Electronic – Wireless Sensor Network Lifetime Estimation

The general rule used is that usable lifetime doubles for every 10°C reduction in operating temperature. This should be considered an upper bound on expected life.

As the capacitor ages its capacitance will decrease and ESR will increase. The increase in ESR can result in a temperature increase if running high ripple currents which in turn increases the rate of aging.

Also make sure that the increase in ESR / decrease in capacitance over the cap's life span is acceptable. For instance, make sure that the circuit still operates when the cap hits -20% of baseline capacitance, or whatever its end of life rating is.

Other factors can increase the rate of aging as well. For instance, the more air flow over the cap, the shorter its life expectancy due to evaporation effects. On the other hand, a reduction in temperature from the air flow can more than offset this issue if the cap is running warm.

The parts most likely to fail first on an electronic device are:

1. Electromechanical parts.

   In an official Arduino, these are the connectors and possibly a pushbutton. Some Arduinos have slide switches and such as well.

   I assume you aren't going to press the reset button very often, so we can ignore it.

   If there are any other buttons or switches — e.g. the settings slide switches on a Seeeduino — I assume they're going to be set and left there for the rest of the device's usage life, so they can also be ignored.

   That leaves the connectors.

   Vibration is a problem in industrial applications. If you let the DC power plug get vibrated out every month, the vibration and repeated re-plugging might put enough wear on the connector to make it unreliable. This won't kill the Arduino, but you can expect intermittent reboots, which many applications won't tolerate. I recommend hot-gluing the barrel connector in place, and strapping the DC power cable down to something solid nearby.

   Be sure to leave enough slack in the power cable that vibration and tugs on the cable don't translate to angled tugs on the connector, stressing the solder joints.

   Shields have enough contact area that they shouldn't come loose on their own, unless there's flexion involved.

   Ethernet and DB-9 serial connectors are fairly tolerant of vibration, but you should still strap the cables down to prevent their weight from dragging on the connector.

2. Electrolytic capacitors.

   Some Arduinos have these, while others get by with lower-valued solid tantalums. Tants are more reliable, particularly at high temperatures.

   As a rule of thumb, electrolytic capacitor life doubles with every 10°C drop in temperature. A really nice electrolytic cap will be rated for 5,000 hours of life at 105°C. 45°C is six such doublings, so 64 × 5,000 hours, giving an expected lifetime of ~36 years.

   Sounds fine, right? Now consider that a cheap cap might only be rated for 1,000 hours at 85°C. That's only 4 doublings, or 16 × 1,000 hours, which is only about two years of continuous use.

   The story is more nuanced than this. By "lifetime," I don't mean that electrolytic capacitors magically cease to be capacitors at the next hour past their rating. Instead, what happens is that the manufacturer has made an educated guess that the capacitor will still be within its rated limits after that many hours of use.

   After the rated lifetime, the capacitor is still going to be a capacitor, but its accumulated drift is allowed to take it out-of-spec once it's given you its rated service life. The more well-made sort of Arduino board will have some design margin that can absorb some spec drift. So, even if you get only exactly as many hours of life out of the cap as the manufacturer guessed you would, the Arduino might continue working.

3. Anything connected directly to a raw power source.

   It doesn't take a very big power surge to kill a delicate microcontroller. To some extent, the fact that you're using the on-board regulator helps. But, consider adding some outboard protection components: zener clamps, maybe a polyfuse, etc.