X-IFU focal plane assembly development model design upgrade and critical technology developments

Within SRON a first Development Model (DM 1.0) of the X ray Integral Field Unit Focal Plane Assembly (X-IFU FPA) has been designed, realized and tested over the past years. Since the start of the FPA DM 1.0, the X-IFU instrument design has significantly evolved. Moreover, the recent Athena redefinition activities have resulted in a new overall planning in which the Mission Adoption Review (MAR) and X IFU instrument PDR are now foreseen for 2027. To ensure the required experimental feedback for FPA EM conceptual design is available by the time of the PDR an update of the existing FPA DM 1.0, called DM 1.1, is planned which better reflects the evolved instrument design. In the DM 1.1 the existing 4 channel Frequency Domain Multiplexing (FDM) readout is replaced by an 8 channel Time Domain Multiplexing (TDM) differential readout. A small sized TES array is replaced by a flight-sized kpixel array including full on-chip wiring, of which a limited subset is read-out. The interconnections between the main TES array and the SQUID Multiplexer (MUX) chips are realized in a manner which is flight scalable. Superconducting multilayer Niobium flex cables are used to route the TDM signals from the T0 detector stage at nominally 50 mK to the amplifier (AMP) SQUID stages and external interfaces at 2 K. This work describes the redesign performed on the X ray Integral Field Unit Focal Plane Assembly (X-IFU FPA) Development Model (DM) with a focus on the mechanical modifications of the T0 detector stage. In addition the development progress of several critical interconnection technologies used within the DM 1.1 is described. These include fixation of the main TES array to the metal support, a dedicated wire bonding process of the main TES array to the side panel MUX carrier chips and interconnection of the super-conducting Nb flex cables to the connecting Printed Circuit Boards (PCB).