The QC Engineer who instituted Murata Power Solution's new Design-for-Reliability Program recently performed routine HALT (Highly Accelerated Life Testing) on a randomly selected digital meter . . . and the device "passed." Devices are not supposed to "pass" HALT. The intent of HALT is to "reveal latent defects or design weaknesses that may eventually cause field failures" by subjecting devices to progressively higher stress levels until something "breaks."
The Engineer decided to see how much it would take to make a second meter succumb. With the maximum applied input voltage, the meter (mounted in a metal panel using a Murata bezel assembly) was subjected to the following: operating case-temperature extremes of +145°C and –75°C; nonoperating cold soak at –75°C (after which the meter turned on successfully); three temperature cycles (–65 to +125°C) at a transition rate of 30°C per minute; 15 minutes of 3-axis random longitudinal and rotational vibration at 10 to 15G's; a temperature spike down to –65°C; and then more vibration.
During the second vibration, the meter pulled away from the plastic bezel (which had melted during the +145°C temperature extreme), and one segment of one LED digit went blank. Upon troubleshooting, the Engineer discovered all meter electronics, including the drive circuit to the failed LED segment, were working fine. The failure was in the display itself. The Engineer's report gave the device a "conditional pass" pending failure analysis of the LED.
Now you know why we confidently specify our digital panel meters with an operating temperature range of 0 to +60°C and boast about their rugged, vibration and moisture-resistant, epoxy-encapsulated packages.