Instructions for protecting the Hubs’ fiber ribbons from wear in the heat-sink channel

These are instructions for wrapping the Hubs’ fiber optic ribbon cables in Kapton tape to protect them from wear inside the heat-sink channel and in one other location on the PCB. We’ve found that after spending a lot of time running in a shelf with fans constantly blowing, vibration in the fiber ribbons is causing the fins of the heat sink to slowly rub through the fiber ribbons, exposing the system to light pollution, light loss, and causing generally poor connections.

With all materials in-hand, this operation should take between 30 minutes and one hour per card.

Needed items:

Fiber-ribbons, pre-marked for lengths and Tx/Rx (Rx is marked with Red)

25mm Kapton tape

Exacto/hobby knife/scissors

Needle-nose pliers

Dental pick

Ruler/straight edge

Remove the MTP connectors from the MPO at the back of the board. It also helps to remove the ROD’s MTP and arrange that cable so it’s out of the way during this process. Be sure to install a protective cover!

One at a time, pop the rubber covers on each Minipod. Using a dental pick, if you apply pressure from one side or the other, you can get the Prism to pop out of its mount. Replace the rubber cover after completing each pod.

Remove the metal clips on the side of the heat sink, which hold the ribbons in that groove. These clips are held in with a small drop of superglue. Grabbing them with needle-nose pliers and giving them a little twist in each direction is usually enough to break the glue, at which point the clips can be easily removed.

With both ends of the ribbon free, you should be able to slide them right out of the channel. Sometimes, glue will have dripped down on to one segment or another and you may need to break it free.

Remove the pull-tabs from the MTPs and install them on the new ribbons’ MTPs.

Feel free to discard the old ribbons, or save them for later inspection.

Cut a piece of 25mm Kapton tape approximately 110mm long. Lay it out sticky side up.

Fig. 1 The Tx ribbon is placed first. (Fig. 1) Notice the black marks on the ribbon. It is this area that will ride inside the heat sink, and which we have to protect. Lay the Tx ribbon such that it is slightly lower than the center line of the tape. It should be as straight as possible, but this is not critical. Make sure both black marks are contained within the tape. Be sure to note connector orientation! The Prism needs to be facing down!

The Rx ribbon (marked with Red) will lay exactly on top of the Tx ribbon. Again, make sure the ribbon’s black marks are within the edges of the tape. Again, watch the connector orientation! Note that Rx’s marks are offset considerably, and that the Rx ribbon will hang out further than the Tx ribbon does. The is done on purpose.

Gently, fold the tape over both ribbons so that they are contained inside the tape. Work out any air bubbles that get trapped. Don’t allow any creases to form. I’ve found it easiest to start in the middle and work outwards.

Fig. 2 See Fig. 2. Using a straight edge as a guide, use your exacto blade to trim off excess tape, but make sure to leave at least 5mm of tape.If you leave too little, the kapton shell we just created will come undone and you’ll have to start over.

At this point, the ribbons are ready to be inserted back into the groove on the heat sink. Be very gentle with them; don’t use too much force. It will be a snug fit, but they will fit.

Fig. 3 At this stage, we set the exact lengths we need at the Minipod end: With the ribbons in the channel, and using the excess tape to pull/slide them back and forth, make sure the Tx ribbon goes just past the center of the top of the Tx pod. Use your own judgment, but 5mm is probably the farthest it needs to go. (Fig. 3)

Make sure at least a little bit of tape hangs out at both ends of the heat sink. None of the bare ribbon should touch any of the metal.

Using needle-nose, break away any superglue remnants left on the metal clips.

Replace the metal clips between the fins on the heat sink. Push them in enough to hold the ribbons in place, but don’t wedge them in too tightly. When you’ve started to deform the tape, that’s it.

Apply a single drop of superglue on top of the metal clip. Either use accelerator to cure it, or let it dry on its own before continuing.

Cut another piece of Kapton. 25-40mm long will be fine.

Fig. 4 At the MTP end of the ribbons, there is another set of black marks. (Fig. 4) Those again have to be matched up, and then wrap both ribbons with the piece of tape you just cut. This will leave the pair with a slight curve, so they can be routed between the components. The tape is to protect the ribbons from the red through-hole capacitor, which sort of holds them in place.

At this stage, the ribbons form loops around the big SMD caps, and the MTPs are seated in the MPOs. Rx is again marked with Red.

With that complete, pop the rubber cover on the Tx Minipod and install the Tx prism. Replace the rubber cover. There should be a little excess to work with thanks to Step 13.

Fig. 5 The Rx pod is the same, but it needs to have a loop put in it to make the left-to-right distance. Be sure to loop the cable downward! (Fig. 5)

The final picture below shows overall routing.

Final ribbon routing