The Large Hadron Collider beauty (LHCb) experiment is one of the main experiments running at the Large Hadron Collider (LHC) accelerator at the European Organization for Nuclear Research (CERN). It is a fundamental experiment as it can provide evidence of physics beyond the limits of other direct measurements performed by the other experiments at LHC. LHCb will undergo a major upgrade during Long Shutdown 2 (2019) with one of the main changes being the full replacement of the main tracking system. The new tracker will use SCintillating Fibres (SCiFi) to cover the complete area of the detector. The 2.5m long fibre mats are built winding 250 micrometers diameter Scintillator Fibres one on top of each other to obtain 6 layers (approximately 1.5mm height). In the middle of the detector a mirror helps to collect all the photons generated in the fibres to overcome attenuation length (damaged after irradiation). When a particle passes through the mat a small and fast light pulse is generated. A custom designed array of Silicon Photo-multipliers (SiPMs) then converts the light burst into electrical charge that is collected and processed by the readout electronics. Each array provide 128 channels (0.25mmx1.62mm each) to be readout by the electronics. The fibres and the sensors have the same size but they are not paired. As a consequence, the scintillation light is typically distributed among several channels, requiring a clustering process after readout. The need to achieve as high as possible efficiency and handling the high occupancy leads to very challenging constraints in space, power and signal timing triggering the design of an Application Specific Integrated Circuit for the readout. The low Power ASIC for the SCIntillator FIbre TraCker (PACIFIC) has been developed for this purpose. PACIFIC integrates 64 readout channels including pre-amplifier, shaper, gated integrator and Analog to Digital Conversion (comparators) prior to a digital readout (serialization). Analog chain behaviour is controlled using a standard I2C slave and configuration registers. The first talk of the talk will summarize the main detector elements and construction status while the second part will focus on the mixed signal design with special emphasis on the design closure process before submission.