I have a 1946 Zephyr that finally needed a reset - the neck joint heel had visibly gapped and the fretboard extension (with that infamous Epiphone truss rod adjuster) had tipped down to where the truss rod was sitting on the top. I was not yet building archtops so I took it to a local luthier. He did the job, and all was fine for around 6 months when the problem re-occurred. Lifetime warranty he said, so he re-did the re-set. Failed again in 6 months, warranty expired he said. This was in 2015. Now, with six archtops under my belt I felt I had the chops to do a reset on the old girl myself. Out to the shop, take the strings and body hardware off to just take a quick look - I don't have a steaming or hot needle setup since I don't do repairs. Neck was definitely loose - wiggled back and forth easily. Three minutes later the neck was off and I could see part one of the problem - the shims were nothing more than full length of the cheek business card strips, separated/delaminated in half. The outside of the card was still glued to the wood on both mortise and tenon, the card had delaminated down the middle. Dismayed that a $600 neck reset job was done with a couple of strips of business card on the one hand, happy to see that failure and no signs of worse (like a cracked neck block or separate/delaminated plates, or worse...). So to start the repair I scraped the old paper off, and it's glue, and made up a couple of maple shims the same thickness as a business card - but solid and hard. They showed part two of the problem - the joint was tight at the top, just under the fretboard, but very loose at the bottom - not firming up at all, the taper was wrong. The neck could rock back and forth on the pivot formed by the tight fit at the top. So that was probably why the first failure happened - the joint was simply not fitted all the way down to the bottom of the joint, which is where the greatest stress is. I just went ahead and worked the fit of the joint to were it locked in place with light hand pressure and there was no rocking at all, and after checking alignment and bridge height I glued it up and clamped. I still have to fix the damaged finish at the heel, I didn't manage to get the cheek fit perfect, but I guess I'll let this one sit for six months just to prove it's now done right...
I would probably never use business cards as a shim, but the main problem was the bad fit of the tip of the heel. That's the one thing that needs to be really tight.
Hardwood for shims is my prefered method, but paper business cards are probably a better shim material than soft spruce or even mahogany. One hidden positive with paper is the fact that it will delaminate when you need to do a reset even if the glue still holds! Hmm.