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The PCB manufacturing process includes etching a copper sheet plated onto a non-conductive substrate and drilling holes through it.
Slots and Cutouts | PCB Manufacturing and Design
Castellated holes, also known as plated half-holes or plated edge slots, refer to semi-plated holes positioned along the edge of a PCB.
6 things you need to know before creating a PCB slot
Sometimes, some PCB slots are non-plated due to a mechanical or electrical reason. Any slot that features properties that don’t meet the plated slot conditions is a non plated slot.
The way to produce Castellated-Hole-PCB | ALLPCB
General requirements: minimum plated slot width is 0.5mm. The length should be equal or more than twice the width. Rectangular slots without rounded corners are not supported.