Here is some brief info I found on a great site,
http://www.learntobreed.com/
Your stud should be:
1. show quality
2. a champion
3. have both parents and grand parents that are champions
4. free from hereditary disease
5. parents and grand parents tested and free from hip dysplasia
6. mature enough to breed
7. tested today as free from brucellosis
There are also other genetic tests that are involved with yorkies, such as luxating patella and liver shunt.
Also, be you'll have to be prepared for temperment issues and changes after breeding.