Understanding material properties is critical in determining requirements for PCB manufacturing. In this blog, we will look at the technical definitions, as well as an overview of why each property may be important to your designs. All of these properties should be specified on the data sheet for most commonly used PCB materials.
PCB Solutions always recommends that you contact the material supplier and review their data prior to making a decision. The data below is available to help you navigate the terms but does not serve as advice on which material to chose for your application because there are so many variables for Rigid, Rigid Flex and Flex PCB designs.
1. Dielectric Constant (Dk or Er):
Technical definition: The ratio of the capacitance of a capacitor with the given dielectric to the capacitance of a capacitor having air for its dielectric but otherwise identical. The Dk value is calculated as the relative permittivity of a material.
Why this is important: The Dielectric Constant is a major factor in calculating and controlling impedance requirements of signals on PCBs. All PCB materials (cores, prepregs, solder masks), have a Dk value. The actual Dk value can vary based on resin content of materials. Values typically range from 3.5 to 5.9. Specific material is available for both very low Dk and Very high Dk values. A low Dk material is often used for RF applications while a high Dk is often used for High Frequency applications.
2. Glass Transition Temperature (Tg):
Technical definition: The temperature at which a polymer changes from hard and brittle to soft and pliable.
Why this is important: The Tg indicates the temperature at which the PCB base material starts yielding. It is important to avoid any yielding of PCB base materials, so the Tg is not an indicator for the operating temperature of the PCB. The Tg temperature can usually only be sustained for a very short time. The actual minimum Tg required for your PCB will depend on many factors including surface finish and assembly process; however, the industry guideline for most ROHS applications in a minimum material Tg of 170 degrees C.
3. Decomposition Temperature (Td):
Technical definition: The temperature at which material weight changes by 5%.
Why this is important: The decomposition is the breaking of chemical bonds in the resin system. The resin in the laminate is basically burning up. This value is widely considered to be more critical than the Tg value with regards to ROHS considerations during the assembly process. Like Tg, the actual minimum Td required for your PCB will depend on many factors including surface finish and assembly process; however, the industry guideline for most ROHS applications in a minimum material Td of 340 degrees C.
4. Loss tangent (Dissipation Factor):
Technical definition: The ratio at any particular frequency between the real and imaginary parts of the impedance of the capacitor.
Why this is important: A large loss tangent means you have a greater amount of dielectric absorption, which can cause the value of capacitance to change with frequency. If clean, consistent, capacitance is a requirement of your design, then look for a material with low loss tangent values. For high speed designs (greater than 1Ghz) it may be recommended to choose a material with a dissipation factor of less than .015.
5. Moisture Absorption:
Technical definition: Maximum percent of moisture absorbed by material in high-humidity conditions.
Why this is important: Absorbed moisture can raise Dk values, expand the board, and cause thermal defects such as substrate blisters, barrel cracking and delamination during assembly. If the PCBs are stored for only short times in low-humidity locations before assembly, then moisture may not be a problem. However if the PCBs are stored in high humidity for long periods of time, then they may need to be pre-baked before assembly.
6. Coefficient of Thermal Expansion (CTE):
Technical definition: A material’s fractional change in length for a given unit change of temperature.
Why this is important: Glass, copper, gold and nickel all have fixed expansion rates up to their respective melting points. A large difference in laminate expansion rates can cause strain on the plated hole wall causing cracked barrels and lifted land patterns during the assembly process. The common unit of measurement for CTE is ppm/°C, parts per million per degree centigrade. 1 ppm is equivalent to 0.0001% of total observed dimension. A material rated at 250 ppm/°C would change 0.025% in dimension for every degree change in temperature. On a .100” thick board over a 100°C temperature range there would be a total thickness change of 2.5% which equates to 0.0025”
7. Thermal conductivity:
Technical definition: Ability of a material to conduct heat.
Why this is important: As the power and density of components on a PCB rises, the need to dissipate heat through the base material of the PCB increases.
Materials that offer greater thermal conductivity can be utilized with designs that have high power, or high heat output devices such as LED’s, coils or relays.
9. Peel Strength
Technical definition: The strength of the bond between base material and copper cladding as measured by IPC-TM-650.
Why this is important: As components become smaller, their footprint or pad patterns also become smaller, resulting in much less area of contact between the base material and the copper land patterns. The strength of the bond at this area will determine the ability of the pads to avoid lifting from the material surface.
10. Arc Resistance
Technical definition: Measure of electric breakdown condition along an insulating surface, caused by the formation of a conductive path on the surface.
Why this is important: Typically a consideration for high-voltage/high power PCBs. Arc resistance is a measurement, in seconds, of the amount of time for breakdown along the surface of the material.
If you have questions regarding materials or anything else PCB- Send us an email at info@PCB-solutions.com and we will be happy to guide you in the right direction.
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Bob Neisis
Quality Manager
PCB Solutions, LLC
bobn@pcb-solutions.com
