Lead Frame: Non-Destructive Layer Thickness Measurement for Micron-to-Nanometer Structures

Lead frames are metal structures used in semiconductor packages to provide mechanical support, electrical connections, and heat dissipation. The thickness of a lead frame is a critical parameter that affects the electrical, mechanical, and thermal performance of a semiconductor package, as well as its manufacturability and cost.

Lead frames must have a uniform thickness to meet the performance and reliability standards that are required of modern electronic components. As X-rays can pass through matter, X-ray Fluorescence (XRF) can be used for the fast and non-destructive determination of layer thickness. With an advanced XRF instrument it is possible to measure thin films with thicknesses ranging between 1 nm and 40 µm.

Using micro-XRF on SEM with Bruker’s advanced XTrace2 X-ray source and XMethod software it is possible to perform layer analysis, in terms of both thickness and composition, at a micrometer-scale spatial resolution.

Figure 1 shows a lead frame sample alongside elemental maps for each layer. The base material consists of copper (Cu) and iron (Fe), with nickel (Ni), palladium (Pd), and gold (Au) layers on top.The element distribution maps reveal that all elements appear are distributed homogeneously.

Using the mapping data presented in Figure 1, different regions of the lead frame sample were analyzed to determine the thickness of each metal layer (Figure 2).

Specifically, three distinct areas were selected: large flat regions (number 1-3), narrow horizontal strips (number 4-6), and narrow vertical strips (number 7-9). The average thicknesses of the metal layers in these regions are shown in the accompanying bar chart.

The analysis showed significant variability in layer thickness between the different regions and layers. The Ni layer exhibits an average thickness of approximately 1 µm, while the Pd layer measures about 55 nm, and the Au layer is approximately 5 nm thick (on average). These findings unveil the non-uniformity of layer deposition across the sample, highlighting the importance of precise measurement techniques for the characterization of thin film structures.

For a better understanding of the thickness distribution across the sample, a surface distribution analysis was performed in the central flat area of the sample (yellow region).

As shown in Figure 3, the thickness of Ni layer is not uniformly distributed. It is thinner in the center and thicker towards the periphery.