II-9. Instrumentation

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Production of bus-extender for sPHENIX INTT detector
T. Hachiya,∗1,∗2 Y. Akiba,∗1 D. Cacace,∗3 K. Cheng,∗4 H. En’yo,∗1 S. Hasegawa,∗1,∗5 M. Hata,∗2 H. Imai,∗1,∗6T. Kondo,∗7 C. Kuo,∗4 H.-S. Li,∗8 R.-S. Lu,∗9 E. Mannel,∗3 C. Miraval,∗3 M. Morita,∗1,∗2 I. Nakagawa,∗1Y. Namimoto,∗1,∗2 Y. Nakamura,∗1,∗6 G. Nakano,∗1,∗6 R. Nouicer,∗3 G. Nukazuka,∗1 R. Pisani,∗3M. Shibata,∗1,∗2 C. Shih,∗4 M. Shimomura,∗1 M. Stojanovic,∗8 Y. Sugiyama,∗2 R. Takahama,∗1,∗2 W.-C. Tang,∗3M. Watanabe,∗2 and X. Wei∗8sPHENIX is the second-generation heavy-ion collision experiment at the Relativistic Heavy-Ion Collider
aimed at studying the properties of strongly coupled
quark-gluon plasma. The intermediate tracker, INTT,
is a silicon strip detector in sPHENIX that is used
to perform precise tracking near the crossing pointof colliding nuclei.1) A special cable for INTT called
bus-extender is required to transmit large amounts ofdata from INTT to the readout electronics at highspeed. The bus-extender is more than 1 m long androuted with a curving path. We developed the busextender based on ﬂexible printed circuit (FPC) technology since no commercial cable is available.
The bus-extender is composed of four Cu layers including signal, power, and ground layers laminatedwith a liquid crystal polymer as a dielectric substrate.
The signal layer contains 62 pairs of low-voltage differential signals within a 5 cm width of the cable, andtheir lines and spaces are both 130 μm.
We built prototype bus-extenders with a length of
1.3 m, as shown in Fig. 1, and evaluated their electrical and mechanical performance. The signal loss
due to a long transmission line was 30%. The peelstrength among the laminated layers was greater than
15 N/cm. These results are summarised elsewhere.2)
From the results, the performance of the prototype is
suﬃcient to satisfy the requirements. We completed
the development and moved to the production stage.
The fabrication procedures are as follows: 1) thepattern of signal lines is printed by photolithography
and etching, 2) all four layers are laminated by bonding
sheets with pressure, and 3) through holes are opened
and Cu-plated to connect the signal lines between theinner and outer layers. During the fabrication, we
found that the yield rate was 30% because of shortand open circuits of signal lines. Figure 2 shows two
examples of short and open circuits found in the ﬁrst
fabrication procedure. This poor yield rate needs to be
improved for the mass production since the yield rate
directly impacts the production cost. We investigated∗1∗2∗3∗4∗5∗6∗7∗8∗9Fig. 1. Prototype bus-extender with a length of 1.3 m.

Fig. 2. Examples of short and open circuits after the photolithography (left) and etching (right).

why the short and open circuits were produced andfound that the major cause of the problem is the contamination of tiny dust particles, which can interrupt
light exposure and the ﬂow of the etching solution. To
solve the problem, we added two steps to the ﬁrst procedure. One is to remove dust particles as much as
possible using a silicon rubber roller. The other is tocheck the open/short lines after etching and to remove
the bad signal layer in the later lamination process if
found. With these new steps, the yield rate was improved to 75%. The visual inspection of signal lines
after etching is useful to improve the yield rate further. An inspection system to search the bad lines
semi-automatically is under development.3)
In summary, the development of the bus-extender
was completed, and new steps added to the fabrication procedure successfully improved the the yieldrate. The mass production of the bus-extender will
be started in January 2022 with the semi-automatedinspection system.
References

RIKEN Nishina Center
Nara Women’s University
Brookhaven National Laboratory
National Central University
Japan Atomic Energy Agency
Rikkyo University
Tokyo Metropolitan Industrial Technology Research InstitutePurdue University
National Taiwan University

1) I. Nakagawa et al., in this report.
2) T. Kondo et al., submitted to IEICE Electron. Expr.
3) H. Imai et al., in this report.

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