Agilent/HP/Keysight 8468C PC Ribbon Cable Repair

[Bruce, KG6OJI]

In opening and closing the case of my 8648C signal generator I accidentally caught a fold of the keypad ribbon cable in the case closure.  This severed at least several of the conductors, leaving the instrument inoperative. 

At first, I thought I might repair the breaks by carefully peeling back the outer plastic layer and soldering jumpers across the break, but quickly learned the very thin foil layer flaked and fell off.  It seems this connecting ribbon is not separately replaceable without replacing the keyboard membrane, so I considered methods of repairing it in place.  At first I thought the 16-pin connector used to connect the ribbon cable to the motherboard was designed to puncture through the ribbon conductors by closing the housing, however, by looking at ribbon cable connectors online, it appears they are designed to contact pads on the printed circuit.  I thought  as there is sufficient slack in the cable, if I could obtain a comparable  pair of male and female connectors, I could cut the ribbon at the damage point and install these connectors to close the gap; however, I am wondering whether I could properly prepare the cut ribbon ends.

There are no markings on the ribbon connector.  Can anyone direct me to suitable products  and installation requirements?  Has anyone else had this happen and devised a suitable method of repair?

I have been considering drilling #80 (0.0135-inch) (0.34 mm) holes through the conductors, on either side of the breaks, filling these with solder paste, inserting fluxed sections of #30 AWG (0.01-inch) (0.25 mm) wire across the breaks, and heating the wire to hopefully connect to the foil and bridge the gaps.  Does this seem worthwhile trying?

Bruce, KG6OJI

[coppercone2]

Not sure without pictures, but what you can do for most ribbon cable is lay them flat on something and drag sand fine paper on it in 1 direction to expose the foil, then solder to that. Very slowly. Don't peel it, and you can use conductive epoxy if the solder is a problem, for a keypad it does not matter. I think your problem is that you peel instead of sand.

[R-1125F]

It's easiest just to make a new cable.   See https://www.digikey.com/en/product-highlight/a/assmann-wsw-components/idc-connectors-cables
One of these should work. I don't know exactly which one you need.

[Bruce, KG6OJI]

This particular flexible PC assembly is rather robust and appears to gave an about 0.01-inch (0.25 mm) layer of clear plastic on either side.  One must be the PC substrate and the other a protective/insulating covering. I may have made a mistake by trying to peel back the plastic covering on the substrate side.  It's possible the other side might be removable without destroying the printed conductor.

Bruce

[R-1125F]

I am confused by your description.   Are you working with an IDC type ribbon cable or an FPC.   These a very different things.  Could post pictures of what you are working on?

[Bruce, KG6OJI]

Fortunately, this printed cable assemble is one-sided; however, I suspect it is an integral part of the flexible-PC keypad assembly.  I cannot see inside the front panel without extensive dismantling.  I'm hesitant to do this, not because of complexity, but because of potential RF leakage and connector contact problems that might develop.  For this reasons I am considering repair methods that avoid the need to remove the keypad membrane.

An HP picture suggests the keypad flexible circuit is two-sided, but seems to show some soldering where the ribbon meets the main PC.  Pictures of low-cost Chinese replacement membranes on eBay show the flex connecting cable as an integral part of the two-sided flex-board. 

It seems like a clean way to make a repair would be to cut-out the damaged section of the flexible PC connecting cable and rejoin it with either a suitable joining connector or a M-F pair of connectors; that is if appropriate connectors are available and I am able to suitably prepare the cut ends.   

Regards,

Bruce

[R-1125F]

Is this what you are trying to repair?
https://www.ebay.com/itm/254660910171

[Bruce, KG6OJI]

Yes,

That's an aftermarket version.  I would much prefer to stay with the original, as if the low-cost Chinese offerings are gold-plated, they are likely just a flash.

Bruce 

[R-1125F]

For $28 I would take a chance.  Repairing an FPC can be difficult.  I have used aftermarket membrane flexes for HP power meters.  They work fine.

[Bruce, KG6OJI]

Thanks,
I went ahead and ordered one.  When it arrives and is on hand, I plan to attempt repair of the damaged cable by drilling conductors, packing in solder-flux, then inserting fluxed tinned wire jumpers and heating.  With the membrane out, I can properly evaluate and test the reliability of the repair.  If this repair is successful, I might fabricate a reinforcement device to support the repaired area.

Regards,

Bruce   

[R-1125F]

Ok, good luck with the repair.  Let us know how it works out.

[tooki]

Thanks,
I went ahead and ordered one.  When it arrives and is on hand, I plan to attempt repair of the damaged cable by drilling conductors, packing in solder-flux, then inserting fluxed tinned wire jumpers and heating.  With the membrane out, I can properly evaluate and test the reliability of the repair.  If this repair is successful, I might fabricate a reinforcement device to support the repaired area.

Drilling and then trying to solder essentially to the edge of metal foil seems silly to me; I can’t imagine it being easy to do (since the tiny contact area has a comparatively huge heat sink behind it) nor reliable. Why not use sandpaper or a fiberglass eraser to expose the tracks, then solder to them, as coppercone2 said? Much larger contact area to solder or glue to. Then you could reinforce it with a layer of epoxy or UV-cure solder mask.

[Bruce, KG6OJI]

Thanks Tooki, your point is well taken.  However, the base plastic substrate and plastic coating are both in the order of about 0.01-inch (0.25 mm) thick, whereas the copper foil thickness may be only about 0.003-inch (.01 mm) - (of course these numbers are only a guess as I have not measured the subject cable).  My concern is that it would be difficult to sand through the plastic layer without cutting through the copper foil too. 

My proposed repair was only a solution of last resort.  I'm thinking drilling might spread the copper, effectively widening the contact area.  My original thought was that if I could obtain a pair of suitable connectors, designed for connection to the foil, I could cut the flex cable on either side of the damage and rejoin it with the connectors.  Better yet, I thought if a simple joining device was available, such as one that would penetrate the foil with barbs, I could cut out the damaged section and rejoin the ends.  I am pretty naive when it comes to these modern connecting devices; so that's why I asked here with the hope of a solution unknown to me. 

Regards, Bruce

[tooki]

Thanks Tooki, your point is well taken.  However, the base plastic substrate and plastic coating are both in the order of about 0.01-inch (0.25 mm) thick, whereas the copper foil thickness may be only about 0.003-inch (.01 mm) - (of course these numbers are only a guess as I have not measured the subject cable).  My concern is that it would be difficult to sand through the plastic layer without cutting through the copper foil too. 

Copper is a soft metal, but it’s still much tougher than the plastic or lacquer of the flat flex. Use a fiberglass eraser (something I’ve used many times to expose copper from under solder mask). It’ll eat through the other stuff faster than the copper.

[coppercone2]

well its not like im theorizing, that's how people repair flat flex cable. Kapton is more durable then the one you have but the same technique works

[Bruce, KG6OJI]

My thanks to Tooki and Coppercone2,

Where does one obtain suitable fiberglass erasers?  Do they look like an old white rubber drafting eraser or are they of a different shape?

Bruce

[R-1125F]

Ebay or Amazon has them.   Search for Fiberglass Scratch Brush Pen

[Bruce, KG6OJI]

Thanks,

I ordered the Eurotool version from Amazon.

Bruce

[tooki]

I ordered the Eurotool version from Amazon.

Very good, they’re really useful things. Pro tip: do not absentmindedly rest your hand on the “eraser dust” from these, nor use your bare hand to wipe it off the bench. (And definitely don’t leave it on the bench!) Your hand will itch for days. I suggest working on a piece of paper you can discard immediately.

[coppercone2]

yeah and get a case to store the pen in (like a case that opens with a lid).
https://www.amazon.com/Stationery-Rectangular-Containers-Storage-Organizer/dp/B07Z3CDJC2

Another trick is to use a moist paper towel as a surface ,as the dust will stick to it.

I tend to go for sand paper whenever possible because the fiberglass is obnoxious to deal with, next to die grinder burrs. the pen of last resort


And the other thing that is good too is abrasive rubber burrs, for like a dremel. They are various grits of abrasive in various softness rubbers that spin and very gently arbade surfaces. That is my favorite tool for exposing soldermask BTW, I think it would be good on flat flex but I have not tried. They do smell and make dust though, so I use those outside. They have them with silicone rubber and abrasive (usually silicon carbide) and also leather/abrasive/glue. Ranging from types that are bearly able to polish a soft surface to types that can debur rough cut stainless steel (I love using a wheel type coarse rubber abrasive disk to deburr holes I drilled in steel), and they even have kinds that can basically contour a stainless steel weld, they just go kinda fast, but for interior corner work I don't blame anyone for losing it and spending 200$ of burrs to finish a project.  :'(

This one is probobly soft and suitable for soldermask and flat flex, so long its not too soft. Hard to quantify just find ones you like and stock up.
https://www.dremel.com/us/en/p/462-2615000462


If you can find the cup ones with fine or better abrasive, I think I would be comfortable using them on flat flex, so long I had the dremel flex shaft attachment so I can very carefully work it.


PACE electronics recommends/sells burrs/points meant for rotary tool for dealing with PCB coating materials BTW. I actually fixed a few charred PCB's using carbide ball burrs and rubber abrasives (were working OK but damn putting burned PCB back into a restoration is just bad).