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The production of High-density interconnect (HDI) circuit boards requires various materials for creating layered structures, etching copper, applying solder masks, and finishing surfaces. While some materials are standardized, dielectric materials for multi-layer PCBA must possess specific properties, such as dielectric constant and thermal conductivity, tailored to the application.
It is crucial to compare PCB material properties during the design process to select the appropriate substrate. The stackup of multi-layer circuit boards typically includes multiple materials, impacting signal and power losses, connection impedance, copper surface roughness, and temperature rise.
Designers should assess different PCB material properties to determine the best substrate. Once the suitable material is identified, PCB stackup design and analysis tools in CAD software can be used to create an effective layout for high-speed HDI PCBs.
HDI PCB Substrate Properties to Consider
1. CTE or Coefficient of Thermal Expansion
This number shows how the board expands as temperature increases. Of course, boards typically expand at different rates in different directions. However, the most important is along the Z axis, or perpendicular to the larger surface area of the PCB.
2. Tg or Glass Transition Temperature
This is the temperature at which the CTE value of the PCB suddenly increases with increasing temperature. Beyond the glass transition temperature, the board material becomes more plastic.
3. Dk or Dielectric Constant
The ratio of the conductivity of the substrate to the conductivity of free space or a vacuum. Dk is a measure of the potential energy that an electric field can store in a given volume of material.
4. Df or Dissipation Factor
Df, also known as loss factor, is a parameter synonymous with insertion loss and increases with increasing frequency.
5. Conductor loss
This is related to the conductivity of the conductors on the circuit board, which is different for AC and DC. For AC, conductor loss depends on the penetration depth, which in turn depends on the frequency of the AC. The roughness of the copper surface also affects the conductor loss, which increases the total loss in the system and changes the impedance of the connection.
6. Thermal conductivity
This is the rate at which the substrate removes heat from the heat source during operation. It determines the temperature rise of the circuit board relative to the ambient temperature. Designers determine thermal management strategies based on the thermal conductivity of the substrate.
Manufacturers are continually exploring new materials for high-density interconnect (HDI) and high-speed PCBs that perform better in high-temperature environments. Key characteristics of these materials impact the propagation of high-speed signals, dispersion effects, and the board's ability to dissipate heat and endure temperature fluctuations and mechanical shock. Due to the cost of some materials, designers often begin with FR4 to evaluate its suitability for their specific applications.
HDI PCB
Standard Stackup Materials
In the electronics industry, FR4 is the primary non-conductive material used between copper layers in circuit boards. FR4 is a glass-reinforced epoxy laminate known for its favorable properties, including a low Z-axis expansion coefficient, high shear and tensile strength, and flame retardancy, which meets safety requirements by preventing fire spread. It performs well in humid and temperature-fluctuating environments.
Circuit boards are constructed by stacking alternating dielectric layers—typically made from epoxy, but sometimes from modified acrylates, cyanate esters, PPE, and BT—between copper foil layers. Laminates serve as substrates with essential dielectric properties, and designers specify substrate thickness based on the dielectric requirements of their applications, referencing the IPC-2221 standard for dielectric constants of various materials, including FR4.
High-Speed Design and HDI Stackup Materials
In the circuit board industry, epoxy resins serve as the primary base material due to their low cost, strong adhesion, and favorable electrical, mechanical, and thermal properties. However, advancements in epoxy chemistry have led manufacturers to explore alternative resin-based materials to address specific shortcomings.
For instance, BT epoxy resin is often used for organic chip encapsulation due to its thermal stability, while cyanate resins and polyimides are chosen for their lower dissipation factor (Df) and dielectric constant (Dk) values. PTFE and polyimide are also utilized as thermoplastic resins to replace traditional thermosetting resins, with the thermoset form of polyimide offering flexibility suitable for flexible circuit boards.
Low dielectric constant materials are particularly beneficial for high-speed high-density interconnect (HDI) PCBs, as they facilitate faster transmission of high-frequency signals and reduce capacitive coupling between traces. This enhances signal integrity and minimizes crosstalk, which is crucial given the dense trace arrangements on HDI boards.
How to Choose HDI PCBs Materials
When choosing materials for HDI PCBs that transmit high-speed signals, it's essential to select materials with low signal energy loss, characterized by a low dielectric loss factor and a flatter loss factor across frequency responses. Designers have four categories of HDI materials to consider:
Normal Speed and Normal Loss: Conventional FR4 materials with high dielectric loss and non-flat dielectric constant values, suitable for applications up to a few GHz (e.g., Isola 370 HR).
Medium Speed, Medium Loss: These materials feature flatter dielectric constant values and about half the dielectric loss of normal speed materials, suitable for frequencies up to around 9 GHz (e.g., Nelco N7000-2 HT).
High Speed, Low Loss: Materials with flatter dielectric constant values and low electrical interference, appropriate for high-speed applications (e.g., Isola-I Speed).
Very High Speed and Low Loss: These materials have the flattest dielectric constant values and the lowest dielectric losses, ideal for applications up to about 20 GHz, including RF/microwave applications (e.g., Isola Tachyon 100G).
Tips for selecting the right Materials
For better signal transmission performance in high-speed digital applications, materials with low Dk, Df, and better SI or signal integrity capabilities may be more suitable.
The lowest Df materials are more suitable for RF PCBs.
If it is important to avoid signal attenuation, then using high-speed, low-loss materials will help.
To reduce crosstalk, try to use low Dk materials.
The use of BT materials may be more suitable for microelectronic substrate applications with small circuit boards and compact layout features.
The higher the performance, the more expensive the material. Therefore, compromises may be necessary.
Choosing the right material is crucial as it significantly impacts the electrical performance of the PCB. It's essential to identify the material best suited for a specific design, considering the diverse range of materials available for HDI PCBs. Selecting the appropriate material from this extensive selection ensures optimal functionality and reliability of the final product.