Before the Sofer begins writing the Tefillin, he must verbally declare that he is writing the text for the sake of Kedushat Tefillin (the sanctity of Tefillin). Additionally, each time he writes any of the Names of Hashem, he must verbally declare that he writes it for the sake of Kedushat Ha’Shem (the sanctity of the divine Name). This applies to all of G-d’s Names, including "Havaya," "Elokim" and "Ado-nai," and all their various forms. (According to some opinions, this declaration is required even before writing the name "Ehyeh Asher Ehyeh" in a Sefer Torah.)
Some Poskim raised the question of whether we may apply the Halachic principle of "Bittul Ba’rob" – that a minority component of a mixture can be considered "nullified" by the majority – to the Names of Hashem in Tefillin. Meaning, if the majority of the Names were written with the required intent, but a minority of the Names were not, perhaps we may employ the concept of "Bittul" to consider the Tefillin valid. The accepted conclusion, however, is that the rule of "Bittul" does not apply in this context, and thus if even a single Name of Hashem in the Tefillin was written without the required intent, the Tefillin are invalid for use. Furthermore, a Sofer cannot make a retroactive declaration of intent. Meaning, if he wrote one of the Names without the required intent, he cannot then declare when he writes the next Name that he intends for the Kedusha to extend to the previous Name. Therefore, once a single Name of Hashem is written in the Tefillin without the required intent, the parchment is invalid and cannot be used. As it is forbidden to erase the Name, there is no possibility of rectifying the parchment, and it cannot be used for the Tefillin.
As we’ve said numerous times, it is imperative to purchase Tefillin from a qualified and G-d-fearing Sofer who can be trusted to follow all the rules and guidelines governing the writing of Tefillin. One obviously cannot determine what the Sofer’s intent was by looking at the script on the parchment, and we must trust the Sofer that he wrote each and every Name of Hashem in the text with the required intent. Therefore, one must ensure to buy Tefillin only from a Sofer with a reputation of proficiency and integrity.
Summary: Before a Sofer begins writing Tefillin, he must verbally declare his intent to write the Tefillin for the sake of Kedushat Tefillin, and before writing each instance of Hashem’s Name, he must verbally declare his intent to write the Name for the sake of Kedushat Ha’Shem. If even a single Name is written without this intent, the parchment is invalid for use, and there is no possibility of correcting it afterward.
