Purpose
The meeting introduced Robert to BrickKiln’s composite stiffener technology, developed for the Mu3e outer pixel ladders. The team wanted to learn from BrickKiln’s experience for a more modular vertex detector design, which was exploratory. Lessons learnt from the outer pixel stiffener development and experience (from the last ~ 2.5 years) was discussed.
Stiffener design and history
- Principle: stiffness comes from section depth/shape, not extra material.
- Original design:
- It used a single ply of 22 µm unidirectional carbon fibre, custom-specified through a Korean supplier. The material has since been discontinued.
- A 8 µm Kapton layer was co-cured on the back and doubled as a cooling flap sealing against the adjacent ladder. 3 µm Kapton exists but costs much more.
- It ended before the chip edge, which left a weak spot between the stiffener and the interposer. Other problems with the current design: limited access to probing points, difficulty reworking bonds.
- Noise problem: testing found parasitic capacitive coupling, with the stiffener picking up noise in the chips. The stiffener was flipped from the chip side to the reverse side of the ladder, which fixed it.
- New design:
- It shifts the stiffener to give chip access and runs it onto the interposer, with a flat foot on each interposer tab for bonding.
- Three geometry variants exist: two smaller lumps, a single raised section, and a lower single curve with a larger bond area.
- A thicker or woven material over the interposer was rejected. The transition from thin to thick material would add material over the chips, unidirectional fibre can’t change shape much without breaking, and it would need different tooling.
- Hybrid carbon-Kevlar “metamaterial”:
- Kevlar sits where the stiffener touches the chip, with carbon on the raised “apex” sections.
- It removed the capacitive coupling, and its cured ply is about 12 µm.
- Adam said it is proven, not theoretical. Kapton would no longer be needed for chip isolation, though it is still wanted as the sealing flap and for handling robustness.
- The resin system used for this material was proprietary, something developed by BrickKiln with the company directly, therefor the details won’t be shared. This applies also to the actual “splicing in” process that they use to make this meta material.
Manufacturing
- The process is now very repeatable but delicate. The production run of 40 (L3 2026) had one failure, against roughly 4 made per good part in early attempts.
- Problems solved along the way:
- Wrinkling on the bag face. A silicone intensifier shrank over time and was replaced by a proprietary rubber, which may no longer be needed.
- Resin shrinkage, which bowed early prototypes by about 20 mm. The fix was confirmed to carry over to the new shape.
- Breaking parts off the tool, now done with an air cushion.
- Adam said the process used to extract the parts from the mold tool are trade secrets and he can’t share them. This was generally true about all aspects of the manufacturing procedure done at BrickKiln, mainly motivated by the fact that they use similar processes for other more secretive projects.
Key discussion points
- Kapton cost and role: Kapton is the largest cost driver. Adam said it also adds handleability and robustness, so he’d want a mechanical-ladder test before removing it.
- Extending the stiffener over the full interposer:
- Extending the current profile in length is easy, because BrickKiln already makes them oversize and trims them. Matt later added that changing the end-piece clamp to have a u profile would be very easy.
- Changing the cross-section along the interposer would be real R&D, and a thicker or different material would also cover the bonds.
- Matt is keen to extend the stiffener all the way out into the clamp region. Decision to be taken once the current prototypes are received and can be tested.
- The conclusion on any significant R&D work: not possible for next year (2027); fine to explore for the future.
- Vertex concept:
- A thick carbon end frame around the interposer, with the thin stiffener glued across the ladder (triangle shape with feet to glue to ladder - chip side?).
- Feasibility of flexure-based ladder mounting (Z-profile springs) to decouple thermal expansion from ring structure — noted as future concept, not scoped for next year
- The vertex detector had failures when it was clamped to rigid frames, and the cause is still being analysed (mismatch stresses are suspected).
- For 2027 the design is essentially locked.
- Feedback on Robert’s early V-shaped concept (prototyped in his earlier CMS work):
- Adam advised a curved apex rather than a sharp one, because sharp edges in unidirectional fibre split the resin.
- He was worried about bond area and about how the V sections would be attached to the ladder, since a stiffener that isn’t attached well does nothing.
- He also found the transition to the end frames hard to judge from the screen.
- Adam/Alex suggested swapping in BrickKiln’s smooth curve and adding a bonding lip.
- Resin:
- Robert asked about cyanide ester, which is common for stability and radiation reasons. Adam said the metamaterial uses a proprietary resin, not epoxy.
- Adam would be happy to develop it with a standard cyanate ester if that gives shared knowledge.
- Adhesives: Adam said radiation-hard structural adhesives qualified for the LHCb VELO exist (an adhesive transcribed as “CAF4” was mentioned, he wants to double-check the grade).
Collaboration, IP and commercial terms
- The Mu3e side is uneasy about relying on a proprietary process or resin, because technology intended for wider use at CERN would need radiation qualification, plus also the general ideas of sharing knowledge across projects/DRDs etc. They can run their own testing at CERN and share results openly.
- Adam is happy to give informal feedback and do some R&D at BrickKiln’s own cost and risk, as it is already doing for the redevelopment of the new stiffener. He needs an indication of where it’s going commercially, such as future orders, to justify the work internally. He noted there may be more orders for the existing stiffeners possible.
- BrickKiln still has the tooling for the original vertex stiffeners and could quickly try different materials or geometries.
- The preference on the room side is to adapt the existing profile, add their own frames, and use BrickKiln for production runs, where yield know-how matters most.
- Joey said the 2027 plan should be clear within about two months. Larger changes would be a 2028 topic.
Status updates
- Prototypes of the new design: 3D-printed tooling produced flat parts (the main thing Adam wanted to check), but the tools are porous and left the stiffeners resin-dry. They were made without Kapton. Adam didn’t know whether the batch due last week had been finished.
- Vertex stiffeners shipped to Joey: received after a customs and payment delay. Joey hasn’t tested them because he is the only mechanics person on vertex.
- Material availability: the Skyflex stock at Oxford is past its expiry date, and a DMA test to confirm its properties is pending. BrickKiln have plenty more of the existing carbon-fibre material that can be used. More Kapton can be ordered quickly from Goodfellows and doesn’t expire, but it is expensive, so Adam suggested not ordering until the prototyping is finalised.
Action items
Adam:
- Check Skyflex data sheet for resin system
- Oxford resin is K50 (epoxy)
- We have H23 (cyanate ester) and K50 and K51 (a slightly higher temp epoxy with a different cure)
- Update later how latest prototype batch came out
- Send radiation-qualified adhesive grade (CAF4) to Robert
- (Alex) waiting on ordering more Kapton material until prototyping stage is completed
Joey/Ashley
- Update Adam with 2027 production intentions once known (after early Dec collaboration decision)
Robert
- When ready, send Vertex design files to Adam for informal feedback and possible prototyping
Oxford MEG (via Alex)
- Complete the DMA test on the expired material