New-Tech Europe Magazine | Q4 2021
Packaging for microelectronics- Controlling thermal expansion and power dissipation Eran Lipp, PCB
Extended functionality, as well as decreased weight and heat dissipation, are at the core of electronic miniaturization technology. For decades, the microelectronic industry has been engaged in shrinking the transistor dimensions to enable this miniaturization for various applications (automotive, aerospace, medical industries, and others). Today, this miniaturization trend is no longer limited by the size of single transistors. Instead, the system size and reliability are dictated in many cases by the chip packaging and PCB sizes. Therefore, chip packaging technology becomes a key factor enabling smaller, lighter systems with increased functionality. To ensure system reliability and space reduction, the package should comply with several thermal, mechanical, and electrical requirements. Semiconductor chips are the main building blocks of any digital signals processing device. Once assembled
on top of a PCB, the gap between the semiconductor coefficient of thermal expansion (CTE for Silicon- 3 ppm/ C˚) and the PCB (typical CTE of 15-20 ppm/C˚) becomes an obstacle. While the PCB dimensions expand during assembly and reflow processes, the chip remains almost intact. These changes result in mechanical stress, affecting the assembly process and hindering the system's long-term reliability. The most effective way to meet this challenge is by assembling the chips onto the PCB indirectly, using a substrate as an intermediate layer (see figure 1). The encapsulated chip-substrate stack, also known as a chip-scale package (CSP) is then soldered to the main PCB. To mitigate the CTE difference between the main PCB and chip, the substrate circuit should be made out of manufacturing- compatible material that has a CTE value of 5-7 ppm/C.
Another significant challenge in attaching a microchip to a PCB is the need for power dissipation. In high- end digital applications, transistor size comes at a price of increased power consumption. Other systems that use lower-end processors typically include analog devices such as receivers, transmitters, and power management components – all handle high currents. High power results in "hot spots" areas where chip temperature rises during operation. Failure to dissipate this heat causes the temperature to increase and reduces functionality and reliability risk. PCB manufacturing utilizes several solutions for heat dissipation- all rely on the thermal conductivity of copper coins used as heat sinks. However, these solutions are incompatible with microchip assembly, due to the high level of Cu CTE (17 ppm/C). An effective solution, combining high thermal conductivity (typical values above 200
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