Speaker
Description
ITk hybrid pixel detector consists of about 10,000 planar “quad” modules formed from 4 ASICs, (developed within the RD53 collaboration) bump bonded to a single sensor operated in serial power chains.
The flex attached to the sensor connects the ASICs to the system and provides module electrical environment while fulling the mechanical specifications.
The flex must provide a signal transmission at 1.28Gbps, and a power domain that minimizes voltage drops. It must be low material to reduce radiation length and stress in the module bump bonds due to CTE mismatch.
We describe the design and design validation of the flex.
Summary (500 words)
The design and development of the flex PCB required a comprehensive approach balancing electrical, mechanical, and manufacturing constraints, with significant challenges addressed at each stage.
One of the primary goals in the design for manufacturing was the development of a common flexible circuit suitable for the entire system, incorporating a 100 μm sensor.
This required careful attention to data routing and wirebonding pads consistent across different location of the flex in the system. Design for track width (100 μm or 75 μm ) along with accommodating via geometries, were essential to ensure manufacturability without compromising performance. Material choices were guided by a comparison of different stack-ups, emphasizing the importance of signal integrity and mechanical stability. The flex panel was designed as a 2x2 layout, optimizing material utilization and cost during fabrication. To ensure effective module assembly, wirebondability was a key focus, needing appropriate surface finishes, accurate fiducial placement, and optimized pad layouts. Additional design features included well-placed test points and MUX outputs for validation and testing, as well as a well-defined connector strategy for both mechanical and electrical interfacing.
Mechanically, the assembly required defined pick-up points and height measurements to control module glue thickness during pre-production. Wirebond protection attachment points, tabs, dowel holes, and adherence to the mechanical envelope ensured mechanical alignment and system integration.
Data transmission and data sharing posed significant challenges due to the high-speed requirement of 1.28 Gbps. A multidrop configuration was used for command signals, while routing from Chip Out to Chip In for MUX (multiplexer signal) ensured proper data flow. Signal integrity was maintained by routing all high-speed lines on a single layer and controlling impedance via stack-up tuning, informed by POLAR simulations. Test coupons were included with the flex manufacturing to validate impedance control and material behaviour, supplemented by functional tests.
Powering strategy focused on minimizing local ground differences between chips to allow for a common voltage offset in the SLDO (shunt low drop out) serial powering circuit. This required simulations, done using power analysis tools by Altium, to model voltage offsets and to optimize power drops were performed. Power dissipation in the flex was minimized through careful copper allocation and routing, via placement and power plane definition.
High voltage (HV) handling required a capacitor supporting approximately 1000V and appropriate track spacing for safety. Rigorous soak and spark tests validated the HV handling capacity of the system.
Bump delamination was another critical area, particularly influenced by material choices. A delicate balance was required between reducing copper content to prevent bump failure and maintaining enough conductivity to minimize power drops. This led to a non-standard manufacturing requirement, which was ultimately validated through extensive reliability testing. The final stack-up included an effective copper thickness tailored to balance electrical and mechanical needs. Long-term reliability was demonstrated through 1000 thermal cycles on a complete module, ensuring robustness against bump delamination.
This complex design successfully brought together electrical, mechanical, and manufacturing requirements into an integrated, high-performance solution, guided by detailed simulations, hands-on testing, and continuous refinement.