. The

approximate breakdown of such a small spark gap is given in Equation 1.

Table 2.

Dielectric overlay materials and their dielectric strength

Material

(V/mm)

1.

The breakdown voltage of an insulating material is the minimum voltage that causes a portion of the

insulator to become electrically conductive.

Min. overlay thickness at 12 kV

(mm)

Air

1200–2800

10

Dry wood

3900

3

Common glass

7900

1.5

Borosilicate glass, e.g. Pyrex®

13000

0.9

Polymethyl methacrylate

(PMMA) plastic, e.g. Plexiglas®

13000

0.9

Acrylonitrile butadiene styrene

(ABS) plastic

16000

0.8

Polycarbonate, e.g. Lexan®

16000

0.8

Formica plastic

18000

0.7

2.

FR-4 is a widely accepted international grade deignation for fiberglass reinforced epoxy laminates that are

flame retardant.

28000

0.4

Polyethylene terephthalate

(PET) film, e.g. Mylar®

280000

0.04

Polymide film, e.g. Kapton®

280000

0.04