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ITG SUBJECT: SCREENING ELECTRONIC COMPONENTS BY BURN-IN.
On several occasions while participating in medical device investigations, I have been asked by both Investigators and the Supervisor reviewing the EIR to explain the meaning and purpose of the term "burn-in". This ITG has been written in response to those questions.
Burn-in is a common term in the electronics industry and is the holding of an electrical device at an elevated temperature for a specified number of hours, generally with bias and an electrical load applied. (Bias is the application of an external d.c. voltage to set the upper and lower operating limits of a device) This is done in an attempt to stress all elements of the device at maximum rated operating conditions in order to reveal all stress and time dependent failure modes. In effect, burn-in is an accelerated aging process and is used particularly for semiconductor devices such as transistors, integrated circuits, etc.
Experience has shown that an electronic device will most likely fail in the early part of its operating life if it is prone to failure. The failure rate then levels off throughout the device's normal life and again increases when the device becomes old. The early part of the device's life is called the infant-mortality life stage. Those devices that make it through burn-in should, if they are operated within rated limits, have low and predictable failure rates.
MIL-STD-883, Method 1015, Burn-in Screen, states that burn-in is performed for the purpose of eliminating marginal devices, those with inherent defects or defects resulting from manufacturing aberrations which are evidenced as time and stress dependent failures. In the absence of burn-in, these defective devices would be expected to result in infant mortality or early lifetime failures under normal use conditions. MIL-STD-883 "Test Methods and Procedures for Microelectronics" is a military standard by which many semiconductor device manufacturers test their devices. Copies of this document may be obtained on written request, from the Naval Supply Depot, 5801 Tabor Ave., Philadelphia, Pa. 19120. MIL-STD-883 Method 1015, is not necessarily the best burn-in procedure, but is followed and quoted by many electronic component manufacturers.
Burn-in may be accomplished in-house or by an outside testing facility. The components used by a firm may be purchased with burn-in performed by the component manufacturer. The firm may then perform another burn-in prior to integrating the components into the firms equipment or device. The equipment or device may then receive burn-in as a completed device.
Burn-in is normally accomplished in ovens or chambers that will maintain the devices at specified test temperature and provide access to specified electrical connections. The burn-in arrangement should include a monitoring system which will record temperature, time, and bias voltage through-out the burn-in test period to ensure these parameters were maintained within specified limits. Burn-in time and temperature may vary but burn-in temperature is normally 125 C with an initial burn-in time of 168 hours.
The burn-in method of screening out defective devices is only as effective as the testing conducted following burn-in. Such testing must be thorough and include measurement of all device electrical parameters specified in the manufacturers specifications. Measurements may be conducted either while the devices are at burn-in temperature, at rated operating temperature limits, or at room temperature upon removal of the devices from the test chamber. In any event, measurements should be conducted per the firms stated procedure. If the device is cooled to room temperature prior to testing, bias voltage should be applied while the device is cooling to prevent reversion of defective circuits to an acceptable operating mode. To be meaningful, test measurements should be made at 0 hours and at the specified endpoint with intermediate measurements if required. Once rejected and verified as a device failure, no devices should be retested for acceptance.
There are presently no regulations that require device firms to employ the burn-in screening method, but it is generally considered a good manufacturing practice.