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Solders & TLPS Interconnects in High Temperature MLCCs

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There is a long-established market for high temperature multi-layer ceramic capacitors (MLCC) that operate at 150°C and higher in down-hole oil & gas exploration, military and aerospace applications. To maximize the capacitance density and achieve a high degree of mechanical robustness, stacks and leaded form factors have been used with High Melting Point (HMP) Pb-containing solders as the preferred interconnects. However, Pb-containing solders are limited to temperatures below 300°C and are banned from many commercial and automotive applications with further legislation limiting their use planned in the future. Common Pb-free solders such as SAC 305 or SnSb alloys are in widespread use but their performance at prolonged exposures at 200°C is limited. Exposures to high reflow temperatures during assembly, especially successive reflow operations, can also compromise interconnect integrity.

Higher temperature gold-containing solders are widely available but these are cost prohibitive and so are not viable for emerging high temperature electronics including higher volume, price sensitive Automotive and Power markets. The development of more energy efficient power converters and inverters based on Wide Band Gap (WBG) semiconductors is driving the adoption of higher temperature electronics in these markets since these operate at higher junction temperatures than traditional silicon. This has led to the development of non-solder interconnects based on sintered silver, nano-metal sintering and transient liquid phase sintering (TLPS) technologies capable of higher temperature performance than common solder based interconnects.

The availability of discrete components, such as capacitors, that can operate under these conditions is a key barrier to the development and adoption of high temperature electronics. In this paper the key property differences between solders and TLPS interconnect technologies are compared in detail for MLCC interconnects. The development of a new range of nickel Base Metal Electrode C0G MLCC stacks rated for 200°C is described and performance compared to traditional Precious Metal Electrode (PME) stacks. Thermal cycling performance to 200°C of BME X7R stacks made with 10Sn/88Pb/2Ag solders are compared to similar stacks made with TLPS interconnects of Cu-Sn and In-Ag. The development of leadless stacks, a new bulk capacitance form factor enabled by TLPS technology, is described and their properties compared to traditional stacks.

This whitepaper, originally presented at HiTEN 2017, reviews a comparison between solders and transient liquid phase sintered interconnects in high temperature multi-layer ceramic capacitors

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