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    PCB Material Selection: How Substrate Properties Impact PCB Performance

    Altium Designer
    |  July 30, 2018

    Circuit Board with flame resisant

    From the early 15th century until the 17th century, artists used the Flemish masters’ method to create life-like paintings. This process begins with a pencil, charcoal, or ink pen drawing and continues with the imprimatura—or the first transparent, olive-tinged stain of color. Then, the method moves to the first umber underpainting, sanding, second umber underpainting, sanding, and a monochromatic underpainting.

    Each underpainting layer focuses on details along with the tonality of painted subjects and requires a precise approach. After the application of those layers plus additional sanding to create a smooth surface, the artist coats the entire canvas with oil and moves on to the first and second color layers. The second layer focuses on details and texture. An additional cleaning sets the stage for a final color layer that emphasizes the brightest of the image.

    PCB Designers: Layout Engineers and Materials Wizards

    Circuit board or printed circuits will largely have commonalities in requirements for copper, and in necessities for board space. If you’re looking to begin designing, or if you’re an experienced layout designer then you’ll know the main pain points of the design process: routing traces to manage copper, component placement to maintain signal and power integrity, and designing a printed circuit that can actually be manufactured with an eye on solder masks and packaging.

    Polygon region in PCB layout

    How do you know how to optimize your designs based on materials?

    Designing a PCB involves the same approach that the “old masters” used when following the Flemish technique. An artist focuses on the finished product while constantly thinking about small details. The “old masters’ approach” to PCB design involves better understanding the of a PCB and how different properties of substrates and conductors impact PCB performance. For example, commonly accepted substrate materials may detract from PCB performance at high frequencies.

    PCB Material Selection: What’s Underneath Counts

    The Flemish technique highly prioritizes a good foundation. Each underpainting layer adds depth to the final work. With the foundation in mind, you should have a greater understanding of the thermal, electrical, chemical, and mechanical properties of the substrate. Listed in table one, each of those can interact and each impacts PCB performance—but in various ways.

    Table One – Substrate Properties

    THERMAL

    ELECTRICAL

    CHEMICAL

    MECHANICAL

    Glass Transition Temperature (Tg)

    Unit of Measure (Co)

    Temperature threshold that causes substrate material to soften

    Dielectric Constant (Dk)

    The capacitance between two conductors on a dielectric material compared to the same two conductors in a vacuum

     

    Flammability

    Flame retardant property of plastics

    Peel Strength

    Measures the bonding between the dielectric and copper layers of a PCB during exposure to thermal stress, high temperatures, and chemicals

     

     

    Decomposition Temperature (Td)

    Unit of Measure (Co)

    Temperature threshold that causes substrate material to lose mass

    Dielectric Loss Tangent (Df)

    The measure of energy lost (or dissipated) as a result of the material; the lower the Df, the less energy lost

    Moisture Absorption

    The capability of a dielectric material to withstand exposure to liquid when submerged

     

     

    Flexural Strength

    Unit of Measure (pounds per square inch)

    The capability of a dielectric to absorb physical stress without breaking

     

     

    Coefficient of Temperature Expansion (CTE)

    Unit of Measure ( per Million - ppm)

     

    Temperature threshold that causes substrate material to expand

    Volume Resistivity

    Unit of Measure (Ohms-Meters)

    The resistance of a dielectric material to insulation or electricity

    Methylene Chloride Resistance

    Measures the chemical resistance of PCB substrate material by measuring resistance to Methylene Chloride

    Young’s Modulus

    Measures capability of dielectric to withstand stress from each direction and determines stress/strain ratio of the substrate

    Thermal Conductivity (k)

    Unit of Measure (watts per meter)

    The ability of the substrate material to transfer heat

    Surface Resistivity

    The surface resistance of a dielectric material to insulation and electricity

     

    Time to Delamination

    Measures duration of dielectric material resisting layer separation when exposed to temperatures above a certain threshold, thermal shock, or moisture

     

     

     

    Electrical Strength

    Unit of Measure

    The ability of a dielectric material to resist an electrical breakdown

     

    Density

    Unit of Measure (pounds per cubic inch)

     

     

     

    Substrate material selection impacts circuit performance. For example, knowing the CTE of substrate materials becomes a high priority because problems can occur if two substrate materials (or even the substrate and components) have a CTE mismatch. Substrates with mismatched CTEs can have defects caused by expansion at different rates or because the dielectric constant of the substrates becomes unstable. While components such as silicon memory chips may have a low CTE, fiberglass laminates have high CTEs. The difference in expansion rates can cause solder joints to crack or damage components.

    It’s Not a Good Mix

    Artists using the Flemish technique choose very specific colors and types of brushes to achieve the optimal tone and depth. Mistakes occur if an artist fails to consider how oils, varnishes, and paints interact. Selecting substrate materials requires a similar approach. Different variables can change the impedance of circuits—especially when those circuits operate at high speeds and high frequencies. For example, moisture absorption and thermal conductivity can affect the dielectric constant. PCBs with moisture-absorbing materials or heat-generating components have a higher dielectric constant, which, in turn, affects circuit performance at high frequencies.

    Changes in impedance or impedance mismatches can cause high-speed digital signals to reflect energy back to the signal source. Referring to Table Two, substrate materials used for high-frequency circuits must have a dielectric constant that remains stable over a wide range of frequencies. PTFE has a low dielectric constant that does not vary with frequency. Because PTFE has a high CTE, you should ensure that other materials used on the PCB also have a high CTE.

    Table Two – Substrate Properties by Type

    Substrate

    Type

    Glass Transition Temperat
    ure

    (Co)

    Coefficient of Temperat
    ure Expansion

    (X-Y)

    Dielectric Constant at 1 MHz

    Moisture Absorption

    (%)

    Electrical

    Strength

    Peel Strength

    Lbs./in

    Applications

    Epoxy Glass

    FR-4 Family

    135-210

    13-17

    4.4 – 4.8

    0.15

    1000-3000

    >2.0

    Computers and Peripherals, Backplanes

    Polyimide Glass

    250

    12-16

    4.0 – 4.6

    0.35

     

    >1.4

    Mobile Products

    Polyimide Quartz

    250

    6-8

    3.5 – 3.8

    0.35

     

    >1.2

    MEMS,

    Solar Cells

    Epoxy Aramid

    180

    7-9

    3.8 - 4.1

    0.44

     

    >1.7

    Avionics, Ultra-thin cell phones

    BT Epoxy

    185

    13-14

    3.8 – 4.0

    0.19

    1200

    >2.0

    Microelectronics

    PTFE

    188

    60

    2.75 - 3.0

    0.08

    1090

    >8.0

    RF/Microwave

    Unlike PTFE, the standard FR-4 substrate material does not work for high-frequency circuits. Furthermore, the dielectric constant of FR-4 does not remain constant with changes in temperature. If the temperature range for the application increases, the variations become more significant and cause the impedance to change. In addition, the variations in dielectric constant also affect the capability to achieve and maintain a flat response as frequencies increase.

    CircuitStudio® offers high-performance solutions with a cohesive, user-friendly interface to quickly and accurately complete your PCB design, regardless of substrate material, temperature changes, or dielectric constant variations. To learn more about developing reliable designs and optimizing your PCB material selection process, talk to an expert at Altium.

    About Author

    About Author

    PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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