Investigation of Geometry, Frequency and Material's Effects in Lock-In Thermography Applications in Semiconductor Packages

Lock-In Thermography (LIT) is a powerful non-contact and non-destructive technique that can be utilized to detect high-resistance defects inside semiconductor package. Its advantages over other non-contact and non-destructive techniques are its high signal-to-noise ratio (SNR), its fast acquisition time, and its capability to pinpoint high-resistance defects in three-dimensional space, or (x, y, z) coordinates. The current LIT applications interpret LIT data based on the conclusions drawn from semi-infinite model. However, semiconductor package has finite dimensions, which is considered a finite model, not semi-infinite model. This paper studies the effects of geometry towards the accuracy of the interpretation of LIT data based on semi-infinite model. The study was performed numerically using finite element method, combined with analytical solution. The results of the study show that finite dimensions in semiconductor package can affect phase angles that are different from those predicted by semi-infinite model. Since defect's z-coordinate is determined by phase angle, inaccurate results of defect's z-coordinate may be obtained from semi-infinite model. Several other factors -such as thermal diffusivity of material, convection heat transfer coefficient (h), and lock-in frequency (f
_lock-in
) - can also affect the value of phase angle.