The Printed Circuit Board Primer

Drilling And Plating

If the manufactured PCB is a multi-layer board that contains buried or blind vias, each layer has to be drilled and plated before they are laminated together. If not, the layers can be laminated together first.

After drilling holes in the boards, which is done by machines according to the drill-files, the inside of the holes must be 'Plated' (Plated-Through-Hole technology: PTH). This metallization of the holes' inner walls creates an electrical connection through the board and to all conductors in the inner layers that the holes touch. Before the plating can start 'Drill Smear' on the inside of the holes must be removed. This is a resinous epoxy coating caused by the heat from the drilling, and must be removed because it covers the conductors in the inner layers. Both Drill Smear removal and Plating are done in chemical processes.

Lamination Of Multi-Layer PCBs

Single layers must be laminated together to form a multi-layer PCB. Lamination involves gluing the layers together with an insulating film in between. For holes that go through all the layers the drilling and plating must be repeated. The conductor pattern of the two surface layers of a multi-layer PCB is often created as described above after all layers have been laminated together.

Solder Mask, Silk Screen And Plating Of Edge Connectors

A Solder Mask is applied over the wires on the outer layers such that solder will not attach outside the solder pads. The Silk Screen is printed on top of this mask to label the component locations. It is important that the silk screen does not cover any solder pads or edge connectors, something that would respectively reduce the solderability and electrical connection. The edge connectors are often plated with gold to ensure a high quality electrical connection when they are inserted in a slot.

Testing

Testing the PCB for short circuits and breaks (broken connectors) can be done both optically and electrically. Optical tests involve scanning the layers to detect defects, while electrical tests most often are done by a 'Flying-Probe' that verifies all connections. An electrical test is more reliable when searching for short circuits and breaks, but the optical test can more easily detect incorrect spacing between conductors.

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