When designing a printed circuit board (PCB), one quickly needs to consider the stackup configuration and the layer arrangement used for the design in progress. When transitioning to production, the question of material selection for the stackup, particularly for the insulating (dielectric) layer, inevitably arises.

Most of the time, PCB designers use a stackup configuration recommended by the PCB manufacturer based on what they have in stock.

As a result, some designers make these choices without fully understanding the pros and cons of selecting one prepreg type over another. However, it can be both interesting and useful to delve into the specifics of the materials used when configuring a stackup.

Let’s explore the key characteristics of prepregs and their importance when it comes to PCB design.

What is a prepreg?

The term “prepreg” is derived from “pre-impregnated,” which literally means pre-saturated in English. In the context of composite materials, a prepreg is a fibrous material, such as fiberglass or carbon fiber, that has been pre-impregnated with a curable resin. This resin can be thermosetting or thermostable, depending on the specific application requirements. The resin saturates the fiber and, once cured, holds the desired shape while protecting the fiber. The fiber, which can be glass, carbon, or other materials, acts as reinforcement and gives the prepreg its mechanical strength.

Key Characteristics of Prepregs

Glass Transition Temperature (Tg)

This indicates the temperature at which the prepreg transitions from a rigid state to a more flexible one. For electronic applications exposed to high temperatures, a high Tg is preferable to avoid deformation of the PCB’s internal layers.

Example: Tg = 150°C (at 150°C, the PCB may deform).

Decomposition Temperature

This temperature indicates when the material begins to decompose. A prepreg with a high decomposition temperature is considered to have excellent thermal stability.

Example: 350°C (at 350°C, the PCB may begin to decompose).

Time to Delaminate

This is the time it takes for the PCB layers to start separating under heat. A longer delamination time is preferable as it indicates better thermal resistance.

Example: 10 minutes at 260°C.

Z-Axis CTE (Coefficient of Thermal Expansion)

The CTE measures how much the material expands or contracts with changes in temperature. A low CTE, both on the X/Y (surface) and Z (thickness) axes, is ideal as it reduces the risk of cracks or deformations in the PCB.

Example: 50 ppm/°C.

Thermal Conductivity

This characteristic indicates the material’s ability to conduct heat. Good thermal conductivity allows for efficient heat dissipation, essential for high-power circuits.

Example: 0.3 W/m·K.

Permittivity (Dk) and Loss Factor (Df)

These values are crucial for the electrical performance of the PCB. A high Dk can slow down electrical signals, while a high Df can result in energy loss.

Example: Dk = 4.5, Df = 0.02.

Volume Resistivity and Surface Resistivity

These metrics indicate the material’s resistance to electrical conduction. High values are preferable as they reduce the risk of short circuits.

Example: Volume Resistivity = 1 x 10^15 ohm-cm, Surface Resistivity = 1 x 10^13 ohm.

Dielectric Breakdown and Electric Strength

These characteristics measure the material’s insulating capability. High values are essential to avoid electrical failures.

Example: Dielectric Breakdown = 500 V/mil, Electric Strength = 2000 V/mil.

Mechanical Properties

These indicators show the material’s toughness and flexibility, which are critical for understanding how the PCB will withstand mechanical stress.

Example: Flexural Strength = 400 MPa, Tensile Strength = 500 MPa, Young’s Modulus = 20 GPa, Poisson’s Ratio = 0.3.

Arc Resistance

Measures how long a material can resist the formation of an electric arc before it erodes the material’s surface.

Example: 120 seconds.

CTI (Comparative Tracking Index)

Measures a material’s ability to resist the formation of conductive paths due to electrical discharges in the presence of contaminants or moisture.

Example: CTI = 800.

Moisture Absorption

Refers to the amount of water or moisture a material can absorb when exposed to a humid environment.

Example: 0.2%.

Flammability

Describes a material’s ability to ignite or burn when exposed to a heat source or flame.

Example: UL94 V-0 class.

Did you know all these parameters related to prepreg selection?

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