Fibreglass PCB: Why FR-4 Still Dominates

What a Fibreglass Board Is

The material at the heart of most circuit boards is a sheet of woven glass fabric impregnated with epoxy resin. The glass gives it mechanical strength, dimensional stability and a low absorption of moisture; the resin binds the fibres, provides the electrical insulation and carries the flame retardant chemistry. Copper foil is bonded to one or both faces, and the resulting sheet is called a copper clad laminate.

The most common grade is FR-4, where FR stands for flame retardant and the resin system is a brominated epoxy. It is not the best material at anything, and that is exactly why it dominates: it is strong enough, insulating enough, heat resistant enough and cheap enough to be the right answer for the great majority of circuits.

The Construction of the Laminate

• Glass fabric. Woven from fine glass filaments, in a style that sets the thickness and the mechanical properties. The weave is visible on a bare board under the mask.
• Epoxy resin. Impregnates and surrounds the glass. The resin is the dielectric, and its chemistry sets the glass transition temperature and the flame rating.
• Flame retardant. Brominated compounds give the material its UL 94V-0 rating, which means a vertical sample stops burning within a defined time.
• Copper foil. Usually one or two ounces per square foot, bonded to the cured laminate under heat and pressure.
• Filler and reinforcement. Some grades add a mineral filler to reduce expansion or lower the dielectric constant.

On the finished board the laminate is covered by solder mask and legend, and the exposed copper carries a surface finish. Those layers are what the user sees, but the laminate underneath sets the mechanical and electrical behaviour of everything above it.

Properties That Matter

Glass transition temperature. Standard FR-4 has a Tg between about 130 and 140 degrees Celsius, and the high Tg grades go from 150 to 180 and beyond. The number matters because the material expands much faster above the transition temperature, which is exactly the range a board sees during reflow. A thicker board with many layers, or a board that will be reflowed twice, benefits from a higher Tg because it expands less and warps less.

Flammability. The UL 94V-0 rating is the standard for boards used in appliances, information technology equipment and many other regulated products. It is a property of the resin system, and it must be maintained by the whole construction, not just the laminate.

Dielectric constant and loss. FR-4 has a dielectric constant around 4.2 and a loss tangent in the region of 0.02, which is adequate into the low gigahertz range for short traces and becomes a problem on long high speed lines. Above a few gigahertz, or on a long line at a lower frequency, the loss forces a move to a PTFE or ceramic filled material.

Moisture absorption. Low enough for ordinary environments, and still enough that boards should be baked before a process that shocks them thermally. Absorbed water turns to steam during reflow and blisters the laminate.

Mechanical strength. High in tension and bending, which is what allows a 1.6 millimetre board to carry connectors and to be handled thousands of times in production. It is also why a fibreglass board does not bend to fit a curved housing, and a polyimide flex circuit is used there instead.

How the Board Is Made

The laminate is cut to panel size and the layers, if there is more than one, are laminated together with prepreg between them. Holes are drilled, copper is deposited in the holes and plated up on the surfaces, and the conductor pattern is formed by imaging a resist and etching the unwanted copper away. Solder mask is printed and cured, legend is printed over it, and the pads receive their finish. Electrical test checks the nets for opens and shorts, and optical inspection checks the pattern and the mask. Our notes on PCB manufacturing describe each step and the controls that keep them in tolerance.

Two properties of the material shape the process. The glass fabric is abrasive, so drill bits wear and their life has to be managed. And the resin absorbs moisture, so panels are baked before lamination and before any thermal process, which is why a fabricator keeps a controlled store and a schedule rather than leaving panels open to the air.

Where It Is Used

• Consumer electronics. Phones, laptops, wearables, televisions and games consoles, in single sided through to high layer count constructions.
• Automotive. Instrument clusters, body control modules, lighting and sensor boards, often in a higher Tg grade to survive the thermal cycles.
• Industrial control. Programmable logic controllers, drives, sensors and power modules.
• Medical equipment. Monitoring and diagnostic instruments, in a high Tg grade where the product is sterilized.
• Lighting. LED strips, drivers and display modules, in FR-4 where the power is moderate and in metal core laminate where it is not.

Its reach is wider than any other substrate, which is the practical meaning of the phrase general purpose material.

Fibreglass Against Other Substrates

Polyimide. Higher temperature capability and available as a thin flexible film, at several times the cost of FR-4. It is chosen for flexible circuits and for boards that must survive extreme heat, not for ordinary rigid boards.

Aluminium backed laminate. A metal base with a thin dielectric, which moves heat far better than fibreglass but is limited in most constructions to a single copper layer. It is chosen for LED and power boards where the thermal path is the problem.

Ceramic. Excellent electrical and thermal stability, very expensive, brittle and only practical in small sizes. It belongs in high power modules and microwave circuits.

Composite materials such as CEM-1 and CEM-3. Cheaper than FR-4, single sided or limited in layer count, and used where the price of the laminate decides the price of the product.

The comparison usually ends with FR-4 winning because the alternative costs more than the improvement is worth. Where that is not true, the reason is almost always thermal, mechanical or high frequency, and those are exactly the three cases in which the choice is made.

What It Costs

Indicative prices for a small fibreglass board in moderate quantity run around 0.80 to 1.50 US dollars for a single sided board of about 100 by 100 millimetres at a hundred pieces, 1.20 to 2.50 dollars for a double sided board, and 3.00 to 5.50 dollars for a four layer board. An electroless nickel immersion gold finish adds roughly 0.30 to 0.80 dollars per board over hot air levelling, and an expedited build adds 20 to 50 percent.

Quantity dominates everything else. Above a thousand pieces the unit price commonly falls by 30 to 40 percent, and the price of the laminate stops being the visible part of the cost, because the drilling, the imaging, the test and the handling are the steps that consume the plant capacity.

Choosing a Supplier

Look for the certifications that apply to the market, a transparent statement of the laminate grade including the glass transition temperature, a design rule review before quotation, and a test regime that covers every board. Our notes on quality management describe the controls to ask about, our notes on PCBA testing cover the verification after assembly, and our notes on PCB design and layout cover the design rules the material needs. For the assembly that follows, our notes on PCB assembly describe the build.

FAQ

Is a fibreglass PCB the same as FR-4? FR-4 is the most common fibreglass laminate, but the family also includes the high Tg grades and the composite materials. Fibreglass describes the reinforcement, FR-4 describes the specific flame retardant resin system.

What temperature can a fibreglass board withstand? Standard grades are rated to about 130 to 140 degrees Celsius at the board, and high Tg grades to 150 to 180 degrees and above.

Can fibreglass boards be used at high frequency? At low gigahertz frequencies over short traces, yes. Above that, or on long lines, the loss forces a move to a PTFE or ceramic filled laminate.

How much does a fibreglass PCB cost? Around 0.80 to 2.50 US dollars for a small single or double sided board in moderate quantity, and 3.00 to 5.50 dollars for a four layer board.

Conclusion

Fibreglass boards are the default because the material is good enough at everything and cheap at all of it. Understand the glass transition temperature, the flame rating and the loss, bake the panels before thermal processes, and choose a different substrate only when the thermal, mechanical or high frequency requirement genuinely exceeds what FR-4 can deliver.