Introduction
This month’s blog is pulled from the product safety playbook. It highlights a potential issue to avoid when considering an optocoupler’s creepage distance when placed onto a printed circuit board (PCB).
Recall that the purpose of an optocoupler is to provide isolation of the hazardous voltages present on one side of a circuit from reaching the non-hazardous elements on the other side of the circuit, and creepage distance is the shortest distance along the surface of a solid insulating material between two conductive parts (IEC 60664-1:2020).
If the proper creepage distance is not maintained, then not only is there a potential safety issue with the end-product resulting in the product failing NRTL certification (and the subsequent delay to releasing it to production), or worse, the issue going un-noticed and placing product in the field that has a potential safety issue.
Pollution Degrees
Recall that pollution in the micro-environment is a parameter to consider when evaluating creepage distances. There are four degrees specified (pollution degrees 1 through 4). We will only consider pollution degree 2 here, where only non-conductive pollution occurs, except that a temporary conductivity caused by condensation is to be expected occasionally. This condensation may occur during periods of on-off load cycles of the equipment.
Working Voltage
The RMS voltage applied to the circuit is also used to determine the required creepage distance. This voltage is the highest value of the steady-state working voltage (see 4.2.5 of IEC 60664-1:2020), the rated insulation voltage, or the rated voltage.
Material Group
Material group also affects determination of creepage distances, and materials are classified into four groups according to their comparative tracking index (CTI) values.
- Material Group I is the best with 600 ≤ CTI
- Material Group II is second best with 400 ≤ CTI < 600
- Material Group IIIa is 175 ≤ CTI < 400
- Material Group IIIb is the worst with 100 ≤ CTI < 175
“The” Potential Issue
Based on a pollution degree 2 micro-environment, working voltage requirements for the end-product, and material group for components found in the circuit, including the optocoupler and PCB, assume we determined that we need to use an optocoupler with a minimum of 8 mm of creepage distance. We review specifications online and find a suitable component with the correct NRTL safety approvals. Everything is good to go, right? The correct answer is almost, but not completely. Consider what would happen if the parts placement diagram for the part was incorrect and the pads on the PCB shortened our required 8mm creepage down to just 7.2 mm, as shown in Figure 1.
As the note in the figure states, “PCB pad to pad only requires material group I creepage, while the copper pads common with the optocoupler reduces the effective creepage distance for the optocoupler’s material group IIIx surface.”
Conclusion
Initially, everything was done right by selecting an optocoupler with the proper 8 mm creepage distance based on pollution degree, working voltage, and material group. However, once placed onto the PCB, the placement method violated spacing rules, and only 7.2 mm creepage was obtained. Placing copper pads in a way that reduces the required creepage distance is something to avoid, not only in regard to optocouplers but other safety-critical components as well.
