PCB Material Selection: How Substrate Properties Impact PCB Performance

August 2, 2018 Altium Designer

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 parts 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 parts 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 (Parts 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 Temperature

(Co)

Coefficient of Temperature 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 the Author

Altium Designer

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

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