I too have seen where sometimes people suggest that idea here BUT I highly doubt that anyone has ever made one for an 850 or any other car for that matter. If I'm wrong, post proof to the contrary here.
I'm quite sure that there are very good reasons why every evaporator I've ever seen (many) is always aluminum by the factory and I'm sure that it has to do with heat absorbtion as copper probably doesn't work as well.
I know that many radiators and heater cores are copper but I have yet to actually see a copper evaporator for any car.

Now.....IF you want a new replacement OEM evaportator to last longer than normal, Volvo does now make an evaporator "kit" which includes a new accumulator, pollen filter kit and blower motor (brush) air restrictor that's all designed to help make it last BUT it's more expensive to buy/install than an evap only and few people want to spend more money on their AC when given that option as most are probably thinking that they'll have another car by then. It'd be a great idea IF you're the type who keeps a car a very long time.