Plating copper through-holes also named vias is a requirement for double-sided and multilayer circuits. Becuase one copper layer to another copper layer stack up as below, the midlayers has a Polymide(PI) regardless of adhesive or adhesvieless stack up.
Copper is a conductor, Polymide(PI) is an insulator. Then we need to drill a PTH hole to make the copper layer to copper layer connect together. Below picture is a multilayers flex pcb with PTH holes.
NPTH (Non Plating Through Hole) refers to a hole without copper in the borehole wall. It is generally used as the positioning hole and screw hole of PCB. The hole diameter is usually larger than PTH. The easiest way to distinguish between PTH (Plating Through Hole) and NPTH (Non Plating Through Hole) in PCB is to see if there are any traces of plating on the borehole wall in the PCB. Please kindly check below picture.
The advantages of hole plating
The point of plated through holes is so you can use both sides of your printed circuit board and connect to other layers of the board. The plating on the through holes is copper, a conductor, so it allows electrical conductivity to travel through the board.
Non-plated through holes do not have conductivity, so if you use them, you can only have useful copper tracks on one side of the board. You cannot connect to the other side or other boards because there is no way for electricity to travel through. You can use non-plated through holes either to affix a PCB to its operational location or to mount components, but not to connect to other boards or the other side of the board.
The risks of hole plating
All products that contain printed circuit boards are subject to the thermal cycling effect. When we power them up, they heat up until powered down, which is when they cool. As the product is heating up, so is the printed circuit board inside it. Over time, with the board continually heating up and cooling down, the copper of the plated through hole can become fatigued and crack.
The thicker the copper plating the through hole, the longer it can go through this thermal cycling without cracking. Since this cracking will ultimately lead to failure, the life of the printed circuit board in the product is linked to the thickness of the copper plating of the through hole.
Plated through holes for IPC class 3 different types:
IPC Class 1: The least thick and the shortest-lasting, usually reserved for consumer electronics that are likely to become obsolete in a couple of years.
IPC Class 2: Longer-lasting, continuous-use holes for products like computers or copy machines that will be in frequent operation for five years or more.
IPC Class 3: The thickest and longest-lasting of plated through holes, for products that are expected to last ten years or more.
IPC Class I and II plated through holes require an average thickness of 20 microns, with spots no thinner than 18 microns, while IPC Class III holes require an average of 25 microns, with spots no thinner than 20 microns.
If you have any questions about PTH or NPTH holes, please contact us at firstname.lastname@example.org