August 13, 1997
Dear Medical Device Manufacturer:
This is to alert you to a failure mode we recently became aware of involving vacuum loss in electronic resonating components. This concerns us, because electronic resonating components are frequently used in medical devices, and their failure could have serious adverse effects on device function. To our knowledge, this problem has not been previously documented in the scientific literature.
Mechanism of vacuum loss
The oscillator circuits of medical devices often employ a crystal resonating component to maintain strict control of timing functions and electrical signal synchronization. A resonating component typically consists of a piezoelectric element, housed in a metal or ceramic case. The interior of the case is evacuated in order to decrease motional resistance. This improves the resonating quality of the component. Slow degradation of the interior vacuum can occur in two ways: gas leakage through case joints and diffusion of helium through the glass that surrounds the electrical pins of some resonating components. The loss of vacuum significantly increases the component parameter known as motional or series resistance, and the slow rise in resistance can result in device malfunction years after the device is manufactured. The duty cycle of the timing signal has been found to change significantly as a result of rising motional resistance. In some cases, oscillation has ceased. Loss of oscillation is generally recognized as a catastrophic failure mode of these components, but changes in timing signal duty may be equally catastrophic, more insidious, and less widely recognized as a failure mechanism. Microprocessors, microcontrollers, and memory circuits are examples of digital circuits that may be affected by changing duty cycle.
How to prevent problems
One way to prevent vacuum loss problems is to immunize devices against changing motional resistance. This can be accomplished by design, making the oscillator circuitry tolerate the expected change in motional resistance. Another way to prevent problems is to eliminate the vacuum. Resonating components can be manufactured with any fill gas and pressure desired. If the gas composition and pressure inside the component matches the expected exterior gas environment, there will be no influx of gas to cause motional resistance change. However, since this will reduce the resonating quality of the component, such reduction must be accommodated in the final circuit design.
If an evacuated resonating component is deemed necessary, then maintenance of the vacuum over the expected life of the device is mandatory. Ensuring vacuum longevity is not easy. If one assumes that gas infiltration is an exponential function of the pressure enveloping the component, then the maximum allowable gas leakage rate for the typical resonating component will be less than ten to the minus tenth atmospheres cubic centimeters per second. This quality of component seal is impossible to verify during production. Thus the only way to ensure vacuum longevity is to pay meticulous attention to the component's material composition, design validation, acceptance procedures, and manufacturing quality.
In many cases, vacuum loss cannot be completely prevented but can be limited and expected to rarely occur. In such cases, redundant or backup circuits can be used to mitigate vacuum loss problems. However, manufacturers must ensure that the switching mechanism from primary to redundant or backup function occurs reliably and in advance of any malfunction that could result from slowly rising motional resistance. Checking for backup circuit engagement at the extreme of expected motional resistance or only when oscillating function is lost has proven to be an ineffective deterrent to vacuum loss problems.
Why FDA is issuing this alert
Resonating component vacuum loss represents a failure mode to which some medical devices are particularly susceptible. This alert is intended to provide notice of this failure mode to the designers and manufacturers of implantable medical products. However, any device that incorporates an electronic resonating component is potentially susceptible. Therefore, we encourage all designers and manufacturers to consider this failure mode during design risk assessment and when devices are analyzed for failure. In addition, we recommend that manufacturers assess the susceptibility of their marketed devices to malfunction caused by slowly changing resonating component resistance and consider remedial action if a device is susceptible.
Getting more information
If you have any questions regarding this letter, please contact Eugene O'Bryan, Electronics Engineer, FDA, 10903 New Hampshire Ave., WO-62 Room 3120, Silver Spring, Maryland 20993; or fax to (301) 796-9925; or E-mail to firstname.lastname@example.org.
D. Bruce Burlington, M.D.
Center for Devices and Radiological Health
Food and Drug Administration