What you need to know about PTH and NPTH holes?

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.

2 layers adhesiveless 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.

4 layers FPC 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.

Holes

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 sales@bestfpc.com

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What should we pay attention to EMI shield design?

Electromagnetic interference (EMI) is associated with every electronic device we use nowadays. If you turn on your radio set and TV simultaneously, you will experience the noisy disturbance from TV interfering with the radio signal and vice-versa. We can also experience this when we board a plane and are asked to switch off the electronic devices by the crew. This is to avoid interference of mobile and electronic device signals with the plane’s navigational signals. This is the reason why EMI/EMC study and analysis is important. Does your product’s radiation disturb other devices present nearby?  

EMI Shielding Design Challenges

As we all know, the flex circuit EMI shielding added will create multiple design challenges that require careful review to ensure a successful part number. All EMI shielding will increase both the total flex circuit board thickness and cost. The thickness increased is most often the critical issue. The normal EMI shield thickness is 22um, but we also has 10um thickness EMI shield. It can easily lead to the bending effect get worse. This creates a reliability/mechanical breakage concern. The added cost is also should concern. The

Shielding is often combined with other electrical requirements; the most common is controlled impedance. This further increases the flex thickness and compounds the challenge of meeting both the electrical and mechanical design requirements.

The flexible circuit industry has multiple solutions that can be applied, which will eliminate both the absorption and or radiation of interference noise.

What should we pay attention to EMI shield design to avoid the interfering?

  1. Keep your signals separate. Keep high speed traces ( clock signals) separate from low speed signals, and analog signals separate from digital signals.
  2. Keep return paths short.
  3. Route differential traces as close as possible. This increases the coupling factor, bringing influenced noise into the common mode which is less problematic for a differential input stage.
  4. Use vias wisely. Vias are necessary because they let you take advantage of multiple layers in your boards when routing. Designers must be aware that they add their own inductance and capacitance effects to the mix, and reflections can occur from a change in characteristic impedance.
  5. Avoid using vias in differential traces. If you must, use an oval anti-pad shared by the two vias to reduce parasitic capacitance.
  6. Singled sided FPC EMI shield is not working, you need design it as a dual flex pcb at least or double sided or multilayers FPC. Below is a dual flex pcb with EMI shield stack up.
dual flex pcb with EMI shield stack up

7. Avoid sharp right-angle bends. Capacitance increases in the 45° corner region changing the characteristic impedance and leading to reflections. This can be mitigated by rounding right angles.

8. EMI minimum solder mask opening should be more than 0.8mm, and the solder mask area need to far away from the trace more than 0.2mm. Below design solder mask opening is less than 0.8mm, it will be not able to arrange production.

EMI open solder mask design issue

If you have any EMI shield design questions, warmly welcome you to contact us. Once we received your news, we will reply to you immediately..

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