It is forbidden to speak, signal with one’s eyes, motion with fingers, or any other means, between putting on the shel yad and the shel rosh. If one spoke he must repeat the bracha of להניח before reciting the bracha of על מצות תפילין, but if one made a silent signal, he does not repeat the bracha. Also, one may not respond to kaddish, kedusha, borchu or any other bracha between the shel yad and shel rosh; rather listen to the chazzan. Similarly, one should not allow any unnecessary delay, for example walking, between putting on the shel yad and shel rosh.