EMC Question of the Week: November 12, 2018
Heatsinks mounted over the top of high-speed microprocessors should generally be
- connected to the circuit board's ground plane
- connected to the chassis
- floated (i.e. not grounded)
- none of the above
Answer
The correct answer is "d". Whether and where a heatsink is grounded can be very important, but there is no general rule. Small heatsinks on microprocessors are often floated from ground. Very large heatsinks are often grounded to the chassis. Large heatsinks that have no ready access to chassis ground may be connected to the circuit board ground.
Decisions about whether and where to ground a heatsink need to be made after evaluating each situation individually. Heatsinks are often located on the fastest and highest-power components in a system. They generally have a voltage on them that varies rapidly with time (i.e. a high dV/dt). The EMC engineer should evaluate the worst-case electric fields between a heatsink and surrounding conductors. The worst-case common-mode current associated with the charging of the heatsink capacitance should also be evaluated. Only after evaluating these potential coupling mechanisms, is it possible to make a good decision about whether and where to ground the heatsink.
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