Fowler Associates Labs

First Published in EOS/ESD Technology May/June 1993

Analyzing EOS/ESD Failures

Without failure analysis, it is difficult to tell whether you actually have a static problem. Here's a guide to the subject.

Owen J McAteer, Advisory Engineer, Westinghouse Electronic Systems, Gp.
Baltimore, MD

The subtle nature of ESD failures makes it difficult to assess the impact of static damage on a manufacturing operation. An accurate appraisal of the extent to which ESD contributes to in-house rejection rates and poor reliability can only be uncovered through failure analysis.

Smaller facilities do not have the laboratory facilities necessary to properly identify physical static damage characteristics. Therefore costly ESD-control tradeoffs are often precariously based upon published data addressing the extent of ESD problems at other companies.

Perhaps the best way for a small company or facility to get a realistic assessment of the problem is through the use of independent failure analysis consultants. Some analytical steps can be taken without specialized laboratory facilities, however, which will minimize the amount of outside help needed.

Approach

Failures which are wrongly assumed to be due to ESD, or which are assumed not to be ESD related, can lead to dire consequences. In either case the real cause of failure goes uncorrected and unnecessary corrective actions are funded. Therefore the approach to failure analysis must be general to be objective, and no presumptions should be made.

The task of positively discerning ESD failures is not as formidable as it might seem in spite of their subtleties in both physical damage and transient paths. Nor is separating electrical overstress (EOS) failures form ESD . except when rare circumstances mask the usually distinctive traits.

More forbidding is the wide spectrum of failure mechanisms other than EOS and ESD that must be acknowledged during the analysis process. The general approach taken allow for analysis of all failure causes, but some specific methods are of particular value with ESD problems.

Failure Analysis Elements

Procedures based upon the scientific method can alleviate much of the analysis effort. A comprehensive failure analysis procedure contains the following major elements:

Notification
Fact Gathering
Part Analysis
Failure Cause Identification
Corrective Action

The single element requiring specialized skill and facilities is part analysis. The remaining four equally important elements are generally left up to the manufacturing facility, regardless of size or experience.

Notification

Notification is the conveyance to the investigating activity of all important details associate with observing, recording, and reporting the failure. This step requires discipline rather than technical skills.

Significant factors include use of proper and up-to-date test specifications, discrepancy or failure definition, and monitoring apparatus. Discrepancy reporting forms must convey the most-useful information clearly and concisely. Procedures must assure that static sensitive parts are removed under good ESD-handling procedures and transported and stored in static protective packaging.

Fact Gathering

Circumstantial evidence found at the scene of the failure often yields the most germane information needed to resolve the cause of the problem. The only degree of skill required is the ability to ask the right questions and to detect pertinent anomalies present by observing general conditions of the area. A review of past failures of the same part or assembly might also be informative.

It is good policy to verify that the removed part is actually bad. Many good parts are removed because of poor solder joints, solder or other conductive particulate shorts, and printed-circuit-board faults. A low power microscopic (or even eyeball) examination can reveal such things as broken leads, fractured seals, signs of over-heating, and possibly other faults. Sometimes removed parts are found to be examples of incorrect installation.

In addition to general questions, where ESD is suspected, the types of things to investigate might include:

The degree of adherence to specified ESD preventive measures
Apparent ESD awareness as reflected in attitude as well as knowledge
Any recent changes in personnel, equipment, or procedures
Static generative materials in the area
ESD sensitivities of the part or assembly
History of static problems with the part and/or assembly

This kind of inquiry can shorten the analysis cycle considerably. If the detective work of fact gathering gives strong evidence of static damage, then a cursory laboratory analysis to verify the damage may be all that is needed.

Part Analysis

Part analysis consists of the dissection steps required to identify the failure mechanism and likely stresses or other catalysts involved in the failure. Static failures sometimes require the use of special methods such as liquid crystals, chemical and/or plasma etching, and scanning electron microscope (SEM) techniques in order to isolate and identify the failure mechanism.

Typically the end result of part analysis is a report containing a summary of the analytical steps used, photomicrographs of the damage site, conclusions about the failure mechanism, and supportive evidence in regard to the suspected cause of failure.

Failure Cause Identification

The primary purpose of failure analysis is to positively identify the cause of failure so that effective corrective action can be implemented. All the information gained in fact gathering and part analysis must be carefully evaluated in order to deduce the most likely cause of failure.

Cause identification of ESD failures is sometimes straightforward. Suppose that fact gathering steps revealed any of the following: poor handling practices, unprotective packaging, improper grounding, or the presence of highly generative material at the work site. If (and only if) part analysis showed that the device had physical traits indicative of ESD, the cause can be assigned to the ESD control impropriety and corrective action is to initiate the procedures and discipline necessary.

Where blatant infractions are not present, it may not be sufficient to merely grove that the failure was due to static. If a reasonable level of ESD control is in place, the analyst must identify the deficiency in these controls in order to recommend corrective measures.

For example, a more careful rein-spection of the scene of failure might uncover that the item had been handled at the single improperly grounded workstation. Other cases might require extensive investigation at the failure scene including the part and assembly design to determine static source and path of the transient.

Corrective Action

ESD Control procedures are typically improved through the iterative detection of additional failures and institution of further corrective measures. The higher the degree of ESD controls present, the more the corrective actions must become specific. Thus proper failure analysis and precise failure cause identification to the localized problem source is required. Corrective actions for ESD can involve changes in design, control equipment, training, as well as control procedures.

Rules of ESD Objectivity

The most important part of ESD failure analysis is to retain an objective viewpoint. The following rules will help assure that failure causes and corrective actions are properly assigned:

1. The analyst must always consider that the cause might be some mechanism other than ESD, even when some indicators of static are present.

2. Suspicion of ESD is justified only in the presence of strong circumstantial evidence.

3. Verification of ESD is attained only with laboratory affirmation of a physical ESD damage trait.

4. Proff of the specific ESD cause is best achieved through duplication of the failure both in regard to symptoms and physical damage by applied ESD.