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Defense systems rely heavily on different electronic materials, whether they happen to be used for circuits, shielding, or packaging. And whether intended for applications on the ground, at sea, or in the air, demands on these electronic materials can be quite extreme. A hasty or improper choice in electronic materials—such as printed-circuit-board (PCB) materials, electromagnetic (EM) shielding materials, or packaging materials—can lead to system failures, often at the worst times. Knowing key features for these different electronic materials can help a great deal at the system design level and contribute to the effective operating lifetime of the systems that rely upon them.

Choosing a PCB material for a defense-based application can be influenced by a variety of needs, with high reliability and consistent performance usually fairly standard requirements. For example, ground-based applications may be hindered by water or moisture absorption while minimizing weight may be a more important need for PCBs heading for satellites or missile-guidance systems.

No one circuit material is ideal for all applications. Some materials may be better for portable use while others have traits that serve better in high-power circuits. Matching circuit materials to the requirements of different applications is a matter of finding the right combination of characteristics that will deliver the best performance and reliability—hopefully for the lowest cost.

Most PCB material selection starts with the material’s relative dielectric constant, which is the ratio of the dielectric material’s permittivity to the permittivity of a vacuum or air. It’s an important circuit-design parameter that will be used in computer-aided-engineering (CAE) circuit design software to determine circuit dimensions for targeted circuit impedances, such as 50 Ω. A circuit material’s relative dielectric constant is usually specified through the thickness (z-axis) of the material and at a standard test frequency of 10 GHz.

Perhaps as important as the value of the dielectric constant is its consistency across the PCB material. Especially for some larger circuit designs, such as antennas, variations in dielectric constant can result in changes of characteristic impedance for transmission lines. This can make it difficult to achieve consistent amplitude and phase responses across a wide temperature range.

Low-cost FR-4 circuit laminates have often served as “ground zero” for less-critical circuit designs; while low in cost, they can provide serviceable and respectable performance through frequencies of about 3 GHz (and even higher). These fiberglass-reinforced epoxy-based circuit materials have a typical relative dielectric constant of 4.8 in the z-direction at 10 GHz, with good strength-to-weight ratio and low water absorption. Unfortunately, FR-4 tends to suffer greater losses at higher (microwave) frequencies than circuit materials with lower dielectric constants and loss tangents, so copper-clad FR-4 circuit laminates tend to be used more for power-supply and lower-frequency circuit applications in RF/microwave circuit/system designs.

As a general guideline, lower values of dielectric constant are typically associated with higher circuit operating frequencies. As a result, circuit designers working at microwave frequencies and beyond typically look for PCB materials with dielectric constants of less than 4.8 in the z-direction at 10 GHz for RF/microwave circuit designs, often with dielectric-constant values around 3.0 or less at 10 GHz in the z-axis of the material. The lower the effective dielectric constant for a PCB material, the higher in operating frequency it can usually be used without unacceptable losses.

Dielectric loss is one of the components of insertion loss for an RF/microwave circuit and it is usually related to the dissipation factor of the PCB material. This particular loss is due directly to the substrate material and not the conductor. The conductor material, such as copper, will also contribute to the insertion loss from a PCB, although often the roughness of a conductor can have as much to do with the loss of the PCB as the type of conductive material. Dielectric loss can be a concern for a PCB material when circuit losses must be minimized. Heat produced by circuit losses may also be a concern at higher signal levels.

Polytetrafluoroethylene (PTFE) has long been favored for use in RF/microwave PCBs for its low loss and low dielectric constant. Physically, it is considered a “soft” substrate and benefits from structural reinforcement. Many PTFE suppliers strengthen their PTFE-based PCBs with additional filler materials, such as woven fiberglass, glass fiber, or ceramic materials. Such fillers help the mechanical strength by controlling the PTFE expansion and contraction in a PCB’s x and y dimensions (its length and width).

An important parameter for PTFE-based and other types of PCB materials is coefficient of thermal expansion (CTE). This is a measure of a material’s physical change in length for a given change in temperature. It is measured and specified in all three dimensions for a PCB in percent (%) or in units of ppm/°C. The goal is usually for the circuit-material’s x and y (length and width) dimensions to have the same CTE values as the conductive metal, such as the 17 ppm/°C CTE of copper.

Although it is physically the smallest of a PCB’s three dimensions, the material’s z-axis can still suffer from the effects of material expansion and contraction with temperature and will also have a specified value for CTE. When plated through holes (PTHs) are formed through a PCB material, by drilling through the dielectric material and metal-plating the holes, conductive paths are formed from the circuit layer to the ground plane. Ideally, the CTE of the dielectric material in the z-direction would be minimal, especially when PTHs are used in the interconnection of different circuit layers in a multilayer assembly.

Significantly different CTE values for the metal plating and dielectric material can result in stress as the materials expand and contract with temperature. They may survive numerous cycles of expansion and contraction, but fatigue can affect the PTHs over time and cause loss of some of the ground connections. Changes in temperature take place regularly in hostile operating environments. However, they also occur due to normal PCB processing when creating and manufacturing a circuit, soldering components and connections, and other production steps.

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