Photonics West: Terahertz Spectroscopy to Detect Changes in Integrated Circuit Packaging Materials

On January 30, 2024, Chengjie Xi of the University of Florida gave a lecture at SPIE Photonics West in San Francisco, California, on how terahertz time-domain spectroscopy (THz-TDS) can be used to detect changes in integrated-circuit (IC) packaging materials (1). The lecture, showed how using THz-TDS can help the monitoring process under various conditions, allowing for insights into counterfeit IC detection.

The presentation began with Xi discussing the status of IC packaging, with each iteration being put through different means of inspection to reduce costs and enhance effectiveness. Wafer foundries and assembly testing usually happens in packaging facilities all over the world, which, according to Xi, creates opportunities for malicious vendors or individuals to negatively affect original designs through practices like implementing unwanted features, resulting in significant hardware insurance challenges. He also pointed out how supply chain vulnerabilities can extend to advanced packaging, so it is important to spend effort on assurance and detecting errors or changes in IC packaging materials.

Xi went on to explain why electromagnetic compatibility (EMC) usage for IC packaging is a better alternative. It is already used in different types of IC packaging and has been found to have multiple applications. For quad flat package (QFP) and dual in-line package (DIP) circuits, it serves as an encapsulant, meaning it helps “protect electronic components from detrimental chemical, mechanical, electrical or thermal environments” (2). Additionally, it can also be useful as underfill material, which “protects electronic products from shock, drop, and vibration and reduces the strain on fragile solder connection” (3). It can act as an underfill between two different chiplets, a chiplet and a package substrate, or a chiplet and an interposer. EMC material properties can also vary in multiple ways, such as material composition, fabrication process, and how they deal with thermal aging and moisture effects. Materials and processes can vary, with the former category including epoxy resin, hardener, and filler, and the latter including different stages of the curing process. With so many different factors at play, scientists must take note on what environmental factors can affect EMC materials, and how.

One factor that can affect EMCs is the surrounding temperature. Most EMC materials are designed to work around 150 °C, though they usually work temperatures under 200 °C. Beyond that, higher temperatures can cause oxidation within EMC components, in addition to cracks, shrinkage, aging speed increase, and internal stress, among other issues. Moisture can also be an issue. Different EMC have different water uptake percentages, and the more time spent in water, the less mechanically strong the components become. There are different ways to characterize EMC components, some destructive, like DSC (measures the hardness of packaging polymers) and DMA (measures the storage), and some nondestructive methods, like X-ray or Fourier transform infrared spectroscopy (FT-IR), that characterize either the structure or materials, respectively. It is difficult to simultaneously characterize both EMC materials and structure.

With THz-TDS, Xi said this solves multiple issues with EMC characterization. It can measure thickness, defection levels, and delamination at the same time, while THz-TDS images can better capture internal components. From his research, he sees that THz-TDS can simultaneously characterize EMC structures and materials, addressing multiple issues without need for interference. Referencing a case study, his team used THz-TDS for thermal loading characterization, specifically on an aging furnace that was used for 4 hours at 200 °C. In this instance, the images and data were able to capture THz-TDS amplitude and phase changes in different locations.

EMC materials can be difficult to characterize, but Xi said there is potential in using THz-TDS in this regard. With its versatility and recorded capabilities in different analysis conditions, he said it can help streamline analysis and prevent malicious interference from outside sources. There is further research to conduct in this regard, but THz-TDS can help better the IC packaging process as we know it.

References

(1) Xi, C.; Varshney, N.; Khan, M. S. M.; Dalir, H.; Asadizanjani, N. THz-TDS for IC packaging material changes detection under real-world conditions. In SPIE Photonics West, San Francisco, California, USA, January 30–31, 2024.

(2) Encapsulant. ScienceDirect 2012.https://www.sciencedirect.com/topics/chemistry/encapsulant (accessed 2023-1-30)

(3) What is Underfill? Nordson Corporation 2024. https://www.nordson.com/en/divisions/electronics-solutions/your-process/fluid-types/underfill (accessed 2023-1-30)