You really need to find the root cause of the scratching as it could be necessary to contact a veterinarian for consultation. You should pay close attention if the scratching lasts for a long time, if you start to see blood or your dog whines for no apparent reason. One or more reasons are usually identified for the scratching:
Delicate skin: atopic, sensitive, prone to irritation or dry.
Allergies: allergic to materials put on their skin, to other shampoos or to insect bites.
Skin infections: scabies or pyoderma, among others.