PCB base materials from FR-4 to Rogers and flex

The base material of a printed circuit board is the insulating layer (the dielectric) that holds the copper. It decides how much heat the board survives, how fast signals travel along it, how well it carries heat away, and whether it bends. Most boards use the same material; you only need another one for a reason.

What a laminate is

A rigid board starts as a laminate: a sheet of insulating material with copper foil bonded to one or both sides. The insulator is usually woven glass fibre soaked in resin and cured. Layers of a multilayer board are held together by prepreg, glass cloth with partly cured resin that melts and bonds during lamination. Three numbers describe a laminate for most purposes:

1. Copper foil, bonded to the face.
2. Resin: the cured epoxy that holds everything together and insulates.
3. Glass cloth: the ovals are bundles of fibre cut across, the wavy strand runs along. It gives strength and a steady thickness.
4. Copper on the other face. A core has foil on both sides, and the copper pattern is etched from it later.

• Tg (glass transition temperature). The temperature at which the resin turns from rigid to soft. Above Tg the board expands much faster, and stresses holes and plating.
• Dk (dielectric constant) and Df (dissipation factor). How strongly the material affects electric signals and how much signal energy it absorbs. They matter at high frequencies and for controlled impedance.
• Thermal conductivity. How well heat flows through the dielectric.

FR-4

FR-4 is the common name for glass-reinforced epoxy laminate with a flame-retardant additive. It is cheap, strong, easy to process, and good enough for most electronics. Its Dk is typically about 4 to 4.6, depending on the resin content and the frequency, and varies from batch to batch and between makers, so a designer who needs a precise value asks for it from the stack-up.

FR-4 comes in grades, mainly by Tg:

Lead-free solder melts at a higher temperature than tin-lead solder, and the board goes through the oven at the higher temperature, sometimes twice. Boards with many layers, thick boards, and boards that will be reworked benefit from a higher Tg. A related figure, Td (decomposition temperature), tells when the resin starts to lose mass, and is another reason to choose a better grade for hard service.

Above its Tg the resin softens and the board expands much faster. The drawing compares two grades.

1. The Tg of a lower grade: above this temperature the board grows much faster.
2. The Tg of a higher grade: the board stays rigid up to a higher temperature.

Other FR-4 variants are sold: halogen-free versions for markets that ask for them, and grades with a better resistance to conductive anodic filaments (a slow failure inside the board). Ask for them when your product needs them.

Metal-core boards (aluminium and copper)

A metal-core board bonds a thin copper circuit layer, through a thin insulating layer, to a metal base, usually aluminium and sometimes copper. The point is heat: the insulating layer is made to conduct heat far better than FR-4, and the metal spreads it. Typical users are LED lighting and power conversion.

Things to know:

• Most are single-sided; double-sided and multilayer versions exist and cost more.
• The dielectric layer's thermal conductivity varies widely between grades, so the figure on the data sheet is worth asking for.
• Aluminium expands differently from the parts on it, which can stress large ceramic parts and solder joints under temperature cycles.
• Drilling and routing a metal board is slower and the board is heavier. Ask about the limits.

High-frequency laminates (Rogers, PTFE and others)

At radio and microwave frequencies, standard FR-4 loses too much signal, and its Dk varies too much. Special laminates have lower and steadier Dk and much lower Df. They fall into two groups: PTFE-based (Teflon type) and ceramic-filled hydrocarbon or similar materials. Rogers Corporation is a well-known maker; other makers sell equivalents. Dk values range from about 2 to 10 across the families, so the choice follows the design.

Practical points:

• They are more expensive per area than FR-4.
• They may need special processing, since PTFE is soft and slippery.
• Many designs use a hybrid: a thin high-frequency layer for the radio part and FR-4 for the rest, which brings cost down but needs a stack-up that works for the factory.
• Ask the factory which brands and thicknesses it can supply before you design the board around one.

You rarely need these below 1 GHz. Between 1 and a few gigahertz, careful FR-4 design works for many products; above that, or when losses matter, a better laminate becomes the sound choice.

Flexible and rigid-flex boards

A flexible board is made on polyimide film, not on glass fibre. It bends, and can be folded once to fit, or bent over and over in a moving part. The two uses are different: a bend made once is a static bend, repeated bending is a dynamic bend, and the design rules (copper type, bend radius, track routing) are not the same. Rigid-flex combines rigid sections and flexible connections in one piece.

Points to remember:

• Flex needs a bend radius that depends on its thickness; designers use a multiple of the total thickness, with larger multiples for more layers and for dynamic use.
• Stiffeners (extra rigid material bonded behind connector areas) are added where parts are mounted or where a connector plugs in.
• Flex and rigid-flex cost much more per area than FR-4.
• Flex absorbs moisture and should be dried before assembly.

1. FR-4: copper, a rigid core of glass cloth in resin (the stripes), copper.
2. Metal core: a copper circuit, a thin insulating layer, then a thick metal base.
3. Flex: a thin film with copper on it. It bends instead of staying flat.
4. Heat from a part on a metal-core board goes down through the thin insulator into the base.

How to choose

1. Start with FR-4. Choose the Tg from the soldering process and the layer count.
2. Choose a metal core when the heat from a few parts is the problem, and move the heat out through the board.
3. Choose a high-frequency laminate when losses or a stable Dk set the performance, usually for radio.
4. Choose flex when the product moves or must fit a shape a rigid board cannot.
5. When in doubt, ask the factory before designing the board around a material.

How this shows up when you order here

The materials, thicknesses and special processes offered are listed on the capabilities page, and the list in force is the list that counts. The factory works with FR-4, aluminium, copper and flexible substrates, and with high-Tg, halogen-free and high-frequency materials. Which of these you can order through the site is set on the capabilities page, and an engineer confirms every order. Stack-ups and impedance are covered in Copper weight, stack-ups and impedance.