ESD Answers

What are the conditions for electrostatic discharge? Some texts say " cold & dry " and some say " hot & dry "? Is there an optimum humidity?

Actually, static electricity is generated under all conditions where two materials are in contact - the question is, what happens to it? If the static charges can dissipate faster than they are generated, then we don't see electrostatic charge build-up. One factor that influences this is the humidity of the air.
Plastics and other insulators don't dissipate static charges quickly. However, when the air is humid, a thin moisture layer forms on the surface of many insulating materials, and this helps the charges dissipate. So, dry air conditions generally show greater levels of electrostatic charge build-up.
There's no specific optimum humidity but anything over about 30% rh will help prevent electrostatic charge build-up. Don't rely on it as your main prevention measure, though.

I take coffee beans from my freezer and scoop them into a grinder from where they are ejected into a plastic container. Some of the grounds always fly out of their container, repelled by static electricity!

It's likely that the beans from the freezer are very dry and charge highly when ground up. When fine ground, the high charge can easily make the particles fly apart by mutual repulsion. Try tipping them into a metal container, and the effect should be much reduced!

We transport pcb assemblies in ESD trays, accompanied by an A4 paper document. What are your views of the possibility of ESD damage being caused by laying sheets of A4 paper over the assemblies?

Paper is a very variable material and can be highly insulating when dry. Documents can be highly charged, especially if they have just come out of a laser printer or copier! I recommend you don't place paper documents inside ESD protective packaging with an ESD sensitive device, or bring them unprotected into the EPA. You can reduce any risk from charged documents by placing them within a dissipative ESD bag.

Is it possible to ground my car in some way so that I won't be shocked every time I get out of it?

Actually, the car is grounded - it is you who are charged and shocking the car! The charge is generated between your body and the car seat - the charges are separated when you get out and your body voltage can rise to over 10kV in the process! Try holding a metal part of the door frame as you get out - the charges can then dissipate as they are separated. Most car tyres are sufficiently conductive to ground the car body with no problem. Occasionally a highly resistive ground surface can prevent this.