PCBMASTER Helps You Understand What the Thermal Expansion Coefficient of a PCB Board Is and How to Measure It

Author: Jack Wang

I. Why Does a PCB Board "Deform" When Heated? Inspiration from a Beer Bottle Cap

When you open an iced beer, have you ever noticed the property of the metal bottle cap shrinking when it gets cold? The materials in a PCB board also exhibit this "thermal expansion and contraction" phenomenon - this is the essence of the Coefficient of Thermal Expansion (CTE).

Key Data in 2024:
According to the IPC - 4101 standard, the CTE of the Z - axis of an FR4 substrate is 70 ppm/°C (from room temperature to the Tg value), which is 4.1 times that of copper (17 ppm/°C). When the temperature difference exceeds 120°C, a 1 m² FR4 substrate will experience a deformation of 1.68 mm (Calculation formula: ΔL = CTE × L × ΔT).

II. The Disaster Scene of CTE Mismatch: A $2.2 Million Lesson from a German Industrial Equipment Manufacturer

In 2023, a German automation equipment manufacturer did not specify the CTE parameter of the PCB material, resulting in the following problems for 2,000 control boards in a high - temperature environment:

①Solder Joint Fracture: There is a displacement difference of 53 ppm/°C between the copper pad (CTE 17 ppm) and the FR4 substrate (CTE 70 ppm).

②Via Tearing: The fracture rate of the copper layer in blind vias is as high as 37% during the thermal cycle test (Data source: Siemens Quality Report).

Solution: After changing to a ceramic substrate with a CTE of 25 ppm, the product failure rate dropped from 19.7% to 0.8% according to the IEC 60068 - 2 - 14 test standard.

III. Practical Comparison of 3 Mainstream CTE Measurement Methods

Method 1: Thermomechanical Analyzer (TMA)

①Principle: Continuously monitor the dimensional changes of the sample in the range of - 150°C to 1200°C with a probe.

②Accuracy: ±0.05 μm (equivalent to 1/2000 of the thickness of a human hair).

③Cost: The cost per test is $80 - $120 (complies with the IPC - TM - 650 2.4. 24.5 standard).

Method 2: Strain Gauge Method

①Advantage: Can monitor the actual deformation of PCB assemblies in real - time.

②Data: A certain automotive electronics manufacturer found through this method that during the - 40°C to 125°C cycle, the shear stress on the BGA solder balls reached 38 MPa. When the CTE difference > 25 ppm/°C, the fatigue life of the solder joints is shortened to 500 cycles.

Method 3: X - Ray Diffraction (XRD)

Applicable Scenarios: Suitable for special materials such as ceramic substrates and metal - core PCBs.

Limitations: Unable to detect the Z - axis expansion of multi - layer boards (detection depth is only 20 μm).

Measurement Method Selection Matrix:

Material Type	Preferred Method	Alternative Method	Test Cycle
Conventional FR4	TMA	Strain Gauge Method	2 working days
Metal Substrate	XRD	TMA	3 working days
Assembled Finished Board	Strain Gauge Method	TMA	Real - time Monitoring

IV. Practical Suggestions for Foreign Trade Buyers

European and American Markets

①Automotive electronics must meet CTE ≤ 50 ppm/°C (AEC - Q100 standard).

②For medical equipment, it is recommended to use aluminum substrates with a matching CTE (CTE 23 ppm).

Southeast Asian Markets

Consumer electronics products can accept CTE ≤ 80 ppm/°C (cost reduction of 18% - 22%).

Negotiation Skills

①Require suppliers to provide the original TMA test curve (be vigilant against manufacturers that only provide text reports).

②Compare CTE values in different temperature ranges (pay special attention to the sudden expansion after the Tg temperature).
(50 TMA measurement 40--Load: 98 mN Z ·Heating rate: 5 °C/min ·Probe: Expansion/compression 30 X TmA / μm 20 10 122.1°C 0 5.77× 10⁻⁵ [1/°C] - 10 - 20 40 80 120 200 160 Temperature /°C)