Is yashica cs-220 auto flash compatible with nikon d3100 dslr?

Is yashica cs-220 auto flash compatible with nikon d3100 dslr?

I don't think so.

I use to sell Yashica cameras in the 60's. I have not been able to find any digital cameras under the Yashica name. This leads me to believe that yours is a film camera. Film cameras and the flashes made for them have a higher voltage than today's digital cameras. Using a flash unit from the film camera days would most likely burn up the contacts on your Nikon DSLR. DO NOT USE YOUR OLD FLASH!

As a side bar, I sold all my studio flash units just because of this voltage difference, even though I used a $50 device called a "Safe-Sync" to lower voltage so it would be safe on my digital cameras.