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I am thinking of a farming project that includes using some board like Arduino, Raspberry Pi or Onion Mega (the list is not exclusive).

As the system will work with sensors and support life of its wards, their health will depend on the work of the whole chain of components.

I am surely going to keep the main board in a safe place, like, put it in a moisture- and temperature-proof case, and isolate contacts where needed, but I understand that these boards are more for education/experiments than for real daily duty. Also, there always a factor of defectiveness in the board coming from the manufacturer.

So, I wonder, if there is information about how durable the boards are and if they are suitable for 24/7 work during weeks/months?

How do I make sure that system has some more or less definite margin of safety and know the moment when I should replace it with a new one?

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  • \$\begingroup\$ Pretty sure any good system has redundancy built in. You might want to look into failsafes. \$\endgroup\$
    – hedgepig
    Apr 18 '17 at 8:18
  • \$\begingroup\$ Absolutely, use watchdog timers and power supply monitors that apply reset signals and make sure the reset turns off outputs that control motors and the like. You may find that more information on MTBF is made available on industrial modules rather than "hobbyist" ones like Pi and Arduino. \$\endgroup\$
    – Finbarr
    Apr 18 '17 at 8:39
  • \$\begingroup\$ but I understand that these boards are more for education/experiments than for real daily duty. I disagree with that. The components used on these boards are designed for normal consumer products so are often designed for a 10 years (guaranteed) lifespan when used within specifications for these, see the manufacturer's datasheet. The reliability will be more determined by your software and how you use it. Mistreat the board and it will fail sooner. When it will fail is extremely unpredictable. \$\endgroup\$ Apr 18 '17 at 8:40
  • \$\begingroup\$ @FakeMoustache, I do see the OP's point about boards, not ICs or software. The large ICs should be of the same standard, as you quite rightly say. But the manufacture, assembly, soldering etc and the investment in review and continuous improvement of the manufacturing process may well not be there. These are very low-cost mass-produced boards after all and they're saving money wherever possible, balanced against the costs of rejects and returns if they go too far, obviously. They're not idiots but they do have different reliability targets. We can guess and suppose but it is an unknown here. \$\endgroup\$
    – TonyM
    Apr 18 '17 at 9:57
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    \$\begingroup\$ Good question (but hard to answer well) and various good comments and answers. || A / the key point is that you can get a feel for the failure rates you can expect BUT you can never be certain and must not design a system based only on reliability projections. | People have mentioned "fail safe" - UNDERSTAND IT. This does NOT mean "safe from failure" BUT that WHEN failure occurs the system is designed to ensure that damage does not occur as a direct result AND the system will accommodate the failure such that it can be dealt with without damage given suitable other designed in precautions. \$\endgroup\$
    – Russell McMahon
    Apr 18 '17 at 11:50
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You have to look for information about RAMS (reliability, availability, maintainability and safety) engineering.

Basic RAMS concepts and techniques

  • Failure rate: number of expected failures of a component, assembly or product per time unit.
  • MTTF (mean time to failure) / MTBF (mean time between failures): the inverse of the failure rate. The expected time your component/assembly/unit will be operating under given conditions until a failure happens.
  • ER (established reliability) vs. non-ER components: so-called hi-rel (high reliability) components are often lot-tested to establish their failure rate, which makes them expensive. On the other hand, for non-ER components a rather pessimistic failure rate is assumed according to tabulated values.
  • Parts Count Analysis (PCA) / Parts Stress Analysis (PSA): a method to calculate the expected value for the failure rate of an assembly/product, deriving it from the failure rate of each component and its associated stress (temperature, moisture, power/voltage/current derating, etc).
  • Derating: the % of the maximum power/voltage/current rating at which the component/assembly/product operates. The higher the derating, the lower the stress and the longer the MMTF.

  • Bath tub curve: a curve describing how failure rate changes along the useful life of the component/assembly/product. See image below.

  • Burn-in: a non-destructive, initial high-temperature (accelerated aging) test intended for precipitating early failures in already defective components/assemblies/products. It's a kind of screening test.
  • Life test: a destructive, high-temperature (accelerated aging) test intended for establishing the reliability of a whole lot of components/assemblies/products from a reduced sample submitted to this test.

Bath tube curve

Image source.

Where do I begin?

  1. Download MIL-HDBK-217F, RELIABILITY PREDICTION OF ELECTRONIC EQUIPMENT. There you'll find almost all tabulated values you'll need. You don't need to implement all the methods described in it from the beginning, so don't panic about its complexity.
  2. Create an excel sheet for basic reliability data from your BOM (bill of materials). The columns must include at least the following information about the components: P/N, description and base failure rate. We'll add more information later, if needed.
  3. Populate the excel sheet with base failure rate data and carry out a basic PCA to calculate your first rough approximation to the failure rate and MTTF of your assembly/product. Don't forget to include the solder joints in the analysis!
  4. Look at he results of your PCA and compare them with the MTTF required by your application:
    • If the PCA delivers an insufficient MTTF, you're already in trouble and should go back to your design, your parts selection or your calculations to check what's wrong with them.
    • If the PCA delivers a MTTF well above your requirement (by a 1000x margin or more) then you might want to stop here. Just check that there aren't any components operating too close to their maximum ratings).
    • If the PCA delivers a MTTF above your requirement, but without high enough margin, then you'll have to calculate the actual stresses for the components.
  5. If your PCA was inconclusive, then you'll need to carry out a PSA with the actual stresses and environmental conditions (temperature, moisture) of your assembly/product:

    • Go back to your excel sheet and add more columns to take into account the pi-factors in MIL-HDBK-217F (temperature, quality, environmental, power rating, voltage stress, etc.). Pi-factor are modifiers of the base failure rate according to actual stress conditions.
    • Populate the new fields in your excel sheet with data for the component datasheets, but also from your own circuit simulation and calculations.
    • Recalculate the modified failure rates for each component according to their pi-factors.
    • Recalculate the total failure rate and MTTF of your assembly/product.
    • Look at he results of your PSA and compare them with the MTTF required by your application. If the results are good, then you're all set. If not, look for the components that contribute the most to the total failure rate and address their problems individually: higher power/voltage/current rating replacement component required? changes in certain design values required to avoid too much power/voltage/current in the problematic component? heatsinking required? etc.
  6. If you've done everything in your hand to reduce the total failure rate but you still can't get a MTTF compatible with your requirement, then you might want to add redundancy to your design, but specifically targeted at subassemblies of your product with high partial failure rates. Redundancy must be introduced only when MTTF calculations demand it, and never in a preemptive manner. Why? Because redundancy needs adding switching elements that can fail themselves and introduce unneeded complexity as well.

  7. Even if your PCA/PSA says everything will be OK, keep in mind that that will be true for random failures only! The PCA/PSA doesn't deal with the early failure rates of defective components/assemblies/products. Therefore, a burn-in of your product is highly recommended before deployment in the field.

  8. If you want to have actual statistical data about the useful life of your assembly/product, you might want to do a life test. But that means spending money in the samples that will be destroyed or worn out during life testing, and having the time (usually around 1,000 hours or more, depending on the testing temperature) and means to carry it out.

Notes below:

  1. There are also specialised reliability prediction software packages that will make all these calculations easier for you. Only you can decide whether you application and business case calls for such an investment.

  2. Here's a free reliability prediction software I've found (disclosure: I've never used it).

  3. I've looked for reliability data (MTBF) for Raspberry Pi without any success...

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I'd look at dual redundancy: two boards with each one monitoring itself, the second board and common resources such as the power supply. The output have to agree to drive critical things and this can be done with simple diode-ANDing. Critical things might include enabling the power to motors or other moving things. The actual implementation really depends on your circuit and application.

Each board can output a 'sign of life' signal to the other and this really can be as deep or shallow as you like. A simple UART-to-other, sending an incrementing count, is shallow as it shows that the other's alive but not to what degree. A deeper scheme would be to exchange a state value for how each board has responded to its inputs and have each gate the diode-ANDed critical outputs off if they don't agree.

I did all this (diode-ANDing, state exchange) for a very critical system once and it worked well. But you have to have a clear idea about what the states are before you start as it's not easy to work with in more 'tweak and adjust' developments.

You could also include an operating life timer, with each unit tracking the number of secs/mins/hours it has been running and the number of hours since first power-up. Building that into your system from the start may be more revealing as you application clocks up a lot of mileage and its often cheap to implement.

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