Choosing the right dielectric material for a PCB is important no matter what application you’re working on. However, the stakes are higher with HDI boards.
High-density materials are categorized into 4 groups: normal speed and normal loss, medium speed and medium loss, high speed and low loss, and very high speed and very low loss. You need to choose the substrates that best suit your design.
Watch the full video to learn how to overcome common challenges in designing HDI circuit boards.
In this article, you will learn how to choose the right dielectric material for your high-density boards.
An HDI PCB features fine lines and spaces (4 mil), microvias, small capture pads (16 mil), and high pad density (>20 pads/cm 2). These boards are perfect for small consumer applications.
PCBs mainly consist of polymer resin (dielectric), along with fillers, reinforcement, and copper foil. The image below represents the core material of the PCB sandwiched between copper layers.
Composition of laminate and its functions. Image credit: HDI Handbook by Happy Holden
To learn more, see how to design an HDI PCB stack-up.
The first step in selecting dielectric materials for your high-density board is determining the operating frequency (Gbps or GHz) and allowable signal loss (dB).
Next, you need to consider the material’s thermal, electrical, chemical, and mechanical properties.
For instance, high-frequency applications require laminates with a low dielectric loss tangent or dissipation factor (D f) and a flatter D k versus frequency response curve.
Here’s the classification of high-density circuit board materials:
These are the most common dielectric materials—the FR4 family. Their D k versus frequency response is not very flat, and they have a higher dielectric loss.
Therefore, their suitability is limited to a few GHz (0 to 5 GHz). The D f of these materials is greater than 0.012.
EM-827(I), EM-370(Z), Nelco N7000-2HT, and Isola FR370HR are examples of normal speed and normal loss laminates.
Medium-speed materials exhibit a flatter D k versus frequency response curve, with approximately half the dielectric loss compared to normal-speed materials.
These dielectrics are suitable for applications operating at 0 to 10 GHz with a dissipation factor of 0.012 to 0.007.
FR408HR and Isola I-Speed fall under this category.
These materials exhibit lower electrical noise and dielectric loss than the previous 2 categories. They also have a flatter D k versus frequency response curve.
High-speed and low-loss materials are well-suited for applications operating in the 10 to 30 GHz band. The D f ranges from 0.0095 to 0.0022.
Example materials include EM-528, I-Tera MT40, and Megtron6 R-5775.
Materials for RF/microwave applications have the flattest D k versus frequency response and the least dielectric loss. They are suitable for applications operating at 20-60GHz. The D f is generally ≤ 0.0021.
EM-890K, Isola Tachyon 100G, and Astra MT77 are very high-speed and very low-loss laminates.
The graph below summarizes the relationship between the signal loss and operating frequency for HDI material categories.
PCB material categories Vs. loss tangent at 10 GHz
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Once you finalize the material category, you need to consider several other electrical, thermal, and chemical properties of the material. Stick to these guidelines:
The dielectric constant is the first thing to consider when comparing different materials for high-density boards. It is the ratio of the material’s electric permeability to the electric permeability of free space (i.e., vacuum).
The dielectric constant also measures the amount of electric potential energy stored in a given volume of material under the action of an electric field.
Mathematical equation for dielectric constant
D k of most HDI materials ranges from 3 to 4. If you’re working on high-frequency applications, you need to choose a material with a lower dielectric constant, preferably 3 to 3.5.
Select materials with similar dielectric constants to avoid signal integrity issues when using different types of laminates in your circuit board stack-up.
Insulation in low Dk and High Dk HDI PCB material.
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Cores and prepregs are made of fiberglass yarn woven into bundles using epoxy resin.
Since the dielectric constants of the fiber and epoxy resin are different, the density of the fiber weaves influences the uniformity of the laminate’s dielectric constant.
Sparsely woven cores and prepregs have non-uniform dielectric constants along the material, causing impedance mismatch and signal loss. The fiber weave structure is important to consider when working with high-frequency applications, as it can cause signal integrity problems.
Opt for densely woven fiberglass weaves such as 1078, 1035, 3313, and 1067 to minimize the fiber weave effect.
D f, also called loss tangent (Tan δ), is a metric for quantifying signal attenuation that occurs during signal propagation along a transmission line.
Signal attenuation is caused by the absorption of electromagnetic waves within the dielectric material.
The loss tangent of commonly used dielectric materials ranges between 0.02 and 0.001, and its value increases with frequency. Even minor signal losses in HDI, high-frequency, and high-speed digital applications can result in diminished signal quality.
Select material with a low-loss tangent to reduce signal attenuation, reflections, and localized heating.
Conductor loss is another factor contributing to material losses. It generally occurs due to resistance in the copper foil. For HDI and very high-frequency designs, it’s important to consider the impact of the skin effect on current distribution.
The skin effect phenomenon causes most of the current to be concentrated on the outer surface of the copper conductor. When the frequency reaches a point where the skin depth closely aligns with the roughness of the copper, the current flow is hindered by the unevenness of the copper surface. This results in a rise in resistance and a subsequent increase in conductor loss.
Effect of copper foil surface roughness on high-frequency signals
Although surface roughness isn’t usually listed in the datasheets from material manufacturers, a typical value for the outer surface is 3μm, and a typical value for the inner surface is 6μm.
To reduce conductor loss, PCBs should be constructed with wider traces and use rolled copper foils rather than traditional electrodeposited (ED) copper foils.
Panasonic’s Megtron6 is a high-performance laminate with a D k of 3.4 and D f of 0.004 at 18GHz. This material is offered in standard ED, low, very low, and hyper-low profiles.
Glass transition temperature (T g) is the temperature at which the material transforms from solid to viscous. The chosen material’s T g should be ≥ the maximum withstanding temperature.
For instance, if the operating temperature = 200⁰C, choose a laminate with T g ≥ 200 ⁰C. Nelco N7000-2HT with T g = 260⁰C is suitable in this case.
The graph shows that the typical T g for a standard FR4 material is between 130°C and 140°C, making it suitable for moderate operating temperatures. High-Tg FR4 offers a T g greater than 170°C. When selecting FR4 for your design, choose the variant that best suits your operating temperature.
Glass transition temperature (Tg) of FR4 variants
If your circuit board operates above T g, it might soften and lose its mechanical and thermal integrity. This can cause dimensional changes, warping, delamination, mechanical failure, and affect electrical performance.
The glass transition temperature should be higher than the soldering temperature to minimize board stress during assembly.
Decomposition temperature (T d) is the temperature at which the material decomposes chemically. To prevent chemical changes and board degradation, the T d should be significantly higher than manufacturing and operational temperatures.
CTE (coefficient of thermal expansion) is the rate of expansion of a dielectric material as it heats up. It is expressed in parts per million (ppm) expanded for every degree Celsius that it is heated.
CTE along the X and Y axes is typically between 10 and 20 ppm due to the material’s woven glass structure. This restricts the material’s expansion in X and Y directions, allowing it to expand in the Z direction perpendicular to the surface layer of a circuit board.
The coefficient of thermal expansion (CTE) is a measure of a material’s expansion. This image shows CTE in the Z direction
Select materials with a CTE that closely matches other components in the assembly. This ensures that the printed board and its components expand and contract at similar rates.
Ideal CTE along the z-axis is 70 ppm or less.
Thermal conductivity (k) measures a material’s capacity to transfer heat. This factor determines the heat dissipation rate during operation and its corresponding temperature rise relative to ambient temperature.
High thermal conductivity materials offer a greater heat dissipation rate
Many PCB substrates typically exhibit a thermal conductivity in the range of 0.3 to 0.6 W/m-⁰C, which is relatively lower when compared to copper’s thermal conductivity of 386 W/m-⁰C. Hence, it can be observed that copper layers in your board tend to dissipate heat more efficiently than dielectric material.
Always choose a material with high thermal conductivity to effectively distribute and dissipate heat, protect components from overheating, and prevent localized hotspots.
For more thermal management strategies, see 12 PCB thermal management techniques to reduce PCB heating.
Evaluate materials based on their ability to withstand circuit board laser drilling procedures and sequential lamination. Choose flat glass clothes with high resin content, such as 1035, 1067, and 1086.
These prepregs have excellent heat absorption properties, allowing for efficient vaporization during laser drilling without compromising structural integrity.
Always check with your manufacturer to identify options specifically tailored for HDI applications.
To learn about laser drilling, see how laser drilling works in PCBs.
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