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Blog / Prevent PCB Insulation Failure: PP Thickness & Stackup Guide

Prevent PCB Insulation Failure: PP Thickness & Stackup Guide

Posted:04:41 PM June 17, 2025 writer: Ivy

Customer Story

We have encountered a PCB case like below:

1. The previous project was a 6-layer board with 1.6mm thickness and 2oz copper, and there was an insulation breakdown problem.

2. It is suspected that the prepreg is too thin (0.077mm), resulting in insufficient interlayer withstand voltage.

3. Now we hope to find thicker prepreg and more reasonable stacking to improve insulation based on TG170 material.

Why does insulation breakdown occur?

Insulation breakdown is essentially: the medium (prepreg or core) between adjacent copper foil layers cannot withstand the voltage difference, resulting in electrical breakdown inside the medium or along the surface of the medium.

The breakdown position usually occurs in: high-voltage routing area, power layer to signal layer, power layer to ground layer, or adjacent routing in the area with voltage difference. PP (Prepreg) and Core are interlayer dielectrics, and their thickness, withstand voltage level, and distribution will directly determine the anti-breakthrough capability.

The thicker the PP thickness, the higher the withstand voltage.

Take the commonly used TG170 material as an example:

0.077mm prepreg, the withstand voltage value is low, suitable for ordinary digital signal isolation

0.12~0.15mm prepreg, the withstand voltage performance is significantly enhanced, suitable for power-ground layer, high-voltage signal isolation

The thickness is doubled, and the withstand voltage is basically doubled.

Try to use "sandwich stacking".

In a 6-layer board, the L3/L4 signal layer is placed in the middle, sandwiching the ground layer and the power layer above and below, which has better isolation. And the thickness of the upper and lower Prepreg is uniform, and the electrical performance is stable.

Try to avoid multiple continuous high-voltage areas.

If there are multiple high-voltage power layers, try to disperse them in different areas or different surfaces, and do not concentrate them in the middle 2 layers to reduce high-voltage superposition.

Suggestions for designers:

1. Confirm the voltage resistance/insulation distance requirements, such as whether there are specific AC/DC voltage resistance levels and minimum insulation distances (creepage & clearance).

2. 2oz copper + 0.077mm prepreg is indeed prone to insufficient interlayer voltage resistance in high-voltage or high-current applications, especially between adjacent power layers and signal layers or GND planes.

3. Recommend several more reasonable 6-layer 1.6mm TG170 stacking solutions, such as:

Use 0.12~0.15mm prepreg;

Increase the Core thickness, or use a dual Core structure (sandwich structure improves isolation);

Ensure that there is complete ground plane isolation between the key signal layer and the high-voltage area;

If you have impedance control requirements, remeber that thick prepreg will affect the impedance value, and help adjust the impedance together.

  • PCB
    Prototype
  • PCB
    Assembly
  • SMD
    Stencil

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Other Quantities:(quantity*length*width is greater than 10㎡)

OK

Layers:

2

Thickness:

1.6 mm
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