What is a PCB?
A printed circuit board (PCB) is a board made of fiberglass, composite epoxy, or other laminate material that connects electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. PCBs are used in virtually all electronic products, including computers, smartphones, appliances, etc.
PCBs contain components like capacitors, resistors, inductors, transformers, integrated circuits (ICs), and more, soldered onto the board. The board provides mechanical support and electrically connects the components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate.
What is FR4?
FR4 (or FR-4) is a grade designation assigned to glass-reinforced epoxy laminate sheets and printed circuit boards (PCB). FR4 is the most widely used insulating material for making PCBs. “FR” stands for flame retardant, and type “4” indicates woven glass reinforced epoxy resin.
FR4 glass epoxy is a popular and versatile high-pressure thermoset plastic laminate grade with good strength to weight ratios. With near zero water absorption, FR4 is commonly used as an electrical insulator possessing considerable mechanical strength.
The material is known to retain its high mechanical values and electrical insulating qualities in both dry and humid conditions. These attributes, along with good fabrication characteristics, lend utility to this grade for a wide variety of electrical and mechanical applications.
Physical Properties of FR4
Key physical properties of FR4 PCB material include:
| Property | Value |
|---|---|
| Density | 1.85 g/cm³ |
| Water Absorption | 0.1% |
| Tensile Strength | 415 MPa |
| Compressive Strength | 415 MPa |
| Flexural Strength | 485 MPa |
| Elastic Modulus | 18.6 GPa |
| Poisson’s Ratio | 0.15 |
| Hardness | 110 Rockwell M |
Thermal Properties of FR4
Some key thermal properties of FR4 PCB material are:
| Property | Value |
|---|---|
| Thermal Conductivity | 0.3 W/m·K |
| Specific Heat Capacity | 1150 J/kg·K |
| Coefficient of Thermal Expansion (CTE) – X Axis | 12-14 ppm/°C |
| Coefficient of Thermal Expansion (CTE) – Y Axis | 14-16 ppm/°C |
| Coefficient of Thermal Expansion (CTE) – Z Axis | 50-60 ppm/°C |
| Glass Transition Temperature (Tg) | 140-180°C |
What is Coefficient of Thermal Expansion (CTE)?
The coefficient of thermal expansion (CTE) is a thermodynamic property of a substance. It relates a change in temperature to the change in a material’s linear dimensions.
Specifically, the coefficient of linear thermal expansion (α) is defined as the fractional change in length (linear dimension) per degree change in temperature at a constant pressure:
$\alpha = \frac{\Delta L}{L \Delta T} = \frac{1}{L}\frac{dL}{dT}$
where
– $L$ is the original length
– $\Delta L$ is the change in length
– $\Delta T$ is the change in temperature
The units of CTE are (°C)⁻¹ or (K)⁻¹ (since the value is the same in both units).
CTE values are typically very small. So for convenience, they are often expressed in parts per million per kelvin (ppm/K) or parts per million per degree Celsius (ppm/°C). To convert to these units, multiply the raw CTE value by 10⁶.
Different materials expand by different amounts for a given temperature change. Materials with a high CTE expand more with increasing temperature compared to materials with a low CTE.
In general, metals have higher thermal expansion coefficients than ceramics, polymers and glasses. Plastics like FR4 have higher CTE values than metals. And CTE values for composites like FR4 are highly anisotropic (different in different directions).
CTE of FR4 PCB Material
The coefficient of thermal expansion of FR4 printed circuit board material varies depending on the direction or axis. This is because FR4 is a composite material made of woven glass fabric impregnated with epoxy resin. The fibers constrain expansion in the XY plane (in-plane) compared to the Z direction (out-of-plane).
Typical CTE values for FR4 are:
| Axis | CTE Value |
|---|---|
| X | 12-14 ppm/°C |
| Y | 14-16 ppm/°C |
| Z | 50-60 ppm/°C |
Note that the CTE in the Z direction (through the thickness of the board) is much higher than the X and Y directions (in the plane of the board). This is because there are no glass fibers oriented vertically to constrain the epoxy’s expansion in the Z direction.
The CTE values also vary somewhat depending on the exact type and grade of FR4. Values may differ between manufacturers as well. But in general, the CTE of FR4 will be in the ranges shown above.
Effects of CTE Mismatch
When materials with mismatched CTE values are bonded together, stresses and strains can develop as the temperature changes due to the materials expanding/contracting at different rates. This can lead to warping, bowing, delamination, fracture and other reliability issues if not properly accounted for.
In PCBs, CTE mismatches can occur:
– Between the FR4 substrate and copper traces/planes
– Between the PCB and surface mounted components
– Between the PCB and the solder joints
– Between different layers in the stackup (especially near transitions between standard FR4 and high-Tg FR4)
Copper has a CTE around 16-18 ppm/°C, while FR4 is around 14 ppm/°C in-plane. This small mismatch is usually tolerable. The epoxy resin helps constrain the copper, resulting in the PCB having an “effective” CTE of roughly 15-16 ppm/°C, close enough to not cause major issues under normal operating conditions.
However, greater problems can arise when surface mounting components that have very low CTE values compared to FR4, such as ceramic chip capacitors (~8 ppm/°C). Or when using lead-free solders which have higher CTE values than traditional tin-lead solder alloys. During thermal cycling, the CTE mismatches can put high stresses on the solder joints leading to cracks and premature failure.
Other factors like component size, standoff height, board thickness, and glass transition temperature (Tg) also affect solder joint reliability. In general, larger components, smaller standoff heights, thinner boards, and lower Tg laminates tend to exacerbate CTE mismatch issues.
Tips for Mitigating CTE Mismatch
Some strategies for mitigating CTE mismatch issues in PCBs include:
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Use PCB materials with lower and/or better matched CTEs to sensitive components
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Be mindful of the board stackup, avoiding CTE mismatches between layers
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Use caution mixing high Tg and standard FR4 layers
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Be careful with very large components like BGAs
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Provide sufficient solder joint standoff height
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Limit overall board size, or use expansion slots for very large boards
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Avoid placing sensitive components near board edges and corners where stresses are highest
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Use compliant lead designs to allow more flexibility
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Limit the temperature range and number of thermal cycles the board will see in service
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Perform modelling and simulation to evaluate stresses and optimize the design
FAQ
What does FR4 stand for?
FR4 stands for “Flame Retardant 4”. The “FR” indicates the material is flame retardant, and type “4” indicates woven glass reinforced epoxy resin.
Is FR4 the same as G10?
G10 and FR4 are similar materials – both are glass-reinforced epoxy laminates. The main difference is FR4 has better flame retardancy due to additives in the epoxy. FR4 is the grade more commonly used for making PCBs.
What is the CTE of copper?
The coefficient of thermal expansion for copper is around 16-18 ppm/°C.
What happens when materials with different CTE are joined together?
When materials with mismatched CTE are bonded together, stresses and strains can develop as the temperature changes due to the materials expanding/contracting at different rates. This can lead to warpage, delamination, fracture and premature failure if the stresses are too high.
How can you reduce CTE mismatch problems in PCBs?
Some ways to mitigate CTE mismatch issues include using PCB materials with lower and better matched CTEs, optimizing the layer stackup, providing sufficient solder joint standoff height, limiting board size, avoiding placing sensitive components near board edges, and using compliant lead designs. Careful modelling and simulation can help evaluate and optimize the design.
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