Please don't use a collar on your yorkies. Buy a good harness. This will lessen the possibility of injury to your yorkie's trachea. Yorkies have a history of having weak tracheas and it is always better to be safe than sorry. Trachea damage can occur to a yorkie wearing a collar when being pulled at on a leash. A proper harness will eliminate this stress to the trachea.
TRACHEA COLLAPSE
There can be a weakness in the trachea and if it collapses the dog cannot breathe. Emergency surgery and inserting plastic devices or coils can be done in some cases to keep the dog alive. This can be caused by severe hacking from a cold or it can be a congenital thing.
Symptoms -- A loud honking, dry cough. Gasping, unable to breath. http://www.shooterdog.com/alexfaq3.htm#COUGH