July 31, 2018
What are PCB Laminates?
Understanding the different types of laminate for a printed circuit board is crucial for the design and development process your PCB manufacturer will go through. The type of laminate you choose will determine the stability and overall performance of your PCB.
The laminate of the PCB is what holds the layers together. Traditionally, there are four layers that make up the board: Substrate, Copper, Soldermask, and Silkscreen (from bottom to top). The laminates are developed by curing under pressure and temperatures of cloth using thermoset resin to create the final piece of uniform thickness.
When you see a type of laminate written out, it typically will look like this: FR-4 (with FR-4 being the most common material used). These numbers and letters are the characteristics of the laminate produced. Some of the important characteristics include: fire retardant, dielectric constant, loss factor, tensile strength, shear strength, glass transition temperature, and how much thickness changes with temperature (the Z-axis expansion coefficient).
Types of PCB Laminate
|FR-4||Common across the board||Most common; good performance across all characteristics; good strength-to-weight ratio reliable.|
|High-Performance FR-4||Best for multilayer PCBs||Increased Tg for better reliability; suitable for high-frequency circuits.|
|High Tg Epoxy||Best for multilayer PCBs||Tolerates higher heat; increase heat, moisture, and chemical resistance.|
|BT Epoxy||Lead-free PCBs; multilayer PCBs||Maintains high bond strength at high temperatures; outstanding thermal, mechanical, and electrical properties.|
|Polyimide||High density, flexible, rigid-flex, multilayer PCBs||Ideal for advanced applications (ex. military); extreme environmental stability; high levels of thermal, chemical, and mechanical properties.|
|Copper Clad (CCL)||High-voltage circuits||Perform well based on performance requirements with appearance, size, electrical, physical, chemical, and environmental performance.|
|Teflon||High Frequency Applications||Used for low loss communication applications.|
The following list is comprised of the different types of PCB laminates and the PCB materials needed, starting with the most common ones. The designation of a laminate comes from the resin used.
What makes FR-4 the most commonly used laminate is how it performs across all characteristics. This material has a good strength-to-weight ratio, is flame resistant, which improves its reliability, and when there is an increased temperature, the material’s mechanical, electrical, and physical properties stay maintained.
You would use a high-performance FR-4 laminate when dealing with multi-layered PCBs. It has a high-reliability level due to the increased TG and is more suitable for high-frequency circuits because of its low dielectric properties.
High Tg Epoxy
For multilayer PCBs, the High Tg Epoxy is most suitable. The Tg is the vitrification temperature, similar to the melting point. The higher the number, the more rigid the board will be. A material with a high TG (over 170o C) has better heat, moisture, and chemical resistance, as well as better stability.
Most suitable for lead-free PCBs, the BT Epoxy laminate is known for having outstanding thermal, mechanical, and electrical properties. This laminate is commonly used for multilayer PCBs. It will also maintain bond strength at high temperatures.
This type of laminate produces extreme environmental stability. You would use the Polyimide laminate when working with high-density, flexible, rigid-flex circuit boards, and multilayer PCBs. It also produces high levels of thermal, chemical, and mechanical properties. A polyimide laminate is ideal for advanced applications, including military, aerospace, automotive, and consumer electronics.
Copper Clad (CCL)
The Copper Clad laminate is made up of a glass fiber or wood pulp paper as the reinforcing material. You would typically use a CCL laminate for high-voltage circuits based on specific aspects, including appearance, size, electrical, physical, chemical, and environmental performance.
The most commonly used material in high frequency applications. Teflon materials come in many different options, the easier materials to use in manufacturing are teflons with woven glass. There are several challenges with manufacturing teflon PCBs that causes issues with a conventional board shop.
Frequently Asked Questions
Material data sheets show different dielectric constants in the same board. How is that possible?
When you look at a data sheet, the applications of the dielectric constants are for a whole series of materials, not just a small section. You can come across a laminate that is made up of four different layers. Although they are all in the same stack-up, they all may have different dielectric constants.
Regarding basic stack-ups, are you able to do higher layer count multi-layers using just high-frequency materials?
Yes, when necessary, high-frequency stack-ups are possible. It will be up to you whether you decide it is necessary and what the signal integrity needs that need to be met are.
Is it possible to v-score high-frequency materials?
Generally, the answer is no. High-frequency laminate materials tend to be soft and fibrous. A V-score takes two circular saws facing each other. Put that against soft material, and they will not hold up very well.
If your high-frequency materials are not in stock, how long will they take?
The time will vary depending on a lot of factors, including how rare the laminate material is, and if we can keep it stocked. Expect for the wait to range between a few days to up to two weeks. The sooner you can get your order in for the supplies the better.
What are the most common characteristics between an FR4 type versus the IPC spec
The FR-4 laminate falls under two categories: high temperature and low temperature. Typically, low-temperature FR-4 is used for old tin, lead, hot air solder level type assemblies. These are the ones that bond with their board when you receive them. The high-temperature FR-4 is meant for lead-free and RoHS-compliant work since it sees higher soldering temperatures.