PCB Laminate FR-4 vs. Steel AISI 4340

Compare PCB Laminate FR-4 vs. Steel AISI 4340 strength, stiffness, weight and thermal properties.

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Mechanical
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Yield strength	not given (tensile used)	1,035MPasourcetypical, transverse
Ultimate tensile strength	246–385MPasourcetypical, in-plane: cross direction .. length direction (ASTM D3039)	1,140MPasourcetypical, transverse
Flexural strength	531–621MPasourcetypical: cross direction .. length direction (IPC-TM-650 2.4.4B)	not sourced
Elongation at break	not sourced	18%sourcetypical, transverse
Young's modulus (stiffness)	21.9–25.8GPasourcetypical, in-plane: cross direction .. length direction (ASTM D790-15e2)	200GPasourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Density	1,920kg/m³sourcetypical, 50% resin content; another FR-4 producer: Park Nelco N4000-6 (high-Tg multifunctional epoxy), not Isola 370HR	7,840kg/m³sourcetypical
Strength to weight	164kN·m/kg24% higher	132kN·m/kg
Stiffness to weight	12.4MN·m/kg	25.5MN·m/kg2.05x higher
Poisson's ratio	0.171–0.177sourcetypical, in-plane: cross direction .. length direction	0.32sourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Shear modulus	10.2GPa	75.7GPa7.45x stiffer in shear
Bulk modulus	12.2GPa	185GPa
Speed of sound	3,525m/s	5,049m/s
Thermal
Thermal conductivity	0.4W/m·Ksourcetypical (ASTM E1952); direction not stated by the sheet	37.5W/m·Ksourcetypical
Thermal expansion	13–14µm/m·Ksourcetypical, in-plane X/Y-axis, below Tg (IPC-TM-650 2.4.24C)	11.3µm/m·Ksourcemean, -18 to 93 °C
100 mm part over a 50 °C swing	67.5µm growth	56.5µm growth19% less
Specific heat	1,200–1,400J/kg·Ksourcetypical (ASTM E1461); another FR-4 producer: Park Nelco N4000-6 (high-Tg multifunctional epoxy), not Isola 370HR	448J/kg·Ksourcetypical
Heats up and cools (diffusivity)	0.16mm²/s	10.7mm²/s66.6x faster
Thermal shock resistance	324W/m	11,678W/m36.1x more resistant
Glass transition	180°Csourcetypical, DSC (IPC-TM-650 2.4.25C)	not sourced
Max service temperature	130°CsourceUL Relative Thermal Index (UL 796)	not sourced
Values for	Isola 370HR FR-4 laminate (Polyclad PCL-FR-370HR, 180 C Tg multifunctional epoxy, E-glass fabric), copper removed; Isola data sheet Revision C, last updated April 15, 2021. Woven-glass laminate: the sheet gives in-plane values per weave direction (length / cross) and the Z-axis CTE separately; the solver is isotropic per material.	Carpenter (Latrobe) Lescalloy 4340 VAC-ARC datasheet; mechanical row for 1100 °F temper; physical properties grade-level

Every value links to the document that states it. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.

Questions

Is PCB Laminate FR-4 stronger than Steel AISI 4340?

Steel AISI 4340 is stronger: its yield strength is 1,035 MPa against tensile strength 315 MPa for PCB Laminate FR-4 (3.28x).

Which is lighter, PCB Laminate FR-4 or Steel AISI 4340?

PCB Laminate FR-4 is lighter: 1,920 kg/m³ against 7,840 kg/m³ for Steel AISI 4340.

Which is stiffer, PCB Laminate FR-4 or Steel AISI 4340?

Steel AISI 4340 is stiffer: Young's modulus 200 GPa against 23.9 GPa for PCB Laminate FR-4, so the same part in Steel AISI 4340 deflects less under the same load.

Which is lighter for the same job, PCB Laminate FR-4 or Steel AISI 4340?

For the same stiffness or strength: PCB Laminate FR-4 is lighter for a stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam, strong panel or plate; Steel AISI 4340 is lighter for a stiff rod or tie (tension).

Part that must be	PCB Laminate FR-4	Steel AISI 4340
Stiff rod or tie (tension)	2.05x heavier	lighter
Stiff beam (bending)	lighter	41% heavier
Stiff panel or plate	lighter	2.01x heavier
Strong rod or tie	lighter	24% heavier
Strong beam	lighter	1.85x heavier
Strong panel or plate	lighter	2.25x heavier

For the same stiffness or strength, mass scales with density over stiffness (or strength) raised to a power set by how the part is loaded: 1 for a rod in tension, 1/2 for a beam in bending, 1/3 for a panel (the standard material-selection indices).

Which conducts heat better, PCB Laminate FR-4 or Steel AISI 4340?

Steel AISI 4340 conducts heat better: 37.5 W/m·K against 0.4 W/m·K for PCB Laminate FR-4.

Which expands less with temperature?

Steel AISI 4340 expands less: 11.3 µm/m·K against 13.5 µm/m·K for PCB Laminate FR-4.

Among 72 materials

Stiffness against density

Strength against density