PCB (printed circuit board) materials have two purposes: conduct electricity and provide electrical insulation between conducting layers of copper. PCB substrate materials are being used for this purpose. In this article we cover:
Essentially, the PCB substrate materials of a printed circuit board (PCB) contain the transmission lines and components that enable radio frequency/microwave circuits. Therefore, it's easy to see why substrate materials are important to the success or failure of your Printed Circuit Boards; your substrate material impact thermal behavior, as well as the electrical and mechanical characteristics of the printed circuit boards.
PCBs are generally made of silkscreen, solder mask, copper and pcb substrates. It's the substrate that offers so many choices to being used in pcb fabrication.
Once upon a time, the choice in radio frequency/microwave circuit-board materials was a simple choice of:
You now have more PCB material to choose from and provide electrical insulation to your PCB board. With the boom in electronics, circuit materials are now available for specific applications, such as antennas, or even frequency ranges, e.g. millimeter-wave frequencies and high frequency.
Still, the base substrate for most PCB substrate material tends to fall into the same categories as before: hard/rigid or soft/flexible.
These PCBs are made out of a solid substrate material that prevents the board from bending. Take, for example, a computer motherboard, perhaps the most common application for a rigid PCB.
The motherboard is a multilayer PCB. It's designed to allocate electricity from the power supply while at the same time, enabling communication between all of computer's parts, such as CPU,GPU and RAM. Hard, or rigid materials, are used whenever the PCB has to retain the shape it was set up to be, for the device's lifespan.
Hard circuit materials are typically ceramic based. Generally, the advantages are:
To enable a PCB to flex and move, plastic is often used. Like rigid PCBs, flexible printed circuit boards can be made in single, double or multilayer formats. They can be folded over edges and wrapped around corners.
It's because of flexible materials that wearables are possible, allowing printed circuitry to insert into compact spaces. Flexible materials save on cost and weight, but they tend to cost more for fabrication.
One advantage to flexible materials is that they can be used in areas with environmental hazards. Flexible materials can be waterproof, shockproof or corrosion-resistant, for example – that's not a feature that most rigid PCBs can offer.
Soft circuit materials such as an epoxy or plastic form a coating around a filler, often a glass weave. This form of glass – or a ceramic filler – provides strength and rigidity to the plastic dielectric material. Here are three typical base-substrate soft/flexible materials:
Yet there is a third option for your substrate: a combination of flexible and rigid materials. Flex-rigid boards consists of multiple layers of flexible PCB, such as polyimide, attached to a rigid PCB layer and is often used in aerospace, medical and military applications.
Ease of machining
Flexible circuit materials based on PTFE such as RO3000 and RO4000 are popular because they're easy to machine and have low dielectric losses at microwave frequencies.
That would be FR-4, glass fabric-reinforced laminates, bonded with flame-resistant epoxy resin.
Only it's not actually a material, but a grade designated by NEMA, the National Electrical Manufacturers Association, a U.S. trade body. ‘FR' stands for ‘flame retardant' and denotes that the material complies with UL94VO.
FR-4, also known as FR4, is low cost and versatile. It's made from sheets of prepreg, which is itself constructed from fibreglass matting, impregnated with the epoxy resin. It represents the industry standard as relates to ease of drilling and metallisation. It's responsible for the low-cost production of PCBs, offering good performance at RF/microwave frequencies.
FR-4 offers:
While FR-4 is flexible enough, easy to machine and position as PCBs within larger enclosures, there’s just one problem:
FR-4 is divided into four subclasses, depending on its properties and applications:
All three are mainly used for single-layer PCBs. So what are the differences?
FR-4 is already a low-cost option, but CEM-1 costs even less. CEM stands for composite epoxy material. Like FR-4, CEM-1 is a classification by NEMA. Made from paper and two layers of woven glass epoxy and phenol compounds, CEM-1 is only used for producing single-sided printed circuit boards, as the laminates are incompatible with the process of metallization in holes. Dielectric properties resemble that of FR-4, but the mechanical endurance is not quite as good. Flammability rating is UL94-V0.
CEM-3 is used in double-sided PCBs with plated holes and typically costs around $1 to $2 U.S. per square meter less than FR-4. Depending on the properties and applications, CEM-3 laminates are divided into the following subclasses:
The purpose of stacking PCBs is to save space, of course. That can be very easy to do with this guide to stacking with PCB spacers.
As to materials, parameters related to dimensional and electrical stability are critical for multi-layer boards.
As already mentioned, this material is well suited for stacking PCBs. It features a low-temperature coefficient of the dielectric constant to minimise phase variations, as well as z-axis coefficient of thermal expansion (CTE), closely matched to copper.
Also known as rigid-flex materials, these work well. For example, PTFE is particularly suited for low-loss microwave circuits, but it doesn’t lend itself well to multi-layer circuits due to its dimensional and dielectric changes with temperature.
A solution to getting the advantages of PTFE’s electrical characteristics is to improve its structural integrity. This has been done by combining the material’s electrical properties with the mechanical properties of polyimide materials.
The four main types of PCB substrates offer different benefits, but of course, the one you choose depends on your application and budget. Here’s a quick reminder of where to start when considering your material choices.
