Derivation of Rule Limiting Open-Circuit Board Area
This appendix contains equations that describe the rationale for the internal charging design guideline in Section 220.127.116.11.6, Fill Circuit Board Material with Grounded/Referenced Metal, which limits open volumes on a standard circuit board to less than 0.3 cm2. The assumptions and resulting design guideline presented in this appendix have not been validated by test.
This derivation has been approached as a volumetric equation; that is, the threat is developed on the basis that when a surface area is dielectric, the circuit board under that dielectric is also a dielectric through to the bottom with no ground planes or traces to interrupt the storage of undesired energy in that volume. If there were a ground plane at some depth, these calculations estimate greater storage of energy than actually would be present.
The new rule additionally allows for the presence of ground planes, which reduces the level of concern. The energy of a capacitor of area A and discharge voltage V is
E = energy (J)
C = capacitance (F)
V = voltage (V)
This capacitor is based on FR4 circuit board material with a relative dielectric constant of 4.7 and a mm (80 mil) thick patch with a 2000-V discharge voltage. The capacitor contains about ...