Hesech Hada'as by Tefillin - Brochos 23b
http://kollelbokerbeachwood.com/articles/250/1/Hesech-Hada039as-by-Tefillin---Brochos-23b/Page1.html
By Avrohom Adler
Published on 03/18/2009
Rosh - chidush in the explanation as to what constitutes hesech haddas by tefillin.
Gemora - 23b