How to Choose PCB Laminates?
When we talk about the PCBs used in the products with a further discussion, inevitably comes to a query–What PCB laminate materials do you use? PCB laminate is a kind of plate-like material made by dipping electronic fiberglass cloth or other reinforcing materials with resin, coated with copper foil on one or both sides and hot laminating. The performance, cost, and stability of the printed circuit boards depend on the choices of PCB laminate largely.
So what should be included in the consideration when choosing PCB laminate?
1. Glass Transition Temperature (Tg) is the temperature at which higher molecular weight materials transforms from glassy state to a viscous and rubbery state. The rigid PCB laminate stops acting at this temperature and begins to behave softer.
Tg is not the value of the maximum working temperature, but rather the critical value. It is an important performance indicator for PCB laminate (Z-axis expansion is an important indicator to describe the impact of how Tg affect the PCB laminate and is considered a proxy for plated through hole reliability), such as the FR-4 materials are usually classified using different Tg values, the higher the Tg value, the better the material.
The commonly used FR4 materials has a Tg of 130°C (In PS Electronics, it is 140°C). One kind of the PCB laminates with higher Tg of 250 or above are polyimides (PI).
2. Thermal Decomposition Temperature (Td), this property varies greatly with the composition of materials, from the mid 300℃ range for many epoxy systems to over 400℃ for some flexible PCB laminate materials.
3. Dielectric Constant (Dk) and Loss Tangent (tanδ), dielectric constant determines the speed at which an electrical signal will travel in a dielectric material, higher dielectric constants will result in slower signal propagation speed. Loss Tangent is a measure of how much of the power of a signal is lost as it passes along a transmission line on a dielectric material.
In order to achieve high-speed transmission, high-frequency high-speed PCB boards must employee the laminates with low Dk and tanδ, such as PTFE, which possesses low Dk and tanδ, now is one of the best for high frequency applications.
4. Thermal expansion is the tendency of matter to change its shape, area, and volume in response to a change in temperature. It is related to the stability of the material dimensions and affects the quality of the soldering.
Generally, FR-4 has a relatively excellent CTE (Coefficient of Thermal Expansion) range of 14 to 17 ppm/C, but it will rise sharply when the temperature exceeds its Tg value.
5.Thermal conductivity (Tc) is the property of a material to conduct heat. It is frequently discussed contra the high-power products such as lighting devices, in which case, it may be the most important performance.
The metal PCB laminates such as copper and aluminum possess relatively excellent thermal conductivity (m/wk).
6.RoHS compliant, the “green” requirement is driven by market considerations and policies, RoHS compliant has become a consensus among electronics manufacturers in different countries and will be a future trend, which should be taken into account when selecting PCB laminates.
An example is a halogen-free application, which means that the PCB laminates do not contain halogen elements, such as chlorine, bromine, etc., which are easily harmful to the human and the environment. Although no clear requirement, if there are related cases in your market, please, give this item priority.