Is expiation obligatory for someone who swore not to masturbate, but then masturbated unintentionally due to itching?

It is unlikely that masturbation would occur unintentionally. Therefore, you must offer an expiation for breaking an oath (kaffarat yameen) and repent to Allah by regretting what you did, abstaining from it, and resolving not to return to it. However, if masturbation were to occur by mistake and without intention, there would be no breaking of the oath (hanath) and no expiation, as Allah Almighty says: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended. And ever is Allah Forgiving and Merciful."