> I'm getting ready to build a device that will
> need to survive significant physical shocks and accelerations. I'd prefer
> surface mount construction for the size advantage, but I'm concerned that
> flexing of the board and other physical stresses may cause surface mount
> parts to separate from the board.

talk to anyone involved in space electronics :)))

typically items for launch will go through a vibration test involving random
vibration for a minute at max acc around 9G. The vibration frequency profile
will be tailored according to the launch vehicle.

Essentially you will need to deal with someone who can do a vibration
assessment for you. Then you will be able to decide if you really need PCB
stiffeners. However if the PCB is less than about 4 inches square then a
stiffener is probably not required, but in view of your particular use I
would set out to have a mounting standoff support the middle of that area. I
say 4 inches square as that seems to be about the size each side of a
stiffener when we do our PCB design. However I do not know what the
requirements were for items like the mars landers which would be needing to
handle shock loadings similar to yours.

SMD components should be OK. larger components will need to be screwed down
or staked. Typically capacitors like CKR06 size would be staked using an
epoxy so they don't move under vibration. A shock from a landing like you
are talking of could shear off the leads of such a component.

I don't think you will have enough PCB movement to cause a problem with SMD
unless there is some other problem inherent in the component or soldered
joint. You should probably also look at using some form of conformal
coating, but rather than epoxy I would look at a silicone coating that is
removable to allow repair, or assessment of a failure at minimum.

Another possibility would be to put the electronics in a box which is
enclosed with some form of shock absorber. This could be something like
polystyrene or air filled bags. Essentially anything that will cushion your
20mph sudden stop into a broader pulse. And do a few test drops before using
the device in earnest. I would be tempted to fit an ADXL type accelerometer
to monitor and log the landing shock on at least one unit if you are
building multiples.

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