“There’s no substitute for a sound understanding of the ESD technology!”
Common Myths
Excerpt from ESD Program Management
ESD Program Management: Request Your Copy
A realistic approach to continuous measurable improvement in static control
Second edition
Author: G. Theodore Dangelmayer
Common Myths (Part 1)
There are a number of common misunderstandings about Electrostatic Discharge (ESD) Program Management that can have significant impact on the implementation and maintenance of the ESD program. Some of these misunderstandings or "myths" result in unnecessary expenditures: others result in a compromise of the program integrity. These same myths are often cited by skeptics not wanting to adhere to certain standard ESD procedures. As a consequence, it is important to identify and dispel these myths.
This discussion outlines 15 of the more common myths.
Myth 1: "Small" Companies Cannot Afford Large Company ESD Programs
It is often stated that comprehensive ESD programs developed for large companies are simply too expensive for smaller companies to implement. For example, it is argued that full-time ESD process owners and ESD auditors represent an added and unacceptable personnel expense.
Truth: Many small and midsize companies have successfully followed the guidelines in this book in the implementation of ESD programs that proved to be far more cost-effective than they would have been otherwise. With increasing frequency, these companies are finding that they must have a reliable and acceptable ESD program in place in order to satisfy the quality assurance requirements of their customers. Moreover, the costs of manufacturing components continue to rise as a result of industry demands for higher operating speeds and improved performance.
ESD withstand (formerly called 'threshold") voltages tend to decrease as the speed of operation increases. The basic principles of the large company ESD program, including staffing, can and should be scaled to meet the specific requirements of each manufacturing operation. For example, once the program has been implemented, the ESD process owner in a smaller company can successfully perform the necessary functions on a part-time basis. Similarly, the responsibilities assigned to the ESD auditor may require only 2 hours each month to fulfill.
In summary, sound ESD programs can and will result in cost efficiencies for all companies, regardless of size.
Myth 2: PWB Assemblies Are Not ESD Sensitive
Many individuals believe that once a component is inserted into a printed wiring board (PWB) assembly, the component is no longer ESD sensitive.
Truth: The ESD failure rate and sensitivity of a component can increase after it is inserted into a PWB assembly.
One reason for this occurrence is that sensitive device junctions may become more easily accessed through the conductor paths of the PWB assemblies. Furthermore, laboratory tests have quantified this phenomenon.
In one such test, the withstand voltage of a sensitive bipolar component was determined to have increased by only 20 percent (that is, virtually no increase) when the component was tested in a PWB assembly. Even a metal shunt on the edge connector of this assembly had little impact on the sensitivity of the component. Thus, the component was equally sensitive on or off of the PWB assembly.
Another reason is known as the Charged Board Event (CBE). Circuit boards are much larger capacitors than components and, thus, when charge and discharged the CDM like discharges from boards (CBE) have far greater discharge currents than components. The damage to devices on circuit boards is also much greater than for classic CDM damage to components. The CBE resulting damage to components appears to be virtually the same as Electrical Overstress (EOS). This damage is almost invariably misdiagnosed as EOS instead of ESD. Andrew Olney, Reliability Director, Analog Devices has published a paper at the EOS/ESD Symposium that documents this phenomenon.
In summary, at the very least, ESD controls are as important for PWB assemblies as they are for components. In fact, the damage to components can be far more extensive and can occur at much lower voltages.
Myth 3: One ESD Sensitivity Classification Is Sufficient for All Areas
Many companies view multiple work area classifications as unnecessary and cumbersome. These companies arbitrarily assign the same classification to all ESD sensitive components or assemblies. As a consequence, they lose the opportunity to maximize the flexibility and cost effectiveness of the program.
Truth: All components are not the same. For example, the differences between Class 0 and Class I components can be significant, and standard ESD procedures are often insufficient for Class 0 devices. Companies manufacturing a diverse range of products should take advantage of at least these two classifications.
In doing so, these companies can utilize less expensive measures for older, less sensitive technologies and reserve the more costly program parameters for only those manufacturing areas that require them. The secret to implementing a successful program is to use multiple classifications in such a manner that training is virtually the same for all employees. Engineering provides the appropriate ESD control tools based on sensitivity and employees are trained to use all tools with the same techniques.
In summary, companies must adopt a minimum of two area classifications.
Myth 4: Human Body Model (HBM) Data Are Sufficient for Detecting Device Sensitivity Levels
Truth: Companies which rely solely on HBM data fail to recognize the importance of Charged Device Model (CDM) Data. Because the HBM was the first model developed, the most readily available data is HBM and the vast majority of programs are based on HBM thresholds. As a result, HBM counter measures have become highly effective. CDM failures, however, are now far more prevalent in factories because it is almost impossible to prevent components and assemblies from becoming charged. This is especially true due to the prevalence of high-throughput automated assembly and test equipment used in most factories. Thus, CDM data are vital to any ESD program.
At the Lucent Technologies facility in North Andover, Massachusetts, no HBM failures were reported during the last 15 years of operation. On the other hand, CDM failures continued to occur on occasion. It should also be noted that some companies do not fully understand the relevance of the Machine Model (MM). The MM is a variation of the HBM and does not provide a reliable simulation of ESD damage caused by machines. MM typically produces the same failures as HBM but at approximately one-tenth the HBM threshold. As a result, the MM data produce little additional information of use in implementing ESD controls and often prompts companies to overreact. However, the MM is a useful tool for Failure Mode Analysis (FMA).
In summary, a sound ESD program must be developed and maintained on the basis of both the HBM and CDM. The MM can be useful as a diagnostic tool.
Myth 5: Air Flow Causes Charging
Truth: As documented in ESD Proceedings, clean air does not cause charging. However, contaminated air can produce severe charging. Although an extreme example, sandblasting will charge most objects to several thousand volts. Conversely, most compressor quality air will not.
Myth 6: Metalized or Highly Conductive Shielding Layers Are Essential
Truth: Experience has shown that sound ESD program management is the most important aspect of a program and that shielding can be successfully avoided in most circumstances. Highly cost-efficient and effective protection requires proper utilization of dissipative materials and air gaps combined with a sound ESD program. Conversely, improper procedures can cause ESD failures regardless of whether shielding is used or not.
Lucent Technologies as well as our client base have documented tremendous savings due to ESD control procedures: and yet, they do not use shielding materials or enclosed containers at our manufacturing facilities nor have they ever experienced any ESD failures as a result of this policy.