I am not sure what you mean with strong and weak sound, but anyway, for all vowels which are pronounced in the front part of the mouth (as e, i, ie, ei, eu, äu, ö, ü), you get the 'ich'-sound. That means, also 'ch' is pronounced in the front. The other vowels (a, o, u, au) are pronounced in the back part, close to the throat, so also the 'ch' is pronounced there.

I'll .. thanks man

Check out this video :) https://www.youtube.com/watch?v=PtfoPeniRM0