three words that will haunt you till you've replaced so many warranty coils that your ready to cry, volitle organic compounds (v.o.c.'s). what they need is a media filter with a uv light and thats about all you can do. you've changed brands and your still having leaks its not the equiptment its the enviornment its in.
and of course this didnt become a problem till the new units were put in. you did almost hit the nail on the head with thinner copper but the problem is now the mfgs have hit the balance point of efficency vs. relieability.
according to bryant/carrier this is the cause for most i/d coil leaks but they also state that cloride compounds are the most harmful, but if i recall correctly back in the day every aerosol propellent was a cfc so why is it happening now??? too thin copper for the high eff. ratings. carrier has come out with the tin plated coils and york has their own way of combatting v.o.c.'s.
a uv light w/media filter is your best option till lennox comes out with a way to protect their coils.
the thing to remember is this will only be true in some residence not all. the ones this will work the best is premature coil failures, ie 2-7 yr. range. i dont know why but there are certain condtions that make this worse than others.
i work for a bryant f.a.d. service co. and believe me im tired of doing warranty i/d coils but all we can do is inform the consumer and let them make their decision.