Circuit reliability, especially tin solder, under wide temperature range -40 degree to +85 degree
Much depends on the specific solder you expect to use.


  If all the components are specified to work at -40 degree to +85 degree, can I say the circuit will surely working at that environment? 


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.