A package on package structure includes a first substrate having a first region and a second region, a bump formed on the first region of the first substrate, a first semiconductor die bonded to the second region of the first substrate, and a semiconductor die package bonded to the first substrate. The optical interlock has an emitter for emitting a signal, a detector for detecting the emitted signal, a reflector for relaying the emitted signal to the detector, and an actuator for selectively positioning the reflector in two or more positions. The method includes placing at least two chips on a metallic layer, depositing mold material on the metallic layer and between the chips, and selectively removing a portion of the mold material from the metallic layer to selectively expose a portion of the metallic layer. The grid is created automatically, without the need for expert human interpretation, can provide visualization of the four dimensional hazard volumes and may be used directly by decision support tools without the need for expert human interpretation. It thus produces a high quality magnetic device without manual intervention and can be part of a fully automated process to fabricate a circuit assembly.