Tests of Doglegs With Stiffeners

Tony Weidberg, V1 28/10/03

Introduction

In order to improve the robustness of the existing “dogleg” flex circuits we have considered adding ceramic stiffeners under the connectors. There is already an 800 μm FR4 stiffener under the module connector. This study looks at the robustness of 250 μm alumina stiffeners under the interface PCB connector and the redundancy arm connectors.

Sample and Tests Performed

A sample of 5 mechanical grade doglegs was selected for which there was continuity of all the thin lines between the interface PCB connector and the module connector and for all the 6 lines between the incoming redundancy arm connector and the module connector. The reliability of the special redundancy arm connector for dogleg 6 could not be checked because there were too many faults in the initial sample[1]. Special tools developed by Minglee Chu were used for the connectors to allow the stress during a demate to be transferred away from the fragile region with thin tracks not protected by a cover layer to the region under the cover layer.

Mate/Demate Tests

The following sequence of mate/demate cycles were performed on each dogleg:

  1. 50 mate/demate cycles for the module connector.
  2. 50 mate/demate cycles for the interface PCB connector.
  3. 50 mate/demate cycles for the redundancy arm connector.
  4. 50 mate/demate cycles for the special redundancy arm connector for dogleg 6.

The continuity between the module connector and the interface PCB connector was checked every 10 cycles. The continuity between the redundancy arm connector and the module connector was checked after 50 cycles. The continuity between the interface PCB connector and the module connector was checked after the 50 mate/demate cycles of the special redundancy arm connector. No failures were observed.

Thermal Cycles

Each dogleg then underwent 50 thermal cycles between +40C and -25C and back to 40C The time for each cycle was approximately 1 hour. The mate/demate tests described above were then repeated for each dogleg. No failures were observed.

Vibration Tests

A simple vibration test was then performed on each dogleg. A power supply was connected in series with a loudspeaker and the current was fed in series through all the thin tracks from the module connector to the interface PCB connector. The dogleg was vibrated vigorously by banging on the desk near to the dogleg and underneath the dogleg, by tapping on the dogleg and on the stiffeners over the connectors. If a permanent crack would have formed then the current would have dropped to zero. If a micro-crack would have opened up for a few ms, then a characteristic crackle would have been heard. The set-up was checked by pulling out the connector to the power supply and a clear crackle was heard when a bad creation was created. The only time such a crackle was heard was when the dogleg was vibrated so hard that the interface PCB connector became demated from the interface PCB. No cracks or micro-cracks were detected in these tests.

Conclusions

The good results of the mate/demate tests give some confidence in the reliability of the doglegs with stiffeners added and the use of the special tools for demating the connectors. The fact that no problems were encountered after the thermal cycles and subsequent mate/demate cycles gives further confidence. Even more confidence in the robustness of the doglegs with stiffeners was gained from the vibration tests in which the stresses given to the dogleg were clearly much larger than those that will be experienced during all of the normal testing and operation in ATLAS.

1

[1] Most of the cracks obsevred were in the region of the special redunancy arm connector for dogleg 6. However earlier trials by Li-Shing Hou showed that if the special tools were used for mating/demating this connector no cracks were created.