Lower CDM Ratings in Large Printed Circuit Boards?

by Andy on September 23, 2010

As presented in this month’s edition of IN Compliance Magazine, “Decreased CDM Ratings for ESD-Sensitive Devices in Printed Circuit Boards” describes the increased vulnerability of electronic devices when placed into printed circuit boards (PCBs). Authored by Roger Peirce of Simco and Jim Colnar and John Trotman of General Dynamics, this article puts a new spin on the ESD world.

HBM (human body model) ratings are typically used more than CDM (charged device model) ratings because they are easier to acquire from manufacturers. However, a device’s CDM rating may be more relevant in situations where a device can become charged by an adjacent insulator. But because CDM ratings are difficult to determine from HBM ratings, the article’s authors sought out to find a correlation between CDM and PCB size.

PCB From exploratory tests discussed within the article, the results show that an electronic device’s CDM rating is drastically lowered when mounted into a larger PCB. The original device in question tested at 3500 volts. When placed into a 4 x 4” PCB, the device tested at 2200 volts. It was then placed into larger and larger sizes of PCBs. When inserted into the largest PCB, at 12 x 12”, the lowest voltage was found at 900 volts (see Table 1).

Although this information is relatively new and experimental, it does bring awareness to the vulnerability of electronic components in printed circuit boards. It also raises the question of whether or not a component’s CDM rating can be found using the multiplying factors from Table 1. With more testing, this information could potentially be used as a future standard for calculating such CDM ratings. Stay tuned!

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