I’ve been thinking about how to reduce the height on the back of a panel for an application like @marcmerlin’s matrix shirt. Looking at some pictures of flexible panels from Aliexpress, the tallest components besides connectors seem to be the capacitors. I believe those are the same 6mm tall capacitors I have on panels in front of me. So, <6mm max connector and controller height is our ideal target.
HUB 75 Connector
The Shrouded HUB75 IDC connector available on most panels is already a lot taller than 6mm - I measure 9.5mm on mine - and it gets a lot taller when you add the mating connector. The thinnest existing option for mating connector I can see without significant innovation is to use the same SMT connector that’s on the SmartLED Shield, but with a new board that keeps components on the bottom side. In that case we’re only adding the height of the PCB: 1.6mm, or potentially lower if this board were made thinner. OSH Park now has a 0.8mm thick board, or ~0.2mm flex board. Even with the flex board, total height is 9.7mm, still 3.7mm taller than our ideal target.
Instead of the shrouded IDC connector, some boards have unshrouded pins, which are slightly lower height, and have a lower base.
I measure pin height of 9.25mm, and base height of 3mm. With pins like this you could slide the SMT connector that’s on the SmartLED Shield V4 a lot lower onto the pins. With custom length pins just long enough to make secure contact with the SMT female connector, this could get fairly low profile. Now we’re at 7.2mm (3mm male base, 4mm female connector, min 0.2mm board thickness), assuming the excess height of the pins gets trimmed down.
Of course, not all boards have this, and not all users will want to replace this connector manually. The edges of the shrouded connector can be carefully snipped off, leaving the pins and a similarly tall 3mm base. (Note, don’t try to just pull the shroud off the connector. Sometimes this works, and sometimes you end up pulling off one or more pads from the panel, ruining your panel.)
Can we do better? Maybe if we eliminate the mating connector, and source a ribbon cable crimp that’s designed to solder directly to a PCB. I can’t find any that are SMT, but found this THT cable from Samtec:
It’s not SMT, but maybe if the THT pins were carefully bent out 90 degrees (as I drew in red on this diagram), we could get as low as 6.6mm high.
I’d say this has significant drawbacks though, as it’s a manually modified part, requires careful soldering, and a damaged ribbon cable needs SMT rework to replace.
I have a panel like this one with pads for a FFC connector on the bottom. If only that were available on more panels.
Maybe a FFC adapter is what we need. If something like this were soldered directly to the no-shroud header, the height would be 3mm + 0.8mm PCB + 2.56mm FFC connector = 6.36mm, real close to ideal.
I’m not sure all boards have clearance right next to the HUB75 connector for the FFC connector, but most probably do.
Can we do better? What if the FFC adapter were made with castellated edges and designed to be soldered directly to the panel PCB? Something like this board but with castellated edges on two sides.
The width of the FFC connector from PCB footprint to opening of the opposite side is 7.4mm. Adding some clearance so the castellated edge pads can be soldered, let’s say the board width is 9.4mm. The width of the HUB75 connector SMT footprint on the boards I have in front of me are about 11.5-12mm, so there’s enough clearance to solder the castellated edges to the pads.
Height is now 1.6mm PCB adapter + 2.56mm = 4.16mm, well under our target. (I’m assuming we’ll use standard thickness PCB here because we have the space for it and it’s more easily sourced)
Downsides? Yes, this requires desoldering the IDC/SMT connector from the panel, and doing some large pitch SMT soldering.
I think with a couple adapter boards, we can have options for several types of users:
- people that are not comfortable soldering at all (SMT female connector to FFC adapter: 7.2mm height)
- people that are comfortable soldering THT but not desoldering (FFC adapter board with THT holes: 6.36mm)
- people that are comfortable desoldering and soldering SMT (castellated edges FFC adapter board, 4.16mm)
This connector is a lot more bulky. I measure it at 11.2mm tall without any mating connector. The base where the pins are exposed is 5mm tall on its own. There’s very little room to work here if we want to keep the existing connector. The total height when adding the mating connector is 16.4mm, not including the added height from cables coming out of the mating connector.
The pins have a 0.156" spacing and are thicker than standard headers which limits creative options. I’m not coming up with anything at the moment, except I did notice that the SMT female HUB75 connector that’s on the SmartLED Shield V4 will slide onto a single 0.156" connector pin. The spacing isn’t enough to mate with more than one pin, and the current limit is I’m sure lower than the 4+ amps that some panels draw, so this idea isn’t going to go very far.
The only good option I can see at the moment is desoldering the existing connector and soldering wires directly to the SMT pads. Worst case if someone isn’t comfortable desoldering or doesn’t have the hot air rework tool - that is almost required for desoldering a connector like this which is connected to large copper pours on the PCB - they can solder to the existing connector on the pads, clip the excess pins and plastic, and get the height down to ~5mm.
If we have FFC cable and wires coming off the panel, the controller board doesn’t need to go under the panel at all, but let’s assume it will be.
SmartLED Shield V4 plus Teensy is about 12.5mm tall, not considering a new HUB75 to FFC adapter that will need to be added. I think this is far too tall to fit under the panel, compared to the existing 6mm max height. The fact that the shield can’t sit flat against the panel because of the height of components on the panel and the risk of shorts from Teensy pins making contact with those components makes the effective height even taller. I don’t see an easy fix here.
If the SmartLED Shield, Teensy, and FFC to HUB75 adapter are moved away from the panel, they’d probably need to be moved far away, as having a bulge in the shirt for the controller next to the panel will be unsightly. I’m not sure how long of a FFC cable can be used with the high speeds of refreshing a panel. I believe ribbon cables with the length of a foot or two are acceptable, maybe the clock rate will need to be lowered slightly. The traces on the FFC cable are thinner which could degrade the signals, and the cable is more likely to be damaged than a ribbon cable. This solution isn’t sounding ideal to me.
More thoughts on this coming in the future…
I’d love to hear some feedback on what I have so far