Materials for HDI PCB and High-Speed PCB Boards

Choosing the right materials for high-density interconnect boards is a crucial step in design that will impact the overall performance of applications, including their longevity and reliability.

Material selection is more important for HDI  PCB boards because they are small, light, and have certain construction requirements. The decisions you make will reverberate throughout the entire process, from planning and design to assembly and manufacturing.

Designers should choose materials after thoroughly studying their electrical, thermal, mechanical, and chemical (among others) properties.

Classification of HDI Boards

The IPC-6012 standard considers a board to be HDI if it reaches 20 connections per centimeter square on either side of the core. They have a higher wiring density per unit area. Furthermore, the tracks between microvia should not exceed 100 micrometers.

Some of the most important features of HDI boards are:

• Microvias less than or equal to 150µm
• Capture pads less than 400µm
• A minimum capture pad density of 20 pads per centimeter square or more
• Fine lines less than or equal to 100µm

So what factors should you consider when choosing materials for HDI PCB and high speed PCB boards?

Factors to Consider

The material’s properties will determine the board’s durability, strength, conductivity, and stiffness. The wear and tear of materials is another concern when designing high speed PCB boards - especially because manufacturing processes may test the limits of a circuit, leading to even more wear and tear.

Fabricators and designers use a methodical process to choose compatible materials and base their decision on several factors including chemical, electrical, thermal, and mechanical properties. Depending on the designer’s intended application, different materials may perform differently under certain conditions.

Before selecting a material, it is important to determine its overall flexibility because it will be impacted during the design and manufacturing process. In other words, designers should be fully aware of the material’s overall limitations in order to make appropriate design decisions.

The following dielectric properties must be properly studied before choosing any material:

Dielectric Constant (Dk): A material’s dielectric constant impacts signal integrity and impedance. These are important factors for high frequency electrical performance. Most PCB materials have a Dk value in the range of 2.5 to 4.0.

Glass Transition Temperature (Tg): This parameter determines the temperature at which a material transforms from a rigid state to a flexible state, or vice versa. A higher value means the HDI PCB will require a higher temperature during lamination. Moreover, the board’s electrical properties will be affected.

As a rule of thumb, a higher Tg translates into superior performance with improved chemical resistance, stability, moisture resistance, and heat resistance.

Coefficient of Thermal Expansion (CTE): The coefficient of thermal expansion (CTE) dictates how much an HDI PCB will expand or contract when exposed to extreme temperatures. Different materials will expand by different amounts, affecting the device’s overall performance.

Thermal Conductivity (K): The term thermal conductivity refers to the material’s ability to conduct and transfer heat. This is an important parameter for HDI PCBs because it dictates how well the circuit will dissipate heat from its components to the surroundings. Smaller boards like HDI PCBs are more vulnerable to overheating because they lack the ability to dissipate heat naturally.

Decomposition Temperature (Td): This parameter determines the temperature at which the HDI PCB material’s chemical bonds decompose. Most of the reactions are endothermic because heat is required for chemical decomposition. Td is usually expressed in degrees Celsius.

It is important to choose an HDI PCB based on the right TD because once the material reaches or surpasses this value, its properties will be changed irreversibly. This is in stark contrast to Tg where the material will exhibit its original properties once it cools below the Tg range.

Copper Loss: Copper loss is associated with the flow of current through the conductors. Electrons don’t always flow through the conductor’s centers. For example, if the copper trace is finished with nickel, electrons may flow through a nickel layer. This effect becomes more pronounced at larger frequencies. This can be minimized by creating a larger surface area using wider traces.

Pro tip: Besides material selection, designers can also improve thermal conductivity by placing more thermal vias and trace geometries to improve heat dissipation.

Popular Materials to Choose for HDI PCBs

Designers can choose from four types of materials that are well suited to HDI PCBs. The criteria is to look for materials with a low dielectric tangent and a flatter dielectric response (Dk) versus frequency response curve.

Let’s take a look at some examples below.

Normal Speed, High Loss

Normal speed materials have a very high dielectric loss with a non linear relationship between Dk and the frequency response curve. This makes them suitable for very limited applications. Isola 370HR is a popular material used for these applications.

Medium Speed, Medium Loss

The dielectric loss for these materials is half that for normal speed materials. They have a relatively flat Dk vs. frequency response curve, making them ideal for more applications capable of 10GHz. Nelco N7000-2 is a popular choice of material for these devices.

High Speed, Low Loss

The dielectric loss for these materials is very low and they have extremely flat Dk vs. frequency response curves. More importantly, these materials don’t generate unwanted electrical noise. Isola I-Speed is a popular material used in this category.

Very High Speed, Very Low Loss

These materials have the least dielectric loss and the flattest Dk versus frequency response. They are ideal for applications of up to 20 GHz. Isola Tachyon 100G is a popular choice of material used in this category.

High speed digital applications use materials that have a low Dk value to achieve superior signal transmission performance. For HDI PCBs, make sure to use materials that have an extremely low Df value. Use a low-loss, high speed material if signal attenuation is important to the device. A lower Dk value is even more important if crosstalk is an issue.

Wrapping Up

Hemeixin PCB is the leading provider of HDI PCB and other high speed PCB boards having worked with businesses from a wide range of industries. For more help with HDI PCB design, get in touch with the experts at Hemeixin PCB here.