Application Note: Voids and Delamination Detection¶
Transient (dynamic) thermal response — the heating and cooling curves a device produces after excitation — contains the full signature of the underlying heat-conduction path. Because voids, delamination, and other subsurface defects change how heat diffuses through a package, transient thermal imaging can be used to detect and localize these defects even when they are hidden beneath opaque layers such as metal.
Localizing buried heat sources¶
When a heat source is buried under a metal layer, the resulting thermal image at the surface is blurred, since heat spreads laterally as it diffuses upward through the intervening material. Despite this blurring, the location of the buried source can still be localized using transient imaging: by capturing the thermal response over time rather than a single steady-state snapshot, Microsanj systems have demonstrated imaging of heating from an internal metal layer (measured at the M1 interconnect level), distinguishing an early-time overlay (1 ms) from the more diffuse steady-state thermal pattern (5 ms).
Package-level 3D IC void test case¶
Microsanj evaluated a package-level 3D IC test sample built to study void detection, using optical heating applied to the top surface of a stacked structure (aluminum and glass layers, including a glass coverslip cut to 170 µm thickness). Thermal images were captured at multiple time points after the start of heating (t = 0 s, 17 ms, 50 ms, 167 ms, and 200 ms), allowing the evolving thermal pattern to be tracked as heat diffused through the stack. [diagram/image in source — not reproduced here]
Defect depth estimation in metal layers¶
The delay between excitation and the appearance of a thermal response at the surface is directly related to how deep a defect sits below the surface. Microsanj's thermoreflectance measurements have been used to show this relationship on a metal-layer test structure: the thermal response for a 140 Ω short showed a delay of approximately 75 µs before the signal was observed at the poly resistor / M1 measurement point, indicating that the defect (short) was buried below the metal layers rather than at the surface.
This depth-to-delay relationship follows the thermal diffusion time equation:
t = d² / 4a
where t is the diffusion (delay) time, d is the depth of the defect below the measurement surface, and a is the thermal diffusivity of the intervening material. In practice, a longer observed delay before the thermal signal rises indicates a deeper defect, which is the basis for using transient thermoreflectance imaging to estimate void, delamination, or defect depth without physically cross-sectioning the part. [diagram/image in source — not reproduced here]