Heterogeneous packaging trends bring on new thermal challenges

The shrinking device features, the growing device complexity and the new device materials introduced by heterogeneous semiconductor packaging have led to very high power densities, the potential for rapidly changing thermal events and the need for more advanced thermal imaging techniques. Using thermoreflectance-based imaging, many of the challenges can be met.

Doug Gray

Heterogeneous semiconductor packaging involves the integration of different types of components (such as CPUs, GPUs and memory modules) onto a single package or substrate. It enables increased functionality, enhanced system performance, reduced form factor and lower costs. Such attributes are in high demand for 5G and 6G telecommunications, the IoT and automotive and defense electronics.

Heterogeneous substrates and 2.5D and 3D packaging comprise several layers of different materials. For efficient thermal management, all of these must be defect-free from the standpoint of layer-to-layer process integrity. Performance can also be severely impacted if defects occur in interlayer electrical connections, whether they’re solder balls for connection to a printed circuit board (PCB) or through-silicon-vias (TSVs) in the higher layers. Additionally, many of the applications driving such advanced packaging concepts are moving to higher and higher frequencies as well as higher power performance. These microwave and mm-wave modules may also include embedded antenna arrays.

The thermal challenges for heterogeneous structures fall into several categories. First, due to the varied power densities from one component to another in the same structure, it’s paramount to account for non-uniform heat distribution. Second, to prevent the occurrence of hot spots, it’s very important to achieve sufficient heat spreading and heat sinking throughout the structure. Third, to match thermal expansion and mechanical properties with the assurance of adequate thermal conductivity, accurate information is needed for thermal interface materials.

Many of the static and dynamic thermal challenges posed by advanced semiconductor and optoelectronic devices can and still need to be addressed at the chip level, by traditional techniques, for example using advanced data processing algorithms. The shrinking device features, the growing device complexity and the new device materials introduced by heterogeneous packaging, however, have led to very high power densities and the potential for rapidly changing thermal events. They call for more advanced techniques, like thermoreflectance-based imaging.

Complementary enhancements

Microsanj’s commercialization of thermal imaging based on thermoreflectance has met many of the thermal challenges inherent in advanced device geometries. The thermoreflectance principle exploits the fact that the reflectivity of a material changes with temperature. By probing wavelengths in the visible band and measuring the reflectivity change, the technique supports thermal measurements with diffraction-limited spatial resolution in the sub-micron range and a temporal response in the nanosecond range.

With thermoreflectance as a foundation, several complementary enhancements can provide a means for addressing the added thermal challenges. One such enhancement is infrared thermography, a concept based on black-body radiation. Combined with thermoreflectance, a dual-mode thermal imaging solution emerges that offers both a high spatial and temporal resolution and an improved thermal sensitivity.

Another complementary enhancement is optical pump-probe imaging. This technique involves the use of a high-energy laser pump to heat the sample under test. Thus, a passive material or thin film that’s being considered for integration into a heterogeneous structure can be thermally cycled. The pulse width is typically in the nanosecond range and possibly even in the picoseconds and the spot diameter is relatively small. After a programmed delay, the probe uses thermoreflectance to measure the thermal response over a field of view larger than the spot heated by the pump. This approach makes it possible to observe and measure the thermal response over time. Measuring the thermal decay at a distance from the pump excitation enables the determination of the sample’s anisotropic thermal properties.

Another non-invasive technique, flash thermography, employs a rapidly pulsed LED array, instead of a focused laser pump, to heat the sample under test. Following the flash pulse, utilizing the principle of heat transfer, an infrared sensor measures the resulting thermal response to enable the detection of sub-surface defects, irregularities or damage in materials or structures over a large field of view. This approach is fast and applicable to a wide range of materials for finding voids, de-laminations, cracks, defective vias or other anomalies that may impact the thermal behavior of the structure or the electrical performance.

For high-frequency front-end modules containing embedded antennas, the so-called Emscope system enables the testing of antennas-on-chip (AoCs), antennas-in-package (AiPs) and antennas on printed circuit boards. With Emscope, a holistic chip-package-PCB-antenna concept is envisioned for fast industrial testing and failure analysis at the module level using thermoreflectance-based or infrared thermal imaging. This approach offers a new non-invasive over-the-air (OTA) testing solution in the near-field, bridging the electromagnetic field intensity with a thermal imaging map.

A proprietary synthesized coating based on quantum spin-crossover (SCO) materials, developed by eV-Technologies, exhibits functional properties that are responsive to multi-physical external stimuli such as temperature, pressure, light irradiation and electromagnetic fields. Using this coating as an overlay for the device or module under test in conjunction with thermoreflectance-based thermal imaging or infrared thermography enables the observation of an electromagnetic field. The SCO material simply acts as an energy transducer converting the electromagnetic energy into thermal energy, allowing the quick detection of faulty or inoperative antenna elements in a microwave or mm-wave array.

Ultimate goal

Thermoreflectance-based thermal imaging with its spatial and temporal resolution has proven to be an effective tool for gaining an understanding of the thermal behavior of advanced semiconductor and optoelectronic devices. Complementing it with infrared thermography, optical pump-probe imaging, flash thermography and holistic over-the-air imaging will address many of the added thermal challenges emerging with further heterogeneous packaging advances. Going forward, more enhancements are considered that pave the way toward the minimization of thermal limitations in determining device, module and/or system performance.

Satisfactorily addressing the added thermal challenges brings us closer to the ultimate goal: performance being constrained by electrical limitations and not by thermal limitations. For specific device technologies, this could lead to an order-of-magnitude improvement in power performance.

For more information or a demonstration, contact Chris Caenen at Hitech. On 29 May, the solution will be demonstrated at the Benelux RF Conference 2024.