Circuit boards are made PCB with copper conductors separated by insulating materials which are typically resin systems with a reinforcement material to add strength and stability. The most prevalent materials are FR-4 Epoxy systems (which means Flame Retardent) reinforced with layers of woven glass cloth, but other resin systems are available like Polyimide (Kapton) or PTFE (Teflon), or with more exotic reinforcements like Kevlar or quartz. A variety of options have been developed to enhance specific desired PCB properties (for example, better thermal performance or lower signal loss), but of course they are more expensive than common FR-4 materials.
An attempt has been made by the electronics industry (a committee of volunteeers organized by the IPC) to classify materials according to their properties and sort them into groups. This has been published as IPC-4101, THE SPECIFICATION FOR BASE. MATERIALS FOR RIGID AND MULTILAYER PRINTED BOARDS, which continues to be expanded and revised as new PCB materials become available. In IPC-4101, materials with similar properties are grouped by number, and each numbered data sheet is referred to in the industry as a "slash sheet". Most material data sheets will list the IPC slash sheet numbers that they conform to.
LAMINATE materials (also called "CORES") are manufactured and sold in large sheets, typically around 36" x 48", and then cut into smaller fabrication panels by the bare board fabricator. They are fully cured and coated with a thin layer of PCB copper on both sides. PRE-PREG materials (sometimes called "B-STAGE"), are glass cloths pre-impregnated with resin, but are not fully cured. One or more pre-preg layers will be inserted between each core to complete the layer stackup, and it is these pre-pregs that melt in the lamination press, forming the adhesive that bonds the layers together.
The following characteristics will be more or less important depending on the particular application. They PCB may have to be prioritized to find a balanced solution that meets the cost and performance targets of the product.
COEFFICIENT OF THERMAL EXPANSION (CTE)
Most materials expand when their temperature rises, and contract when they cool. CTE is a way of expressing the amount of change in a material's volume during a temperature change, expressed as parts per million per degree centigrade (ppm/%deg;C). One way of classifying materials is by comparing their CTE. Epoxy resin has a CTE (35-45) that is higher than copper (17-18). This would be a mismatch during thermal cycling that could lead to early failures, but the fiberglass reinforcement material has a much lower CTE (5-6) that limits the expansion in the X-Y directions. This added glass brings the overall CTE of a laminate very close to that of copper. Nothing constrains the expansion in the Z-direction, however, so many designers use materials that are rated based on their z-axis CTE. For example, many designers specify materials that expand no more than 3% of their thickness (z-axis) over a temperature change from 50%℃ to 260%℃.
TG - GLASS TRANSITION TEMPERATURE
As temperature changes, the expansion or contraction of a board material will exhibit a fairly linear rate of dimensional change, as shown in the figure below. At the glass transition temperature, the material changes from being in a hard PCB and relatively brittle condition to being in a viscous or rubbery condition, and the rate of change will vary drastically.
In the figure above, you can see that both materials behave very nearly the same at low temperatures, but the material with a lower glass transition temperature begins PCB to change significantly more above 170°C. Materials with low Tg might be okay for applications that use eutectic solder (which melts at 183°C) but might not be acceptable for lead-free solders that require temperatures above 220°C.
TD - DECOMPOSITION TEMPERATURE
The temperature at which a base laminate material experiences an established percentage PCB of weight loss using Thermogravimetric Analysis (TGA). Td is the temperature at which a material begins to degrade, most commonly specified as the temperature at which a material has lost 5% of its original weight due to decomposition. In the past, the glass transition temperature (Tg) has been used to classify materials by their thermal properties, but at the higher temperatures required for lead-free soldering (to meet RoHS requirements), Td has also proven to be a useful indicator of board material stability.
T266 T288 AND T300
T260, T288 and T300 values represent the length of time that a clad laminate will survive a particular temperature (respectively 260°C, 288°C and 300°C) before it begins to delaminate or blister. These values are considered good indicators of the PCB short term resistance of the material to the soldering process.
DIELECTRIC CONSTANT (ER)
The PCB Dielectric constant (or "permittivity") determines the speed at which an electrical signal will travel in a dielectric material. Signal propagation speed is expressed relative to the speed of light in a vacuum, which is defined as having a dielectric constant of 1.00. Higher dielectric constants will result in slower signal propagation speeds. A common dielectric constant for FR4 materials would be in the range of 4.2 to 4.8, but for high-speed applications, materials have been developed that are PCB in the range of 3.0 to 3.6. Other materials may be available with lower Er, but could be a significant cost adder to the product. For designs with Impedance Controlled signals, you will need to know the Dielectric Constant to calculate the appropriate trace widths to use.
Related to dielectric constant is Dissipation Factor, also known as Loss Tangent. This is a measure of the percentage of the total transmitted power that will be lost as power dissipates PCB into the laminate material.
