Materials of Printed Circuit Board

Date : 2017-02-28 16:37:38Back to list

Excluding exotic products using special materials or processes all printed circuit boards manufactured today can be built using the following four materials:

1. Laminates

2. Copper-clad laminates

3. Resin impregnated B-stage cloth (Pre-preg)

4. Copper foil

Laminates

Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermosetresin to form an integral final piece of uniform thickness. The size can be up to 4 by 8 feet (1.2 by 2.4 m) in width and length. Varying cloth weaves (threads per inch or cm), cloth thickness, and resin percentage are used to achieve the desired final thickness and dielectric characteristics. Available standard laminate thickness are listed in below Table:

Standard laminate thickness per ANSI/IPC-D-275
IPC Laminate
Number
Thickness
in inches
Thickness
in millimeters
IPC Laminate
Number
Thickness
in inches
Thickness
in millimeters
L10.0020.05L90.0280.70
L20.0040.10L100.0350.90
L30.0060.15L110.0431.10
L40.0080.20L120.0551.40
L50.0100.25L130.0591.50
L60.0120.30L140.0751.90
L70.0160.40L150.0902.30
L80.0200.50L160.1223.10

Notes: Although this specification has been superseded and the new specification does not list standard sizes, these are still the most common sizes stocked and ordered for manufacturer.

The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate's type designation (FR-4, CEM-1, G-10, etc.) and therefore the characteristics of the laminate produced. Important characteristics are the level to which the laminate is fire retardant, the dielectric constant (er), the loss factor (tδ), the tensile strength, the shear strength, the glass transition temperature (Tg), and the Z-axisexpansion coefficient (how much the thickness changes with temperature).

There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known prepreg materials used in the PCB industry are FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4 (woven glass and epoxy), FR-5 (woven glass and epoxy), FR-6 (matte glass and polyester), G-10 (woven glass and epoxy), CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven glass and epoxy), CEM-5 (woven glass and polyester). Thermal expansion is an important consideration especially with ball grid array (BGA) and naked die technologies, and glass fiber offers the best dimensional stability.

FR-4 is by far the most common material used today. The board with copper on it is called "copper-clad laminate".

Copper Thickness

Copper thickness of PCBs can be specified as units of length (in micrometers or mils) but is often specified as weight of copper per area (in ounce per square foot) which is easier to measure. One ounce per square foot is 1.344 mils or 34 micrometres thickness.

The printed circuit board industry defines heavy copper as layers exceeding 3 ounces of copper, or approximately 0.0042 inches (4.2 mils, 105 μm) thick. PCB designers and fabricators often use heavy copper when design and manufacturing circuit boards in order to increase current-carrying capacity as well as resistance to thermal strains. Heavy copper plated vias transfer heat to external heat sinks. IPC 2152 is a standard for determining current-carrying capacity of printed circuit board traces.

Safety Certification

Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as components in devices or appliances. Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat deflection, and direct support of live electrical parts.