Abstract
Residual deformation evaluation of underfill (UF) materials in flip chips is crucial to improve the reliability of electronic packages. In this study, we propose to evaluate the residual thermal strain distributions using an inverse method based on the sampling moiré technique. Even if a grid pattern is fabricated on the specimen at room temperature, the residual strain distributions at an arbitrary temperature relative to the specimen formation temperature can be successfully calculated. The residual strain distributions relative to the free contraction state at an arbitrary temperature can also be measured when the coefficient of thermal expansion is available. A thermal chamber for flip chips was designed under a laser scanning microscope. Using the proposed method, the normal, shear and principal internal strain distributions and deformation characteristics of two kinds of UFs in flip chips were investigated relative to 150 °C. The strains of the UF with low glass transition temperature (UF-A) concentrate near the die material, especially at the die corner, while the strain concentration of the underfill with high glass transition temperature (UF-B) mainly occurs at the die corner and the buffer layer. The maximum principal strain of UF-A is greater than that of UF-B around the die corner. The residual maximum principal strain distributions relative to the free contraction state at 25 °C were compared with the simulation results by the finite element method. The residual strain distribution trends from experiments are consistent with those from simulations.
Highlights
With the development of high-density semiconductor packages, to achieve high functions including heat resistance, toughness, heat dissipation, etc., more and more attention has been focused on the underfill which is a semiconductor encapsulant resin [1]
3) Dynamic deformation measurement: The proposed moiré technique is suitable for dynamic deformation measurement and able to display results in real time; 4) Wide strain measurement range: Both large deformation and small deformation can be accurately measured even if the periodic pattern is inclined at a large angle
The inverse approach can be combined with other grid-based methods for residual strain measurement, including geometric phase analysis (GPA) using Fourier transform (FT) or windowed Fourier transform (WFT)
Summary
With the development of high-density semiconductor packages, to achieve high functions including heat resistance, toughness, heat dissipation, etc., more and more attention has been focused on the underfill (hereinafter referred to as UF) which is a semiconductor encapsulant resin [1]. Due to the strong nonlinearity of the UF physical properties and the complex micro-nano compositions, optimum design of UF is quite difficult. UF is a liquid thermosetting resin and injected into a flip chip package (hereinafter referred to as FCPKG) at normal temperature, cured at high temperature, and cooled to room temperature again. Since stress and strain are directly related, measurement of internal strain in FCPKG is useful for the optimal design of UF and life extension of FCPKG
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