All About Materials for HDI PCBs

Manufacturing HDI PCBs requires a variety of materials. They are necessary to create a layer stackup, etch the unwanted copper, apply a solder mask, apply a surface finish, and print the silkscreen. While the other materials are fairly standard, dielectric material for creating the multilayer PCB stackup must have different material properties like dielectric constant and thermal conductivity to suit the application. Rush PCB Inc. recommends making a thorough comparison of the properties of PCB materials when designing for specialized applications, as this helps to select the right base material for the board.

A multilayer circuit board will have a stackup that typically includes multiple materials. PCB manufacturing also requires different materials. The chosen materials in the stackup determine the signal and power loss, interconnection impedance, copper surface roughness, and temperature rise in the PCB. The designer must choose the right base material for the PCB stackup that will balance the performance in these areas.

To determine the best base material to use, the designer must thoroughly compare the properties of different PCB materials. Once they have determined the best material for their board, they can use the PCB stackup design and analysis tools in their PCB CAD software to create a suitable layout for high-speed HDI PCBs.

HDI PCBs Base Material Properties to Consider

To select a base material for their PCB, the designer must consider various material properties and verify if they fit into the board’s application. Some important properties they must consider for a high-speed PCB design are:

1. CTE or Coefficient of Thermal Expansion

This figure is a representation of how the board will expand as its temperature rises. Of course, the board will typically expand at different rates in different directions. However, the most important is the expansion along the z-axis or perpendicular to the larger surface of the board.

2. Tg or Glass Transition Temperature

This is the temperature at which the value of the board’s CTE suddenly increases with the increase in temperature. Beyond the glass transition temperature, the board material becomes increasingly more plastic.

3. Dk or Dielectric Constant

A ratio of the electric permeability of the base material to the electric permeability of free space or vacuum. Dk gives a measure of the amount of electric potential energy that an electric field can store in a given volume of the material.

4. Df or Dissipation Factor

Also known as loss tangent, Df is a parameter synonymous with insertion loss, which increases with higher frequencies.

5. Conductor Loss

This is related to the electrical conductivity of the conductors on the board and is different for alternating and direct current. For an alternating current, the conductor loss depends on the skin depth, which in turn, depends on the frequency of the alternating current flow. Copper surface roughness also affects the conductor loss, which increases the overall loss in the system, and changes the impedance of the interconnect.

6. Thermal Conductivity

This is the rate at which the base material can remove heat from the heat-producing source during operation. It determines the temperature rise in the board with respect to ambient temperature. Designers determine their thermal management strategy based on the thermal conductivity of the base material.

Fabricators are continuously discovering new materials better suited to high-temperature environments, HDI PCBs, and high-speed circuit boards. The properties mentioned above affect the way high-speed signals propagate within a substrate, how dispersion affects them, and how well the board is able to dissipate heat and withstand temperature rise and mechanical shock. As some materials are expensive, the designer must start with say, FR4, and evaluate whether it suits the application.

Standard Stackup Materials

The electronic industry builds printed circuit boards primarily using FR4 as the nonconducting material between layers of copper. FR4 is an epoxy laminate material reinforced with glass. The NEMA-grade designation FR4 represents the ratio of fiber to resin. It also indicates that FR4 has characteristics of z-axis expansion coefficient, glass transition temperature, shear strength, tensile strength, loss factor, dielectric constant, and flame retardant. For instance, FR4 is flame retardant, meaning it is suitable for safety requirements by not allowing fire to spread. It is adequately robust in humid environments and varying temperatures, increasing its quality of performance.

In a PCB, the major components are metal foil, reinforcement, and polymer resin, which is the dielectric material, and it can be with or without fillers. The manufacturer forms a PCB by placing alternate layers of dielectric, with or without reinforcement, stacked in between copper foil layers. Although manufacturers typically use epoxy as PCB material for a majority of high-speed designs, others use modified acrylates, cyanate ester, PPE, and BT.

The manufacturer places the laminates between layers of copper. These act as substrates with dielectric properties relevant to the application of the board. Designers specify the thickness of the substrate to meet the dielectric requirements for the application. Designers refer to the IPC-2221 standard that specifies dielectric constants for FR4 and other laminate materials.

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High-Speed Design and HDI Stackup Materials

The PCB industry uses the epoxy resin as its backbone. Manufacturers prefer epoxy as it is inexpensive, has strong adhesion both to itself and metal foils, and has desirable electrical, mechanical, and thermal properties. However, there has been a dramatic change in the basic epoxy chemistry over the years.

Now, manufacturers address specific shortcomings by selecting alternative resin-based stackup materials for PCBs. For instance, manufacturers commonly use BT-Epoxy for organic chip packages as it has adequate thermal stability while using cyanate ester resins and polyimide as they have low Df and Dk values.

Manufacturers also use PTFE and polyimide as thermoplastic resins, in place of thermosetting resins. Although polyimide in its thermoplastic form is relatively brittle, its thermosetting form is flexible and is useful for making flexible PCBs.

Materials with a low dielectric constant are ideal for high-speed HDI PCBs. The low dielectric constant allows high-frequency and high-speed signals to travel faster while lowering capacitive coupling between adjacent traces. Not only does this ensure good signal integrity in the boards, but it also reduces crosstalk between adjacent signal traces. This is significant, as HDI boards have traces closely packed into small spaces.

How to Choose HDI PCBs Materials

HDI PCBs carrying high-speed signals must use PCB material with low signal energy loss. This is possible with PCB material having a low dielectric loss tangent or dissipation factor, with a flatter dissipation factor versus frequency response curve. Four types of HDI materials are available for the designer to choose from:

Normal Speed and Normal Loss

These are the regular PCB materials from the FR4 family. They have dielectric constant values that are not flat with changes in frequency, and they also have a higher dielectric loss. That makes FR4 suitable for applications limited to a few GHz. Isola 370 HR is an example of this type of material.

Medium Speed and Medium Loss

These materials offer a flatter dielectric constant value with a change in frequency. Their dielectric loss is about half that for normal-speed materials. That makes these materials suitable for applications up to about 9 GHz. The material Nelco N7000-2 HT is a suitable example of this category.

High Speed and Low Loss

These materials have still flatter dielectric constant values with changes in frequency. Unwanted electrical noise generation is also very low in this type of material as compared to others. Isola-I Speed is an example of this type of material.

Very High Speed and Low Loss

These materials have the flattest dielectric constant values with changes in frequency. They also feature the lowest dielectric loss. Suitable for applications going up to about 20 Ghz, these materials are suitable for RF/Microwave applications. Isola Tachyon 100G is an example of this type of material.

Tips for Selecting HDI Materials

• For better signal transmission performance in high-speed digital applications, materials with low Dk, Df, and better SI or Signal Integrity features may perform better.
• For RF PCBs, materials with the lowest Df will be more suitable.
• When it is important to avoid signal attenuation, using a low-loss high-speed material may help.
• To reduce cross-talk, try a material with low Dk.
• Using BT materials may be more suitable for applications using microelectronic substrates with small-sized PCBs and compact layout features.
• The higher the performance, the costlier will be the material. Therefore, a tradeoff may be necessary.

Conclusion

Selecting the proper material is essential as this will affect the electrical performance of the board. Rush PCB Inc. can help to determine the type of material best suited for a specific design. We can help you choose specific materials from a large list of suitable PCB materials for HDI PCB.