A Better Way to Characterize Copper Foils for Flexible Printed Circuits, Advanced Packaging

Estimated reading time: 3 minutes

Editor’s note: These interview questions are based on a paper titled "Young's Modulus of Rolled Copper Foils Obtained from Stress-Strain Curves Measured by Digital Image Correlation" by Okabe, Kammuri, and Koike.

Finite element analysis is only as accurate as the material properties behind it. Kenji Koike discusses research showing that digital image correlation produces more precise stress-strain curves and Young's modulus measurements for rolled copper foils, giving engineers greater confidence when designing flexible printed circuits and advanced packaging for demanding applications.

What challenge in flexible printed circuit (FPC) design and reliability motivated this study?

This study was motivated by the need for more accurate material properties for Finite Element Analysis-based FPC design and reliability prediction. As FPCs become increasingly flexible and are used in applications requiring high fatigue resistance, the accuracy of stress–strain curves and Young’s modulus of thin copper foils becomes critical. However, conventional tensile-testing methods often cannot accurately measure the true strain of thin copper foils, leading to errors in Young’s modulus and reduced reliability of FEA predictions.

How does the increasing demand for highly flexible and fatigue-resistant FPCs influence the need for improved mechanical characterization techniques?

As FPCs are increasingly used in applications requiring extreme flexibility and long fatigue life, such as wearable devices, FEA has become essential for predicting mechanical reliability to reduce time-consuming fatigue testing. Since FEA accuracy depends heavily on the accuracy of the copper foil stress–strain curve and Young’s modulus, more advanced characterization techniques such as digital image correlation (DIC) are needed to directly measure the true strain of thin copper foils and provide reliable material data for design and lifetime prediction.

Research Methods and Experimental Design

What advantages does Digital Image Correlation (DIC) offer over traditional strain measurement techniques such as strain gauges, resonance methods, indentation methods, or bulge testing?

DIC offers three major advantages: non-contact strain measurement for thin copper foils, direct measurement of true specimen strain without errors from machine compliance or grip slippage, and acquisition of a full stress–strain curve rather than only a Young’s modulus or localized property.

The study evaluated three rolled-annealed (RA) copper foils with distinct metallurgical textures (RA1, RA2, and RA3). Why was it important to include materials with varying crystal orientations and grain structures?

We included RA1, RA2, and RA3 because crystallographic texture and grain structure significantly affect the mechanical behavior and Young’s modulus of copper foils. By testing both cubic-textured foils (RA1 and RA2) and a randomly textured foil (RA3), we verified that the DIC method can accurately and reproducibly measure stress–strain curves regardless of metallurgical texture, making it broadly applicable to various copper foils for FPCs.

Key Findings

What evidence demonstrated that the DIC method produced more accurate Young's modulus measurements than the conventional tensile-testing approach?

The strongest evidence was that the Young’s modulus values obtained by DIC closely matched independent resonance-method and electron backscatter diffraction (EBSD) predictions, whereas the conventional method consistently underestimated the modulus.

The paper reports that RA1 and RA2 had relatively similar textures, yet DIC could distinguish differences in Young's modulus. Why is this capability significant?

This capability is significant because the cube-texture fraction is related to both Young’s modulus and fatigue performance. The ability of DIC to detect subtle modulus differences suggests it may serve as a practical indicator of microstructural differences relevant to fatigue behavior.

How did the results obtained from RA3 strengthen confidence in the DIC methodology?
RA3 had a completely different texture and grain structure from RA1 and RA2. Despite this, the DIC method still produced results consistent with the reference methods, whereas the conventional method showed greater deviation. This demonstrated that DIC can accurately evaluate copper foils with different metallurgical textures, confirming its robustness and broad applicability.

Implications for Advanced Packaging Technologies

If you were implementing this methodology within an advanced packaging development program, what additional studies or validation work would you perform before incorporating the DIC-derived material properties into production-level simulation models?

I would evaluate a broader range of copper foils and packaging materials, including different thicknesses, textures, and processing conditions, to confirm that the methodology remains robust for production materials.
Kenji Koike began his career in 2004, joining JX Advanced Metals after graduating with an engineering degree from Tohoku University. He received a master’s in metallurgical engineering from the Colorado School of Mines, and has continued his career at JX Advanced Metals, working in marketing and development