Copper clad laminate (CCL) is the base material a printed circuit board is made from. It is a sheet of insulating dielectric, usually woven glass cloth soaked in resin, with copper foil bonded to one or both sides under heat and pressure. The PCB factory images the circuit pattern onto that copper, etches away what is not needed, and laminates several of these cores together with prepreg to build a multilayer board.
The laminate decides much of what a board can do. Its resin and glass set the glass transition temperature (Tg), how much the board expands with heat, its dielectric constant (Dk) and loss (Df), and its resistance to surface tracking (CTI). Change the laminate and those properties change with it, even if the Gerbers stay identical.
This guide explains what copper clad laminate is made of, the main types of copper clad laminates, which properties matter for your design, and how to specify a PCB laminate so that a substitution does not surprise you.
Key takeaways
- Copper clad laminate is the raw material of a PCB: dielectric core plus copper foil. Prepreg is the uncured bonding layer used between cores.
- Most rigid boards use FR-4 CCL. High-Tg FR-4, low-loss laminates, PTFE and hydrocarbon RF laminates, polyimide, and metal-base laminates exist for specific thermal, electrical, or mechanical needs.
- The properties to compare are Tg, Td, CTE, Dk/Df at your frequency, CTI, and copper foil type. Datasheet values only compare fairly when the test method and frequency match.
- Specify laminate either by exact product name or by a property set, and say whether equivalents are allowed.
- A laminate substitution is safe only when the properties that matter to your design are equal or better and the change is recorded.
What is copper clad laminate made of?
A copper clad laminate has three main ingredients.
| Component | What it is | What it controls |
|---|---|---|
| Reinforcement | Woven glass cloth on most rigid laminates; paper or composite on low-cost grades; none on PTFE film or polyimide film types | Mechanical strength, dimensional stability, and part of the dielectric behavior |
| Resin system | Epoxy (FR-4), modified epoxy, polyphenylene ether blends, hydrocarbon, PTFE, polyimide, or BT | Tg, Td, CTE, moisture absorption, Dk/Df, flame retardancy |
| Copper foil | Thin copper sheet bonded to the surface, sold by weight per area (for example 0.5 oz or 1 oz) | Conductor thickness, etch definition, peel strength, and conductor loss at high frequency |
Copper clad laminate vs prepreg
A core is fully cured laminate with copper already bonded on both sides. Prepreg ("pre-impregnated") is glass cloth carrying resin that is only partly cured. During multilayer lamination, prepreg melts, flows, and cures to bond cores and outer copper foils into one board. So the finished multilayer board is a stack of CCL cores and prepreg layers.
Both come from the same material family. When you specify a laminate, you are usually specifying the core and the matching prepreg together.
Copper foil on CCL
Foil is sold by weight. APT's published stackup catalog lists 0.5 oz base copper at 0.0175 mm and 1 oz at 0.035 mm, with heavier weights for power boards. Outer layers usually start with thinner foil and are plated up to their finished weight.
The foil's surface profile also matters. Rougher foil grips the resin better. Smoother low-profile foil reduces conductor loss at multi-gigabit and RF frequencies. If you work above a few gigahertz, the foil type belongs in the material callout alongside the laminate name.
Types of copper clad laminates
These are the main copper clad laminates, and so the main copper clad PCB material options, a board designer chooses between. Values shown are those APT publishes on its own material and capability pages; product datasheets give the full test conditions.
| CCL type | Construction | Values APT publishes | Typical use | Substitution risk |
|---|---|---|---|---|
| Standard FR-4 | Glass cloth and epoxy | Standard Tg 135 °C | Consumer and general electronics, low layer count | Low, if Tg and thickness tolerance match |
| Mid-Tg FR-4 | Modified epoxy | Mid-Tg 150 °C | Industrial products, moderate reflow counts | Low to moderate |
| High-Tg FR-4 | Multifunctional or phenolic-cured epoxy | Tg 170–210 °C laminates with Td ≥340 °C; Dk ≈4.0 on common grades such as 370HR and IT-180A | Thick multilayers, multiple lead-free reflows, automotive and industrial | Moderate; Td, CTE, and CAF behavior differ between products with the same Tg |
| Low-loss / high-speed | Modified resin systems, often low-profile foil | Megtron 4 Dk 3.6, Df 0.005 at 1 GHz; Megtron 6 Dk 3.4, Df 0.002 at 1 GHz | 10–25 Gbps and faster digital channels, backplanes | High; Dk and Df shift impedance and loss |
| Hydrocarbon / ceramic-filled RF | Thermoset hydrocarbon with ceramic filler | RO4003C Dk 3.38 and RO4350B Dk 3.48 at 10 GHz (test-method value) | RF front ends, antennas, radar | High; Dk tolerance and loss are design inputs |
| PTFE | PTFE with glass or ceramic filler | RT/duroid 5880 Dk 2.20, Df 0.0009 | Microwave and mmWave | Very high; processing and Dk differ by product |
| Polyimide (flex) | Polyimide film with copper, adhesive or adhesiveless | Product-specific | Flex and rigid-flex circuits | High; bend life and adhesive system |
| Metal-base (IMS) | Thin thermally conductive dielectric on aluminum or copper | Product-specific; thermal value depends on test method | LEDs and power devices | High; thermal resistance and voltage rating |
The Dk numbers for Rogers materials are a good example of why test conditions matter. APT's Rogers materials page gives both the clamped stripline test value (3.48 for RO4350B at 10 GHz) and a separate "design Dk" for field solvers (3.66). The material did not change; the measurement method did.
CCL properties: Tg, Dk/Df, CTI
Tg and Td
Tg is the temperature where the resin changes from rigid and glassy to soft and rubbery. Above Tg the laminate expands much faster through its thickness, which strains plated holes. Td is the temperature where the resin starts to lose weight through decomposition. For lead-free assembly with several reflow cycles, both matter. APT's high-Tg PCB page recommends specifying Tg, Td, CTE (x, y, z), and CTI together when selecting laminates, rather than Tg alone.
Dk and Df
Dk (dielectric constant, or relative permittivity) sets the speed of a signal and, with trace geometry, its impedance. Df (dissipation factor, or loss tangent) sets how much signal energy turns into heat. Both vary with frequency, resin content, and glass style. For impedance-controlled boards, the Dk used in your field solver has to match the laminate actually built. That is why changing laminate can move impedance even when copper geometry is identical.
CTI
The comparative tracking index describes how well the laminate surface resists forming conductive tracks under voltage and contamination. It matters for mains-connected and high-voltage products where creepage distances are set by safety standards. APT's high-Tg page lists CTI ≥ 600 options for programs that need high tracking resistance. If your safety standard references a material group, include the CTI requirement in the material callout.
Other properties worth checking
Z-axis CTE, moisture absorption, CAF resistance, flammability rating, and halogen content can each be the deciding property for a particular product. Do not assume two laminates with the same Tg behave the same way on these.
How to choose a PCB laminate
Use the design's hardest requirement to pick the family, then the rest of the requirements to pick the product.
- Signal speed or frequency. If the board carries multi-gigabit links or RF, loss and Dk tolerance come first. Standard FR-4 may still be fine for short, slow channels.
- Thermal history. Count the reflow cycles, rework, and operating temperature. Thick boards with many layers and several lead-free reflows usually push toward high-Tg.
- Voltage and safety. Mains, high-voltage, or creepage-limited designs need the CTI or material group the safety standard expects.
- Mechanical needs. Flex, rigid-flex, or metal-base requirements change the material family altogether.
- Supply and cost. Some laminates are stocked widely; others have long lead times. A hybrid stackup, with low-loss material only on the layers that need it, can control cost.
The PCB material selection guide covers this decision in more depth, including when to move from FR-4 to RF, metal-core, ceramic, or flex materials. If you prefer to work from a specific supplier range, the Kingboard PCB laminate guide maps one supplier's grades by Tg and loss.
Specifying PCB laminate on the drawing
There are two ways to call out a laminate, and the drawing should say which one you are using.
- By product name. For example "Isola 370HR" or "Rogers RO4350B". This leaves no doubt, but it can block a fabricator from using an equivalent when stock is short.
- By property set. For example a minimum Tg and Td, a Dk/Df range at a stated frequency, a CTI requirement, a flammability rating, and halogen-free if required. This allows equivalents but only works if the properties listed are the ones that matter to your design.
In both cases, state:
- The finished board thickness and tolerance
- Copper weight on each layer
- Whether "or equivalent" is allowed, and who approves it
- Any IPC-4101 specification sheet you want referenced
- Foil type if it matters to loss or impedance
APT's FR4 PCB page lists the FR-4 options in regular production: standard Tg 135 °C, mid-Tg 150 °C, high-Tg 170/180 °C, and low-loss FR-4 such as 370HR, IT-180A, EM-370, and S1000H. If your requirement fits one of those, naming it shortens material confirmation.
When laminate substitution is OK
Substitution happens when the named laminate is not available in time or a cheaper equivalent is proposed. It is acceptable when the replacement matches or exceeds every property that your design depends on, and the change is approved and recorded.
Ask these questions before approving one:
- Is the Tg, Td, and z-axis CTE equal or better?
- If the board is impedance-controlled, is the Dk at your frequency close enough that the stackup still meets the impedance target? Will the fabricator recalculate and verify with coupons?
- If loss matters, is Df equal or lower at your frequency, and is the foil the same type?
- Is the CTI, flammability rating, and halogen status the same, especially if the product carries a safety listing?
- Is the thickness of each core and prepreg available, so the finished thickness still holds?
- Will the change be recorded on the traveler and the certificate of conformity?
If you cannot answer yes to the ones that apply, keep the original material or re-qualify. A board that passes electrical test can still fail in the field if the replacement laminate has a weaker Td or CAF resistance.
What to send for a PCB laminate review
- Laminate name, or the property set you require, and whether equivalents are allowed
- Stackup with core and prepreg thicknesses, copper weights, and finished thickness
- Impedance targets and the frequency of interest
- Reflow count, assembly profile type, and operating temperature range
- Voltage, creepage, or safety-standard requirements that affect CTI
- Any reliability tests required (for example thermal shock, T260/T288, or CAF)
Why work with APTPCB on laminate selection?
APT reviews the laminate together with the stackup before release, so impedance, finished thickness, and thermal requirements are checked against a material that is actually available. For FR-4 builds, the FR4 PCB manufacturing team can confirm whether a standard, mid-Tg, high-Tg, or low-loss grade fits, and APT's material sourcing agreements cover standard, high-Tg, and low-loss FR-4. For low-loss and RF materials, the material pages list the laminate families APT processes. Any proposed substitution is raised as an engineering query before production, not after.
Relevant standards and references may include:
- IPC-4101: specification for base materials for rigid and multilayer printed boards
- IPC-4103: specification for base materials for high-speed and high-frequency applications
- IPC-4204: flexible metal-clad dielectrics for flexible printed circuitry
- IPC-TM-650: test methods manual (Tg, Td, Dk/Df, peel strength)
- IPC-6012: qualification and performance of rigid printed boards
Frequently asked questions
What is copper clad laminate used for?
Copper clad laminate is the raw material for printed circuit boards. The PCB factory patterns the copper into traces and pads, then stacks and laminates cores with prepreg to build multilayer boards. Nearly every rigid and flexible PCB starts as copper clad laminate.
Is FR-4 the same as copper clad laminate?
FR-4 is a grade of material, glass-reinforced flame-retardant epoxy, and FR-4 copper clad laminate is the most common type of CCL. Other copper clad laminates use different resins and reinforcements, such as PTFE, hydrocarbon, polyimide, or a metal base.
What is the difference between a core and prepreg?
A core is cured laminate with copper foil bonded on both sides. Prepreg is glass cloth with partly cured resin and no copper. During multilayer lamination the prepreg melts and cures to bond cores and outer foils into one board.
Which properties matter most when choosing a PCB laminate?
Start with Tg and Td for thermal survival, Dk and Df at your operating frequency for signal behavior, CTE for plated-hole reliability, and CTI for high-voltage products. Compare values only when the test method and frequency are the same.
Can a fabricator substitute a different laminate?
Only if your documentation allows it. If you name a product and allow equivalents, the substitute should match or exceed every property your design depends on, and the change should be approved and recorded. Impedance-controlled boards usually need the stackup recalculated for the new material.
How thick is copper on a copper clad laminate?
Copper is specified by weight per area. APT's stackup catalog lists 0.5 oz base copper at 0.0175 mm and 1 oz at 0.035 mm, with heavier weights available for power boards. Outer layers are often plated up from a thinner starting foil.
Copper clad laminate quote checklist
Use this when you request a CCL or multilayer laminate quote from a material supplier or board house.
- Base material: FR-4, High-Tg, Rogers, Megtron, PTFE, or Hybrid (state grade and product line)
- Tg value: 135 / 150 / 170 / 180+ °C (state which)
- Td value: ≥320 °C preferred (state minimum if different)
- Dk and Df at operating frequency: state value and test frequency (1 GHz / 10 GHz / other)
- CTI (Comparative Tracking Index): PLC 0–5 or voltage rating
- Inner layer copper: HOZ, H1, H2, or H3 (state which)
- Outer layer copper after plating: state oz or µm
- Prepreg product number or牌号 (state)
- Glass cloth style: 106 / 1080 / 2116 / 7628 or other
- IPC-4101 slash sheet and UL94 flame class
- RoHS and REACH compliance required Y/N
- Stackup symmetry requirement (state tolerance if any)
The laminate is part of the design
Copper clad laminate is not a commodity input that the factory can swap freely. Its Tg, Dk, Df, CTI, and foil type are design parameters, the same as trace width or layer count. Pick the family from the hardest requirement, name the product or the property set on the drawing, and decide in advance which substitutions you will accept.