Case Study: Custom BGA for MIT Particle Physicist

Physicists explore the fundamental constituents of matter by accelerating subatomic particles to speeds approaching that of light and colliding them head on or against stationary targets. The reaction products are then observed in various types of detectors. Recent experimental results from the CERN accelerator facility in Switzerland revealing the long-sought Higgs boson have been widely publicized. Researchers there, and at accelerators elsewhere, investigate the particles produced in such collisions.

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The particles produced in these collisions are often bent by powerful electromagnets surrounding the detectors in order to determine the particle charge and momenta. The detectors incorporate tracking devices that capture faint electrical signals the particles produce during their transit. Various types of tracking devices are employed among the experiments at the different facilities, but many of them have in common a readout chip, the APV25, whose basic design was jointly developed for trackers at CERN nearly 20 years ago by a British university and a national laboratory.

The first prototype module was a composite of two boards laminated together, which distributed the chips’ inputs via bond wires to edge connectors. It was very expensive to fabricate and assemble, and permitted no rework once a chip was bonded to the board.

The APV25 has 128 analog inputs that each connect directly to a tracker channel. The input signals are amplified and shaped, then sampled, and the results are fed into a pipeline of programmable length. The conditioned signals are read out after receiving a trigger request, further amplified, and multiplexed such that the signals from all 128 channels can be read out on a single line that has a differential current output.

The Compact Muon Solenoid is one of four detectors at CERN and one of two that identified the Higgs boson. Its tracker system has approximately 10,000,000 channels, which are read by the 128-channel APV25 chips (about 75,000 of them). The CMS tracker contains many concentric layers of sensors surrounding the interaction point. The APV25 sensors are wire-bonded directly to PCBs and encapsulated, not packaged, because the mass of a package could compromise particle detection in succeeding outer layers.

The wire-bonded, chip-on-board construction, though unavoidable in the CMS detector at CERN, is inconvenient. Board rework is impractical to replace a failed device; instead, the entire readout board would be replaced, even though the other devices on board are good. However, in some trackers at other facilities, detection would not be hampered if the APV25 readout chips were packaged. Such is the case at the Brookhaven National Laboratory Relativistic Heavy Ion Collider (RHIC) in Long Island, NY, for example.

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