Design Note
System Level ESD Mitigation
B. Making Your PCB Layout More Resistant To ESD:
Proper layout and design of a PCB can be a very effective weapon against ESD. Use the following PCB layout
techniques:
1. (Recommended) If a multi-layer PCB is used, then create a separate ground plane. If possible, also create a
separate power plane.
2. (Recommended) If a ground plane is not practical, then use copper pour grounds on the PCB. Do not create
any isolated copper pour islands! It is mandatory that all large copper pour ground sections have at least
two connections to the rest of ground.
3. (Recommended) Put solid vias into power planes and ground planes and copper pours. Use thermal vias for
plated through-holes for components.
4. (Recommended) Keep bypass capacitors close to each power pin of all ICs.
5. (Desirable) Keep the loop area between power and ground as short as possible.
6. (Desirable) Keep areas that can be most affected by ESD (typically ICs) away from the edges of the board
and away from external wires, connectors and power.
7. (Desirable) Place all external wires and connectors on one edge.
8. (Desirable) Bring power into the center of the PCB.
9. (Desirable) Create a star grounded guard ring around the outside of the PCB, with no components on the
outer edge of the PCB. The PCB should contain no other ground loops.
The current from an ESD arc first charges the parasitic capacitance of the metal it hits, and then it follows every
conductive path available. This current prefers to flow in sheets, or short, wide straps, instead of wires. Bonding
establishes low-impedance paths between pieces of metal, minimizing the voltages between them, while
grounding provides a path to eventually drain off the accumulated charge. For grounding and bonding to be
effective against ESD, keep the ESD current density and ESD current path impedance as low as possible by:
1. (Recommended) Use multipoint and thicker grounds where you want ESD current to flow.
2. (Desirable) Use single-point and thinner grounds where you don't want ESD current to flow.
C. Component Selection and Construction To Improve ESD Resistance:
The choice of components and package types and the techniques used in manufacturing can be a big factor in
ESD protection:
1. (Recommended) Microphone wiring must be a shielded cable. Foil shield is preferable to braid shield.
2. (Recommended) Ensure that all bare metal on external components, such as microphones, switches,
speakers, etc, are shielded with non-conductive plastic (perhaps heat shrink.)
3. (Recommended) Use ICs in the smallest available package. Bonded die is the best choice for ESD
protection.
4. (Recommended) Avoid bonding out any signals in a die that will not be used.
5. (Recommended) Keep cables as short as practical.
6. (Recommended) Use surface mount and short-lead components whenever practical. Lead inductance runs
about 0.6 to 1.0 nH/mm.
© 2006 Sensory Inc.
P/N 80-0276-F
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