My answer is more for obsessively checking your first (or second) personal "low speed" PCB rather than verifying your design.
- Make sure your DRC is checking for off page connectors
- Print out the datasheet pages for each IC/transistor etc and go through with a highlighter and verify every pin against the pin on your schematic.
- Do the same obsessive check with your layout
- Check all three pin devices like transistors against their diagram in the datasheet make sure that the pin numbers on your PCB are correct. It's easy to do these SOT-23 style devices wrong ( I usually draw a diagram for my layout guys).
- Do a similar check for polarized components, did you get diodes correct and are they marked which way they go in? What about polarized caps?
- Is this a four layer board? No? It probably should be :) signal-gnd-power-signal Even at low speeds this will help you. Sometimes you just gotta go 2, but do it because you know the consequences or that it won't hurt you.
- Open your gerbers/drill in a gerber viewer like the free GC-Prevue, now print them to a laser printer 1:1. Order your more complex parts and see if they fit on paper. Is it perfect? No. Will you catch using the wrong footprint or wrong spacing? There's a good chance of that.
- What did you do for power did you verify your regulators can supply enough current? How about how much heat they will burn off trying to do it? How about power dissipation for a linear regulator or switchers fets. You want to make sure you don't burn them up. We could go deep into the power section but I'll leave it at that.
- Sounds like your design is cost conscious, consider only mounting parts on the top of the PCB. It will save a whole step during automated assembly if that's where you hope to get to, and that will lower the price.
- Make sure your gerbers files are clearly labeled for the fab house, a drawing usually helps showing what layer goes where, some people go as far as labeling the layers in copper to make sure it's done right.
- If possible have someone else review your schematic and pcb, peer review, peer review.
- Have your 30 Gauge rework wire handy for when the boards come back, I like to use blue for rework :)
The list goes on but I'm tired and I hear the baby crying :) Hope some of that helps.
Some useful information about different types of solder: NIST Metallurgy
The main important tables are 1.12 (coefficient of thermal expansion/elastic properties of leaded solders) and 1.14 (tensile/shear strength of leaded solders).
I believe the document has information for lead-free solders as well (that is what it is called after all), I didn't look too hard for these.
The key properties for 63/37 leaded solder:
Coeff of thermal expansion:
$$
\alpha = 24 \frac{10^{-6}}{K}
$$
Elastic Modulus (I'm using the 20 degree figure, it will be slightly higher near 0 degrees, not exceeding 38.1 GPa at -70 degrees):
$$
E = 30.2 GPa
$$
Tensile strength:
$$
\sigma_{max} = 56.19 MPa
$$
The worst case scenario is if the solder is mounted onto something completely rigid. Suppose we were to take the 0 stress state as room temperature (25C).
The contraction due to thermal expansion is:
$$
\epsilon = \alpha (25C - 0C) = 0.0006
$$
And the appropriate tensile stress is:
$$
\sigma = E \epsilon = 18.12 MPa
$$
This is well below the tensile strength of the solder.
However! Even better is that the PCB board itself will contract with the solder as it cools down. Depending on the actual layup direction, this closely matches the 63/37 solder CTE (~20e-6/C for the primary direction), so the actual stress will be lower.
tl;dr: you'll be fine. You might have to worry more about moisture/condensation, as well as having components which are rated for below 0C operation instead of worrying about solder joints cracking.
Best Answer
This is just general stuff, you should really try to put a bound on the expected acceleration forces, the period and duration of those forces, thermal conditions, and expected angles of impact to get the information you need to make the design robust.
What is the most force that would be OK on a board with no impact hardening measures taken? (Am I worrying too much about a non-issue?)
This is very difficult to put a single number on, it depends on the types of components used and the direction/frequency of the hits.
Are there any design practices that should be followed for the PCB?
Lots of attachments to something solid. One of the most likely failure modes is the PCB flexing which can cause the solder joints on the PCB to crack causing intermittent or complete failure of the connection. I would try to keep the PCB as compact as you can while providing as much attachment to something that won't flex (steel enclosure) as you can. The smaller the PCB the smaller the 'overall flex' of the board. Something like 4+ layer design with solder copper power and ground planes should also add to the rigidity of the PCB but can cause additional thermal flex. Depending on what your needs are, there are specialized PCB substrates that are more rigid than your stock off the shelf FR-4, such as substrates which employ carbon fiber composites vs fiberglass.
What are the weak points in a design that lead to mechanical failure?
Are there parts that should be avoided for a more robust design?
See the list above but keep all parts as lite and as close to the PCB as possible.
At what force levels should I start worrying about the safety of the parts themselves?
Again this is hard to put a number on. If the device is getting hit 'edge on' to the PCB than your concern is lateral shear forces. What force causes a problem there is dependent on the IC. A large heavy IC with few, small attachments to the PCB is probably the worse case. Maybe a tall pulse transformer or something like that. A lite weight, short IC, with many attachments is probably strongest. Something like a 64pin QFP, even better if it has a large center pad. Some useful reading on this topic: http://www.utacgroup.com/library/EPTC2005_B5.3_P0158_FBGA_Drop-Test.pdf
Some parts may be internally damaged by high G-forces, this would be on a part by part basis but would mostly be limited to devices with movable internal parts. MEMS devices, transformers, mag-jacks, etc, etc.
Comments
Have you considered using 2 boards? One small board with the accelerometer which is actually stiffly attached to the enclosure and a second board with the rest of the electronics on it which can then be mounted with a shock absorption system. The shock system could be as simple as rubber supports or as complex as the systems used in hard drives depending on needs.
Your going to need a pretty fast processor and a pretty fast, wide range accelerometer if you want to get accurate measurements of impact events such as getting hit with a hammer.