Conformity Magazine - February 2009 - (Page 22)

ESD Open Forum Dealing with Charged Board Model Events Provided by the ESD Association © Janaka Dharmasena | Dreamstime.com Q: A: We have experienced a significant amount of unexplained “component on board” failures that resemble ESD at times and EOS at other times, but we have not been able to duplicate these failures using any of the existing ESD models. There is literature that suggests that these are charged board model (CBM) failures. Is the ESDA doing any work to produce a standard method/ procedure for the industry? This is a very good question. In an attempt to bring awareness for the need to develop a universal procedure to test for component failures on printed circuits boards (PCBs), the ESDA standards body has had three ad hoc meetings to explore the possibilities. In reviewing existing publications, such as the ESDA white paper [1] and others [2, 3, 4], we found that, up to 1983, only anecdotal evidence existed for ESD failures of integrated circuits (ICs) that are mounted on PCBs. These failures had occurred after improperly grounded personnel handled the ICpopulated PCBs during the many stages of manufacturing, up to and including actual placement in the completed electronic equipment/system. Between 1984 [4] and 1987 [7], it was reported that most components which reportedly failed for the ESD transients on the PCB had failed functional testing (that is, the board was inoperable). Failure analysis (which included SEM analysis) of the ICs removed from the PCB showed that the physical damage type was dielectric/silicon punch-through. From 1985 [5], this PCB ESD discharging event was referred to as a charged board model (CBM) ESD event, and CBM ESD testing was initiated. The measured PCB capacitance (> 125pF) was always larger than that (25 pF) for IC devices being built at that time. In 1986, Enoch and Shaw [6], in their study of board-mounted ICs, used the field induced method to charge the board (PCB), then grounded the PCB via one of the input connectors. Koyler et al in 1987 [7] regarded the PCB to be an extended device package, but with higher capacitance. They suggested two modes by which the board-mounted IC can fail: (i) during the insertion of the device into the board; and (ii) when the PCB discharges into the device, an external to internal mechanism. Paasi 2003 [8] found board mounted IC failures 22 Conformity february 2009 after they were charged by transportation on conveyor belts. Olney et al [9] in 2003 used a regular CDM tester to perform the field-induced stress testing of the components on the board. Conceptually, the charged board model is similar to the charged device model (CDM). During a CDM event, the charge stored by a packaged IC discharges (typically < 100 picoseconds) just before contact is made with a conductive object at or near ground potential. During a CBM event, the charge stored by an entire PCB discharges (100s of picoseconds) just before contact is made with a conductive object at or near ground potential. Thus, the CBM can be thought of as an extension of the CDM where the PCB is the “device” that stores the charge. It is suggested that CBM be renamed CBE (for charged board event), because CBE does not actually represent a new model, but rather just a more severe CDM event – so severe that the failure can be mistaken for electrical overstress (EOS) damage. Yes, we need a document that the industry can rely on. The question is which industries need this? Within the Device Testing Working Group, we have discussed the PCB industry, the cell phone industry, the “board stuffing” industry,” the automotive industry, and the medical industry. CBE is not as well known as other ESD models but it represents a major real-world ESD threat. Even if all the individual components used for a given PCB have high device/component-level ESD robustness, one or more of these components could be susceptible to ESD damage after mounting to a PCB, since a PCB in general has much higher capacitance than an individual device [10-16]. CBE damage

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Conformity Magazine - February 2009 Contents Editor's Note FCC Clarifies Closed Captioning Requirements for DTV Receivers Commission Cracks Down (For the Fifth Time!) On Junk Fax Marketer FDA Requests Comments on Global Harmonization Documents Australia Now Requires Solar Panel Testing and Certification EU Drafting New Directive on Toy Safety EU Commission Revises Standards List for EMC, R&TTE Directives Additional Standards Published For the EU's Medical Devices Directive EU Commission Publishes Additional Standards for Directives on Active Implantable, In Vitro Medical Devices EMC For the Functional Safety of Automobiles ESD Open Forum The ILAC Arrangement: Facilitating Trade Efficient Methodology for the EMI Analysis of Optical Transceivers Including PCBs and/or Flex Circuits The EuP Directive - Implementation, Design Requirements and Issues Chambers, Shielding, and Accessories Buyer's Guide CPSIA Lead Requirements Apply to Existing Inventory, Says CPSC CPSC Clarifies Certifcation Requirements Other CPSC Actions in the News IEC Standards Update Product Reviews UL standards Update Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - February 2009

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