I want to implement a control system I designed for a light electric motorbike. I would like to mount it as professionally as possible on the chassis. It consists of a power system pcb and a SAMC21 development board which I'll merge later once this all works properly. I have tried screwing it directly on the aluminium chassis but have found that shocks and vibrations from the horn disturbed the system or even destroyed it upon large shocks like a few hammer hits. Is there a specific way car and motorcycle manufacturers mount their electronics to make their system as robust as possible? Thank you for your help!
Avoid ceramic capacitors and ceramic components for applications under environmental conditions that include vibrations and shocks. If you can't avoid them, choose components that use materials or constructions techniques targeted to minimise the so-called "microphonics". Also, use microphonics minimisation design strategies on your own.
Microphonics is an unwanted behavior due to the piezoelectric effect of ceramic materials. Usually, it will manifest itself as spurious voltage burst when the component is subject to mechanical vibrations or shocks. Obviously, it can wreak havoc in your circuit, upsetting digital circuits and/or triggering switching in analog circuits.
Apart from microphonics, electromechanical devices (relays, swtches, etc.) can suffer upsetting under vibration/shock.
Also, mechanical stress can dramatically affect reliability and precipitate failures unless the PCB and its enclosure is specifically design to withstand those vibration and shock levels:
- Solder joints may break up due to tension/compression forces.
- Solder joints may not break up but can apply tension force to some components and break them up (ceramic chip capacitor are a classic).
- Glued components may loose (if glue is too stiff).
- Electromechanical components may latch-up (permanent damage), especially relays.
- PCBs may break up, either externally or internally and traces may get damaged.
All these issues are usually addressed at design level, having the environmental specifications in mind. The specifications drive all the following:
- The selection of components and materials (reliable under vibration, no microphonics).
- The electrical design of the circuit (tolerance to upsets).
- The mechanical design of the PCB (higher clearance to PCB edges, etc.).
- The footprints in the PCB (bigger footprints for better solder joints wetting, etc.).
- The soldering process (reliable joints with good wetting, no cold joint, etc.).
- The mechanical interface between the circuit and its enclosure. (stiffening mechanical parts may be required to avoid resonant vibration modes).
- The mechanical design of the enclosure (to avoid vibration amplification).
- The mechanical interface between the enclosure and the place where it has to be mounted (to avoid vibration amplification and to damp vibrations and shock as much as possible).
As you can imagine, if you don't address all these things from the beginning, then you can only pray for it to work under vibration/shock and/or try to mitigate it by replacing some parts (ceramics) and trying to reduce the mechanical energy your board is getting.
For prototype devices (which should last several months and are OK to fail occasionally), it is usually enough to mount the PCB on rubber inserts instead of bare screws). This greatly reduces peak accelerations your PCB sees, while keeping your prototype accessible and requiring minimum design changes:
For commercial products which should last 10 years or more, you want something more robust. I have seen automotive electronics where parts are assembled (or even the whole device is potted) with RTV silicone, which greatly helps with both mechanical sturdiness and heat transfer.
Board flex will crack ceramic capacitors and other brittle parts, like SMD resistors (which have a ceramic body). It will also crack BGA solder joints, if any.
Lower-intensity board flex and vibrations will make X7R and related high-K ceramics act piezoelectric. This means any high impedance nodes (like a sensor, or a voltage reference) filtered by a High-K ceramic capacitor will be very noisy. Using other capacitor types solves this.
Now, there are several strategies to mitigate this:
- Use flex-resistant components (like J-lead caps, flex-termination caps, SMD parts with leads, or even thru-hole parts)
- Minimize flex and shock reaching the board itself
You can make your board stiffer by mounting it inside a solid enclosure or on a metal plate, with lots of screws. Thermal management issues must be considered.
You can orient the board in such a direction that the main shock force will be parallel to it. Forces perpendicular to the board will make it flex more than forces parallel to its length.
Now that you have a rigid metal-backed board, you can mount it (or its enclosure) to the chassis using rubber shock-mounts, or even rubber shock washers.
The idea is to avoid resonance, and create a mechanical lowpass filter without a peak in its frequency response.
One thing you'll notice in many cars is that the electronics are often mounted in an enclosure, and the enclosure is mounted to the vehicle. The enclosure may be metal or plastic, metal enclosures are connected to chassis ground and used for devices sensitive to electrical noise (e.g. radios, GPS/tracking devices).
Quite often both the mounting of the electronics to the enclosure, and the mounting of the enclosure to the chassis have some degree of damped flexibilty in them. A pcb may for example be mounted on rubber bushes into a box that's attached with plastic clips. Some retrofit devices use sticky foam pads to mount the PCB. The box also absorbs some of the large-scale flexing in the PCB.
Prototypes are often worse than production devices, as they may use bigger boards and heavier components/connectors (even through-hole).