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MARCH 2024 I DESIGN007 MAGAZINE 59 cause mechanical stresses that weaken bonds between these elements and, ultimately, lead to warpage, breakage, and premature failures. Effectively managing CTE mismatch is espe- cially important when implementing 2.5D and 3D semiconductor packaging technology. ese innovative packages involve vertically stacking heterogeneous dies to achieve sub- stantial increases in functionality and perfor- mance within smaller form factors. To realize the full potential of 2.5D and 3D packaging, advanced multi-layer high-density intercon- nect (HDI) PCB fabrication technology is needed to create very fine and precisely placed die-to-die interconnections. To ensure suc- cess, modeling and simulation of all elements in these three-dimensional systems is highly recommended to gauge the effects of power output, chip placement, and CTE disparities. Metal Heat Sinks and Thermal Interface Materials (TIMs) When a component is susceptible to over- heating, a common practice is to employ a heat sink or a thermally conductive TIM to assist with heat dissipation. ese passive cooling devices, characterized by their ability to cool without moving parts, play a crucial role in thermal management of electronic systems. Heat sinks, typically craed from alumi- num alloys or copper, feature fins or pins that increase their surface area to facilitate effi- cient heat transfer to the surrounding air or fluid. Whether off-the-shelf or custom fabri- cated, heat sinks are typically attached to the top of heat-generating components like CPUs or GPUs using a thermally conductive paste. While highly effective at enhancing heat dissi- pation, heat sinks come with the drawback of occupying a considerable amount of space—an aspect that contradicts the prevailing market trend favoring smaller and more compact elec- tronic devices. In addition to heat sinks, thermally conduc- tive TIMs are widely used to draw heat from hot components. Composed of composite materials (polymers with thermally conduc- tive fillers), TIMs are formulated to adhere and conform to a variety of surfaces. Strategically positioned between the top surface of a heat- generating component and a heat-spreading substrate, such as a metal enclosure, TIMs pro- vide an effective heat transfer path. Because they are conformable, they offer additional benefits such as stress relief and sealing. PCB Materials as Heat Sinks or Heat Spreaders Substrates and conductive materials in PCBs can be designed to dissipate heat similar to a heat sink or heat spreader. To use a board as a heat sink or heat spreader, specialized materi- als and construction methods may be needed. Some options include: • Polyimide: Widely employed in the fab- rication of flexible and rigid-flex circuit boards due to its exceptional flexibility, polyimide is best known for achieving an optimal balance between flexibility and rigidity. Polyimide PCBs demonstrate exceptional thermal endurance and resil- ience, showcasing stability across a wide range of temperatures, supporting opera- tions up to 260°C. ey have very good thermal conductivity, and in certain appli- cations, bonding the polyimide PCB to a metal base further improves heat dissipa- tion. • PTFE-based laminates: Polytetrafluo- roethylene (PTFE), known by the trade name of Teflon®, is a synthetic fluoropoly- mer highly favored for RF electronics due to its high frequency and heat resistance. PTFE-based laminates are specialized composite structures composed of PTFE with the inclusion of carefully selected additives and fillers that determine the electrical, mechanical, and thermal behav- ior of the laminate. Ceramic powders are the preferred filler materials for RF applications due to their superior thermal