What’s the proper soldering iron temperature for standard .031" 60/40 solder?
There is no proper soldering iron temperature just for a given type of solder - the iron temperature should be set for both the component and the solder.
When soldering surface mount components, a small tip and 600F (315C) should be sufficient to quickly solder the joint well without overheating the component.
When soldering through hole components, 700F (370C) is useful to pump more heat into the wire and plated hole to solder it quickly.
A negative capacitor lead to a heatsinking solid pour ground plane is going to need a big fat tip at a much higher temperature.
However, I don't micromanage my soldering temperature, and simply keep mine at 700F (370C). I'll change the tips according to what I'm soldering, and the tip size really ends up determining how much heat gets into the joint in a given period of contact.
I think you'll find that very few soldering jobs will really require you to change your tip temperature.
Keep in mind that the ideal situation is that the soldering iron heats up the joint enough that the joint melts the solder - not the iron. So the iron is expected to be hotter than the melting point of the solder so that the entire joint comes up to the melting point of the solder quickly.
The more quickly you bring the joint temperature up and solder it, the less time the soldering iron is on the joint, and thus the less heat gets transferred to the component. It's not a big deal for many passive or small components, but it turns out that overall a higher tip temperature results in faster soldering and less likely damage to the component being soldered.
So if you do use higher tip temperatures, don't leave them on components any longer than necessary. Apply the iron, apply the solder, and remove both - it should take just a second or maybe two for surface mount, and 1-3 seconds for a through hole part.
Please note that I'm talking about prototyping, hobbyist, and one-off projects. If you are planning on doing final assembly with the iron, repair work for critical projects, etc, then you'll need to consider what you're doing more carefully than this general rule of thumb.
Are you planning on using PCB fab house and then hand-assembling?? If so, 0402 might be a tad ambitious if you've never done anything like this before.
Each board house will have a list of specifications of what they can make. Some have two sets of rules or more - rules that can be used for fast and cheap, for fast and expensive, and for anything you'd like, but it gets slow and fairly expensive.
http://www.pcb-specification.com/uk are the rules for pcb-pool.com--you'll see them if you click "Design Rules". They give you files to use to set up design rules in Eagle and in Altium, two popular schematic and board layout tools. They'll even include a free metal stencil for solder paste.
If you're planning on using a fabricator, make them earn their NRE's! Send them your design, mention your concerns, and ask them for advice. It won't cost you a dime more to use their expertise in this way. If you plan on a pick-and-place process, be sure to ask them about where they want their fiducial marks.
Lastly, don't be afraid to post the board on this site, and ask for a quick design review. Be sensible, though, and make sure you provide good detail for the problem areas you suspect, and maybe tightly focus the question on that.
Best Answer
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.