Your PCB substrate: a guide to materials
6.5 minutes | 16 Mar 2020
PCB materials have two purposes: conduct electricity and provide insulation between conducting layers of copper.
Essentially, the 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 materials are important to the success or failure of your PCB; your materials impact thermal behaviour, as well as the electrical and mechanical characteristics of the circuit.
PCBs are generally made of silkscreen, solder mask, copper and substrate. It’s the substrate that offers so many choices.
Hard or soft PCB materials?
Once upon a time, the choice in radio frequency/microwave circuit-board materials was a simple choice of:
- Hard, or rigid circuit material
- Soft, or flexible circuit material
You now have more to choose from. With the boom in electronics, circuit materials are now available for specific applications, such as antennas, or even frequency ranges, e.g. millimetre-wave frequencies.
Still, the base substrate for most PCB materials tend to fall into the same categories as before: hard/rigid or soft/flexible.
Hard/rigid PCB materials
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:
- High thermal conductivity, 170W/mK
- Strong dielectric
- High operating temperature >350ºC
- Low expansion coefficient <4 ppm/C
- Smaller package size due to integration
- Hermetic packages possible, 0% water absorption
- Limited to no outgassing
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The most popular ceramic-based PCB materials include:
Most common ceramic PCB. Strong thermal dielectric with low expansion. Exceptional high-frequency performance. Operating temperature up to 350˚C.
High thermal conductivity; high thermal strength with low expansion. Widely used as a substrate or package.
Highly toxic during machining. Used for its high thermal conductivity and low expansion.
Substrate: Soft/flexible materials
To enable a PCB to flex and move, plastic is often used. Like rigid PCBs, flexible PCBs can be made in single, double or multilayer formats. They can be folded over edges and wrapped around corners.
Flexible materials: rise of the wearables
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:
The best-known PTFE-based brand name is Teflon. This material offers temperature stability and low dissipation factor.
Good electrical properties, wide temperature range and high chemical resistance.
High melting point and resistant to chemicals, radiation and extreme temperatures.
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.
The most popular PCB material?
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.
What’s so great about FR-4?
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.
- Electrical insulation with high dielectric strength
- High strength-to-weight ratio
- Moisture resistance
- Relative temperature resistance, so you can expect it to perform well in most environmental conditions
What’s not so great about FR-4?
While FR-4 is flexible enough, easy to machine and position as PCBs within larger enclosures, there’s just one problem:
- High dielectric loss (the dissipation factor) at microwave frequencies
- This means it’s a poor choice for high-speed digital circuits or high-frequency analog applications above a few GHz
A closer look at FR-4
FR-4 is divided into four subclasses, depending on its properties and applications:
- Standard, with a glass transition temperature Tg of ~ 130°C, with UV blocking or without it. The most common and widely used type. Also the most affordable FR-4
- With a higher glass transition temperature, Tg ~ 170°C - 180°C. Compatible with the lead-free reflow technology;
- Halogen-free and compatible with lead-free reflow technology
- With normalized index of CTI ≥ 400, ≥ 600
FR-1, FR-2 and FR-3
All three are mainly used for single-layer PCBs. So what are the differences?
- FR-1 Tg of ~ 130°C
- FR-2 Tg of ~ 105°C
- FR-3 Tg of ~ 105°C, but instead of phenolic resin of FR-2, FR-£ uses an epoxy resin binder
CEM-1: a cheaper alternative to FR-4
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: very similar to FR-4
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:
- Standard, with UV blocking or without it
- Compatible with the lead-free reflow technology
- Halogen-free, without phosphorus and antimony, non-toxic
- With normalized index of CTI ≥ 600
The rest of the CEM family
- CEM-2: cellulose paper core and woven glass fabric surface
- CEM-4 quite similar as CEM-3 but not flame-retardant
- CEM-5 (also called CRM-5) has polyester woven glass core
Which materials for multi-layer PCBs?
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.
• Flex-rigid materials
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.
At a glance
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.
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