Solder joints for flexible connection working at low temperatures

This paper presents flexible electrical connections operating at low temperatures. This kind of connections is used in packages where two devices operate at different temperatures - in this case, the IR detector and electronic control circuit. The IR detector operates at a temperature as low as 200 K, while the control circuit is operating in the temperature range of 290 K–300 K. The challenge is to create an elastic electrical connection between those devices. These connections must have low resistivity and at the same time should transfer as little heat as possible from the control circuit to the IR detector. 35 mm epoxy-phenolic foil with 5 mm Ni film was used as a flexible connection. The In-Sn solder was applied to create electrical connections between the Ni film and Au pads on both IR detector and I-Tera MT RF laminate. Electrical and mechanical properties of the InSn solder joints in temperatures from −40 °C to +30 °C were investigated. Two types of solder joints were taken into consideration: the flip chip type and the lap type. It was found that the InSn flip chip connections are able to work properly at DC mode in low temperatures. Further investigation will be conducted in AC mode for the lap solder connections.