I've said this before, and I'll say it again: How shiny the solder looks is not a reliable indication of the quality of the solder joint.
Even before lead-free solder the shine wasn't a reliable indicator. More reliable than lead-free solder, but not reliable enough for most people.
Here are some things that I look for when evaluating the quality of the solder:
- Consistency: Do all the solder joints look the same? If not, it indicates a variation in the solder process and thus it is more likely to have problems.
- Wetting/Wicking: Did the solder melt evenly and flow onto the mating surfaces, or does it look like the solder beaded up like water on a newly waxed car? Solder that is beaded up could have problems, and hidden cracks under the bead.
- Smooth finish: I'm not asking if it is dull or shiny, but rather is it smooth or are there lumps in it? Lumps are a sign of uneven or incomplete melting.
- Conductive Flux: This one is rare, but important. Some types of flux are conductive, but not everyone is aware of this. Sometimes a board will be reworked with conductive flux but the flux will not be cleaned off correctly (water for water soluble flux, etc.). Check that the proper flux was used in the proper way. Note: Some flux leaves a residue and this is OK as long as the residue is not conductive even though it might look bad.
- Cracked Solder Joints: Often this can only be seen using a microscope, and sometimes not even then.
The thermal time constant of 1 square of PCB GROUND plane, of size 10cm (4"), is 96 second. The time constant of a square meter of foil is 9,600 seconds; of 1cm is 0.96 seconds; of 1mm square is 0.0096 seconds (9.6 milliSeconds).
If we model the heat flow as from one edge of the foil, thru the 4" of foil, and exiting the opposite edge, then that square of foil has a Thermal Resistance of 70 degree Centigrade per watt.
Thus worst case temperature rise, with 1 watt of heat generation spread along one edge of the PCB, and the heat flowing only through the foil (no heat exiting into the air), to exit into the air (or thru metal mounting posts into the case) at the opposite edge, will be 70 degree C /Watt * 1 watt = 70 Deg C.
However, if the heat has to flow through long narrow traces to reach the Ground (or power) plane so the heat can spread out, then you can have much hotter local regions, and the slowly moving air flow will easily cool regions that are 100C or 150C hotter.
SUMMARY:
reach thermal equilibrium? yes, with one TAU being 96 seconds.
assume uniform heating on the PCB? at 70 degree C per watt of heat flow? not likely. You need at least ONE plane, to really spread out the heat, or a bunch of WIDE traces exiting your hot-spots.
That 70 degree C per watt is PER SQUARE. A trace 2cm long and 2mm wide has 10 squares, thus is 700 degree C per watt. Use a plane, or wide-and-short traces. And you need to dump heat THRU the FR-4 epoxy fiberglass substrate, to move heat into the plane.
Draw some sketches of the PCB heat flow, using a bunch of resistors to constrain the heat movement.
Best Answer
Much depends on the specific solder you expect to use.
Not necessarily. The operating capability of the parts is usually limited by certain factors (particularly microcontrollers). You will need to evaluate any self-heating effects to ensure they are in the range specified by the part manufacturer. That said, you will probably be ok with parts rated for those ambient temperatures.
Note that many passives (resistors and capacitors) need to be derated; many resistors are linearly derated abive 70C. In addition, thermal stresses due to CTE mismatch have to be evaluated. Most FR4 PCBs are between 14 and 18ppm in the X and Y axis, but ceramic devices (such as MLCC capacitors) are 7ppm and this can be an issue for large parts (above about the 2012 case size).
If you are using lead free solder then you will need to take some precautions depending on the expected life of the product. Note that lead free solder (particularly SAC varieties - SnAgCu) degrade galvanically, so the notes on conformal coat below take on significant importance.
The subject of tin whiskers is extensive, and the problem is that nobody is really sure of what the root cause really is.
Components with fine pitch leads are more susceptible, although conformal coat can help to impede it somewhat under some circumstances. There is no proof that lower temperatures cause more issues with tin whiskers; the evidence is scattered and contradictory. The only really known thing is it occurs.
If you are going to be operating at -40C to +85C then conformal coating is not going to be a nice to have feature; if you have humidity then it will be a requirement because as the temperature comes up through the dew point the PCB will get very wet (I have seen equipment being tested in chambers where water is literally running off the cards).
Without knowing what industry you intend to put your product into, I cannot be specific about what environmental testing you may need to conduct, but I would expect that ESD and radiated emissions would be the absolute minimum.
You may need to actually put the equipment in an environmental chamber to prove the temperature range is valid (this depends on just where the equipment is going to end up).
Note that even the PCB layout can have an impact on whether something will operate across temperature.
The most stringent testing (again, industry dependent) can be required as proof the equipment can operate and may need qualification testing. This can be an extreme set of tests that you probably will not need to go through.
I would expect your equipment to have some subset of those tests to show it will operate in the expected environment.
If you have nobody on staff with the necessary skills I would strongly suggest getting someone in (at least temporarily) to guide you through what can be a very rigorous process.