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We present a very compact hybrid detection module based on an advanced liquid-cooled low temperature cofired ceramic (LTCC) substrate. The double sided hybrid combines 144 photo detectors and four specialized flip chip readout ASICs (Application specific Integrated Circuits) used for the readout of scintillation crystals with application in time-of-flight positron emission tomography (PET) combined with magnetic resonance imaging (MRI). If MRI images and PET images are combined, completely new medical diagnostic and treatment prospects are feasible because the two techniques are complementary and they will offer both anatomical and functional information. One of the biggest challenges is the development of miniaturized detector modules that are highly functional and MRI compatible. Our SiPM (Silicon Photomultiplier) module has an area of 32.8 by 32.0 mm2 and contains 12 × 12 SiPMs in a pitch of 2.5 mm2. The SiPM readout of the 144 channels is performed by four PETA6 ASICs. The LTCC substrate with a 2.1 mm thickness has been manufactured using the most advanced technologies developed at Micro Systems Engineering GmbH To guarantee the manufacturability in serial or mass production, DP951 P2 green tape has been used. For the cooling channels, special technology has been developed by MSE. The liquid cooling channels inside the LTCC substrate provide excellent cooling for the ASICs, the SiPMs, and thermal insulation between ASICs and SiPMs and allow a very compact design of the detector modules, reducing their height by 50% compared with other technical solutions. We can insert a ring of our modules in an existing MR (Magnetic Resonance) scanner. Operating the SiPMs at low temperature improves their performance, reducing the effects of dark count rate and improving image quality. There is no heatsink, heat pipe, or other cooling element attached to the back side of the ASICs. To avoid interference between the PET and MRI system, short signal length is required for minimizing pickup loops and eddy currents. The 12 SiPM arrays with 2 × 6 geometry are wire bonded only at the edges of the SiPMs to the LTCC, enabling the use of nearly the whole detector area for photon detection, which is of paramount importance for excellent image quality. At the opposite side of the substrate, four ASICs with 272 μm bump pitch are flip chip solder assembled to the LTCC substrate including underfilling, and a few SMD (Surface Mount Device) components are mounted. A scintillator crystal array on top of the SiPMs converts gamma rays (511 keV photons produced from positron-electron annihilation) into light. We assume that the LTCC substrates and all components are fully MRI compatible, which is important for the integration of PET with MRI without mutual interference. The paper elucidates the impact of the used technology on the performance of advanced PET/MRI detector modules.

We present a very compact hybrid detection module based on an advanced liquid-cooled LTCC substrate. The double sided hybrid combines 144 photo detectors and 4 specialized flip-chip readout ASICs used for the readout of scintillation crystals with application in time-of-flight (TOF) Positron Emission Tomography (PET) combined with Magnetic Resonance Imaging (MRI). Positron Emission Tomography is mostly known for its use in oncology applications, cardiovascular disease, and imaging of brain functions. If MRI images and PET images are combined, completely new medical diagnostic and treatment prospects are feasible since MRI delivers precise anatomical information. One of the biggest challenges is the development of miniaturized detector modules that are highly functional and MRI compatible. Our SiPM module has an area of 32.8 by 32.0 mm2 and contains 12 × 12 SiPMs in a pitch of 2.5 mm2. The SiPM readout of the 144 channels is performed by four PETA6 ASICs. The LTCC substrate with 2.1 mm thickness has been manufactured using the most advanced technologies developed at MSE. In order to guarantee the manufacturability in serial or mass production, DP951 P2 green tape has been used. For the cooling channels, special technology has been developed by MSE. The liquid cooling channels inside the LTCC substrate provide excellent cooling for the ASICs, the SiPMs, and thermal insulation between ASICs and SiPMs and allow a very compact design of the detector modules, reducing their height by 50% compared with other technical solutions. This makes both smaller scanners and larger detector rings possible, the latter being needed for heavier patients. We can insert a ring of our modules in an existing MR scanner. Operating the SiPMs at low temperature improves their performance, reducing the effects of dark count rate and improving image quality. There is no heat sink, heat pipe, or other cooling element attached to the back side of the ASICs. In order to avoid interference between the PET and MRI system, short signal length is required for minimizing pickup loops and eddy currents. The 12 SiPM arrays with 2×6 geometry are wire bonded only at the edges of the SiPMs to the LTCC, enabling the use of nearly the whole detector area for photon detection, which is of paramount importance for excellent image quality. At the opposite side of the substrate, four ASICs with 272 μm bump pitch are flip-chip solder assembled to the LTCC substrate including underfilling, and a few SMD components are mounted. A scintillator crystal array on top of the SiPMs converts gamma-rays (511 keV photons produced from positron-electron annihilation) into light. LTCC substrates and all components are fully MRI compatible, which allows integration of PET with magnetic resonance imaging without mutual interference. The performance of a PET detector is characterized by its time resolution, energy resolution, detection efficiency, and spatial resolution. Each of these factors has a huge influence on the quality of the final PET image. All of these factors are improved with our novel technical solution. With measurements on prototypes, state-of-the-art coincidence time resolution (CTR) for pairs of identical detectors in combination with high spatial resolution have been obtained. The paper elucidates the impact of the employed technology on the performance of advanced PET/MRI detector modules. Novel features of the detector modules will help to enhance the “molecular sensitivity” of PET/MRI scanners.

This tutorial examines the reliability of ultra fine-pitch flip chip assemblies with lead-free solder bumps for which there are two major reliability aspects to consider: Thermo cycling performance and electromigration (EM). Isothermal aging and humidity should not cause significant problems, providing that suitable materials have been selected, see [1] and chapter "Results". First, dry heat storage, temperature humidity bias testing, and temperature cycling were conducted, showing excellent reliability of these ultra fine-pitch assemblies. Second, electromigration (EM) performance of the flip chip assemblies (with solder sphere sizes mentioned above) was investigated. Experiment The test coupons used in this study are specially designed flip chip packages. SAC305 Solder spheres with 40 µm, 50 µm, or 60 µm diameter were attached to silicon chips 10 x 10 x 0.8mm in size, using the wafer level solder sphere transfer process (often called "gang ball placement") and 30 µm SAC305 spheres using a laser based single-sphere process, respectively [2]. Even exotic solder compositions are readily accommodated using these methods.