sort (order) posts by category name (title)

Question

I have a parent category and 45-50 sub categories under that. I am trying sort posts by their child category name in archive pages on Wordpress.

By default, wordpress sort them by publishing date; i need them alphabetical (by their category name, NOT "post title")

Let's say A is parent category; i have 45-50 child category under A. And i need to sort posts which belongs to these categories. Order by child category name.