Yew sapwood needs to be a consistent thickness... it isn't necessary to follow a growth ring on the back as it is with some woods (Osage), but you need a smooth even layer with no discontinuities (about 4mm thick). If it is a small localised problem you may be able to patch it, if the whole back is iffy, it may be easier to rasp/plane the back down flat and glue on a backing of 1/4 sawn Ash or Hickory. or a backing of Bamboo or maybe Hazel.
This sort of job is tricky and requires patience, but it is a good learning exercise and can produce a long lasting repair.
It may be possible to do a repair by binding the area with fine linen thread and then touching it with low viscosity superglue (available on E-bay) this will wick into the thread and make it V solid and strong.
There are plenty of examples on my blog (Bowyers Diary) is you search for back patch. backing a bow etc.
This post shows a tiny crack in the back of a bow which I repair, (detailed in subsequent posts)
https://bowyersdiary.blogspot.com/2019/04/busy-time.html
The back of a bow needs to be good and clean as it's the back that stops it exploding... if the belly fails you will just get a lot of set or compression fractures (chrysals), but it will still shoot... if the back fails... BOOM
Del