By: Joel F. Flumerfelt, Ph.D., Metallurgist
A client who manufactures copper heat sinks had one returned from their customer that displayed three surface discontinuities, Figure 1. Scanning Electron Microscopy (SEM) examinations conducted on Discontinuity 1 and Discontinuity 2 showed them to be arc spots created by an external energy source, such as may occur during an electrical discharge. The SEM examinations of Discontinuity 3 showed features that were ambiguous. A Scanning Acoustic Microscopy (SAM) inspection performed on Discontinuity 3 indicated an internal void, Figure 1, partially enveloped by delamination, i.e. cracks, within an alumina dielectric material between the top copper layer and the copper substrate, Figure 2.
A Computerized Tomography (CT) inspection of Discontinuity 3 facilitated an additional non-destructive characterization of this internal discontinuity, Figure 3. The permanent, digital CT records provided the possibility to review virtual cross-sections of the void after a destructive analysis. The CT slices showed the void had a spheroidal geometry. The CT slices and clipped isoview images also indicated the presence of a metal “uvula” hanging from the top copper layer and metal “nuggets” laying on the copper substrate. The dielectric material was not apparent in these images due to its low radiopacity relative to the surrounding copper.
The CT inspection, coupled with the complementary SAM data, provided a priori knowledge about how best to orient Discontinuity 3 in a metallographic cross-section to optimize further portrayals of the discontinuity’s physical nature. Without this non-destructive data, an otherwise random cross-section may have destroyed internal features that would truncate an understanding of how and why the void formed.
A cross-section plane through the center of the void showed evidence of solid-state plastic deformation within the grains of the copper metal surrounding the void. The cross-section examinations also revealed the “uvula” and “nuggets” were copper, Figure 4 to Figure 6. The porosity present in these particles was due to them making a transition from a molten state to their solid form. All these features observed in the cross-section were the result of a high energy, explosive event that caused a localized temperature spike in excess of the melting temperature of copper, which is 1,085°C.
A postulated mechanism for the explosive event was an electrical short circuit that took place between the two copper layers. The potential cause of the short circuit path was the presence of copper particles unintentionally embedded in the alumina dielectric material during the fabrication of the heat sink. Evidence to support this opinion was found in the form of another group of copper particles entrenched in the alumina dielectric material, observed elsewhere within the heat sink after peeling away the top layer of copper, Figure 7.