PCB DEPARTMENT MANAGER
When it comes to PCB body material, many alternatives come to mind. The parameters that need to be considered when choosing a PCB material can often be confusing. In this article, we will mainly focus on Solid (Rigid) PCBs, and talk about how to choose materials and what to consider. The housing material is an important part of a PCB. This material determines the performance of the design and the scope of application. The development and demands of today’s technologies cause us to face more difficulties in PCB board designs and drives the development of Copper Clad Laminates (CCL).
The material classification of the most commonly used PCBs is as follows:
1- Paper-based (FR1, FR2 ..)
2- Glass fiber – epoxy based (FR4, FR5, FR6 , CEM3 ..)
3- Composite-based (CEM1,CEM2 ..)
4- Multi-layer structure (PET, PE film.)
5- Special material based (Metal, Ceramic ..)
Non-Flammability Classification
- The flammable properties of PCB materials are classified on the basis of UL94, and the selection should be made accordingly.
- Slow combustion in the 94HB horizontal sample; the combustion stops at or earlier than (thickness < 3mm) < 76 mm/min or 100 m.
- In the 94V2 vertical sample, the combustion stops within 30 seconds. The flaming parts are allowed to drip.
- In the 94V1 vertical sample, the combustion stops within 30 seconds. Dripping is allowed unless the parts are capable of ember/ignite. In the 94V0 vertical sample, the combustion stops within 10 seconds. Dripping is allowed unless the parts are capable of ember/ignite. The standard accepted grade is 94V0.
Halogen material
It refers to the materials that do not contain fluorine, chlorine or bromine. Halogen-containing materials produce toxic gas when burning. Therefore, halogen-free material may need to be preferred.
TG – Glass Transition Temperature
The Tg value is the temperature at which the material transforms from a reasonably hard, glass-like substance into a more flexible and bendable, plastic-like material. This value is important both for board-placement and the working environment. A normal Tg is considered to be 130°C, medium Tg is considered to be 150°C, and high Tg is considered to be 170°C and above.
Td – Decomposition Temperature
This is a measure of the deterioration of the material. The analysis method measures the time when 5% of the material is lost by weight, which leads to an excess of reliability and creates delamination. High reliability requires “PCB Td ≥340°C”.
Thermal Conductivity
It is the property of the material to conduct heat. Low thermal conductivity means a low heat transfer, high conductivity means a high heat transfer. The unit of measurement is expressed in watts per meter (W/m°C). Most PCB materials have a thermal conductivity in the range of 0.36 ± 0.6 W/m°C, while copper materials have a value of k = 386 W/m°C. Therefore, with the copper plane layers on the PCB, more heat will be removed from the housing material.
Coefficient of Thermal Expansion (CTE)
It is the expansion rate formed during the heating of the PCB material. CTE is expressed as parts per million (ppm) heated for each °C. As the temperature of a material exceeds Tg, CTE will also rise. The CTE of a material is usually much higher than that of copper, which can cause coupling issues when the PCB is heated.
Dielectric Constant
The dielectric constant of a material is important for signal integrity and impedance, which are two critical factors for high-frequency electrical performance. For most PCB materials, this value varies between 2.5 to 4.5. The dielectric constant varies with frequency and usually decreases as the frequency increases.
The Loss Factor
The loss factor of a material gives a measure of the strength lost due to the material. The lower this value, the less power is lost. The Tan δ value of most PCB materials (Loss Tangent) varies from 0.02 for the most commonly used materials to 0.001 for very low-loss materials. It can also vary with the frequency. The loss tangent is important in very high frequency circuits (usually above 1GHz). However, it is not a critical factor for other low-digital circuits.
Surface Resistance (pS)
It is a measure of the electrical or insulation resistance of the surface of a PCB material. The PCB materials should have very high surface resistance values like 10°C – 10¹°C Megaohms per square meter. They are also slightly affected by humidity and temperature.
Electrical Strength
It measures the ability of the PCB material to resist electrical deterioration in the Z-direction (inter-surface) of the PCB. It is expressed in Volts.
Peeling Strength
It is a measure of the bond strength between a copper conductor and a dielectric material.
Flexural Strength
It is the measure of the ability of the material to withstand mechanical stress without fracture. It is expressed in Kg per square meter or pounds per square inch (KPSI).
Delamination
It indicates how long a material would withstand against delamination (i.e. How long it will take for the resin from the laminate, foil or fiberglass to separate at a specified temperature). Delamination can be caused by thermal shock, incorrect Tg in the material, moisture, poor lamination etc.
