If you have not created your custom field, go here for instructions. Do not make the field required at this level. Take note of the API name of the field. For example, if the field is Capacitance, the API name will likely be Capacitance__c.
Go to Lifecycles tab
Click Lifecycle Name
Go to Phases
Click Edit by Phase Name
Go to Item Phase Details
Enter API name of field, such as Capacitance__c in Required Fields
If entering multiple fields, separate with a comma
Tip: This field will be required only for the category and phases(s) you specify. For example, if you have added the custom field "Capacitance" to only one category, "Capacitor", and only specified it as a required field for the "Production" lifecycle phase, then the field will be required for the union of the two: only Capacitors in the Production phase.