Attaching flex cable to PCB

[Franc Zabkar]

I've been given a Mercedes-Benz instrument cluster where a flex cable
has come adrift due to mishandling. The PCB is microcontroller based
and interfaces with an odometer via a flex cable. The other gauges
connect via long pins. AFAICS, the flex cable is attached with some
kind of glue - there are no clamps, no screws, no rubber strain
relief. The traces have a very fine pitch, and the cable is almost too
short to work on. The PCB is single sided, so I could probably drill
two holes and add a clamp with a rubber sandwich, but I need to ensure
that the copper traces make reliable contact. Any suggestions?

BTW, the mechanic that gave me this job was doing a freebie for a
friend. AFAICT, he was trying to cover the empty lamp socket of an
annoying glow plug indicator with black tape.


- Franc Zabkar

 

[NSM]

| I've been given a Mercedes-Benz instrument cluster where a flex cable
| has come adrift due to mishandling. The PCB is microcontroller based
....
| BTW, the mechanic that gave me this job was doing a freebie for a
| friend. AFAICT, he was trying to cover the empty lamp socket of an
| annoying glow plug indicator with black tape.

What did the indicator light say? "Don't disassemble this"?

N

 

[Gerard Bok]

I've been given a Mercedes-Benz instrument cluster where a flex cable
has come adrift due to mishandling. The PCB is microcontroller based
and interfaces with an odometer via a flex cable. The other gauges
connect via long pins. AFAICS, the flex cable is attached with some
kind of glue - there are no clamps, no screws, no rubber strain
relief. The traces have a very fine pitch, and the cable is almost too
short to work on. The PCB is single sided, so I could probably drill
two holes

Are you very sure ?
'Very fine pitch' and 'single sided PCB' sound like a
contradiction to me. (Or as a euphemism for 'multi layer

 

[Franc Zabkar]

| I've been given a Mercedes-Benz instrument cluster where a flex cable
| has come adrift due to mishandling. The PCB is microcontroller based
...
| BTW, the mechanic that gave me this job was doing a freebie for a
| friend. AFAICT, he was trying to cover the empty lamp socket of an
| annoying glow plug indicator with black tape.

What did the indicator light say? "Don't disassemble this"?

What it should have said was "CAUTION: PCB obscures very poor design".

I do quite a few jobs for this mechanic and find that he and his staff
are very competent. Unfortunately I don't have the unit with me,
otherwise I would post a photo so you could see it for yourself. As I
said in my OP, there is no strain relief for the flex cable, and no
connector. The printed traces of the cable and the PCB rely purely on
a tiny, almost imperceptible dab of glue to keep them in contact. I've
never seen anything so abominably bad. Even children's toys are better
built.

BTW, this abortion is manufactured by Magneti Marelli.


- Franc Zabkar

 

[NSM]

| What it should have said was "CAUTION: PCB obscures very poor design".
|
| I do quite a few jobs for this mechanic and find that he and his staff
| are very competent. Unfortunately I don't have the unit with me,
| otherwise I would post a photo so you could see it for yourself. As I
| said in my OP, there is no strain relief for the flex cable, and no
| connector. The printed traces of the cable and the PCB rely purely on
| a tiny, almost imperceptible dab of glue to keep them in contact. I've
| never seen anything so abominably bad. Even children's toys are better
| built.
|
| BTW, this abortion is manufactured by Magneti Marelli.

Is that Italian for Lucas Electric?

http://pw1.netcom.com/~krk/lotus/humor/lucasjokes.html

http://members.tripod.com/~tcotrel/lucas.html (think about it)

NM

 

[Ross Herbert]

What it should have said was "CAUTION: PCB obscures very poor design".

I do quite a few jobs for this mechanic and find that he and his staff
are very competent. Unfortunately I don't have the unit with me,
otherwise I would post a photo so you could see it for yourself. As I
said in my OP, there is no strain relief for the flex cable, and no
connector. The printed traces of the cable and the PCB rely purely on
a tiny, almost imperceptible dab of glue to keep them in contact. I've
never seen anything so abominably bad. Even children's toys are better
built.

BTW, this abortion is manufactured by Magneti Marelli.


- Franc Zabkar

I don't like your chances Franc. The only thing I an think of is
electrically conductive epoxy.

 

[NSM]

| I don't like your chances Franc. The only thing I an think of is
| electrically conductive epoxy.

If it's a zebra connection wouldn't that be worse? You'd short everything to
everything.
He could just try to align it as best he can and hold it there to test. Glue
it if it works.

N

 

[Ross Herbert]

| I don't like your chances Franc. The only thing I an think of is
| electrically conductive epoxy.

If it's a zebra connection wouldn't that be worse? You'd short everything to
everything.
He could just try to align it as best he can and hold it there to test. Glue
it if it works.

N

Franc made no mention of a zebra connection otherwise I wouldn't have
suggested epoxy. He did say that the original seemed to be attached
with some sort of adhesive so that's the reason for my suggestion.

 

[NSM]

| Franc made no mention of a zebra connection otherwise I wouldn't have
| suggested epoxy. He did say that the original seemed to be attached
| with some sort of adhesive so that's the reason for my suggestion.

But if it's a multi conductor cable of any sort conductive epoxy seems like
the worst choice.

N

 

[Franc Zabkar]

Are you very sure ?
'Very fine pitch' and 'single sided PCB' sound like a
contradiction to me. (Or as a euphemism for 'multi layer

There are no thru-holes. All components are smt. The mechanic says an
instrument specialist told him that these flex cables are known to
separate in normal use, probably due to heat and vibration. He claims
these units are irreparable.

Here is a photo of the PCB:
http://members.optusnet.com.au/~fzabkar/pcb_trace.jpg

The connection point is above and between the two lamps. The traces go
directly to a microcontroller. Note that there are numerous test
points which look like thru-holes, but I believe these could be for
bed-of-nails testing (???).


- Franc Zabkar

 

[Franc Zabkar]

| I don't like your chances Franc. The only thing I an think of is
| electrically conductive epoxy.

If it's a zebra connection wouldn't that be worse? You'd short everything to
everything.

It's not a zebra elastomer connection. The copper traces of the flex
cable bear directly against the copper traces of the PCB. The two are
retained with some kind of glue. It's a very peculiar and IMO very
poor connection method. In fact, I've seen more adhesive on a postage
stamp.

He could just try to align it as best he can and hold it there to test. Glue
it if it works.

On closer inspection some of the printed copper traces on the flex
lead have delaminated. It appears this job is a lost cause. :-(

Anyway, here are some pics. The original 5MP photos have been
compressed to 90%, so the quality is poor.

http://members.optusnet.com.au/~fzabkar/label.jpg
http://members.optusnet.com.au/~fzabkar/instruments.jpg
http://members.optusnet.com.au/~fzabkar/flex_lead2.jpg
http://members.optusnet.com.au/~fzabkar/flex_lead.jpg
http://members.optusnet.com.au/~fzabkar/pcb_trace.jpg


- Franc Zabkar

 

[NSM]

| It's not a zebra elastomer connection. The copper traces of the flex
| cable bear directly against the copper traces of the PCB. The two are
| retained with some kind of glue. It's a very peculiar and IMO very
| poor connection method. In fact, I've seen more adhesive on a postage
| stamp.
| On closer inspection some of the printed copper traces on the flex
| lead have delaminated. It appears this job is a lost cause. :-(

I'd say so. I only see two choices, fix it, which may not be possible, or
replace it with something else. Depends on how much time and effort you want
to put into the task and how much value you will get for it.

You could start collecting the dead ones and use them as a garden border.

NM

 

[Jason D.]

There are no thru-holes. All components are smt. The mechanic says an

instrument specialist told him that these flex cables are known to
separate in normal use, probably due to heat and vibration. He claims
these units are irreparable.

Here is a photo of the PCB:
http://members.optusnet.com.au/~fzabkar/pcb_trace.jpg

The connection point is above and between the two lamps. The traces go
directly to a microcontroller. Note that there are numerous test
points which look like thru-holes, but I believe these could be for
bed-of-nails testing (???).

Yes, those "bumps" are for the bed of nail test points.

Looks like the ribbon and the pads on the circuit board are good, Why
not do a low-temp soldering?

Next time use a good older program like photo shop pro 5.x (ebay
hopefully) to crop it (this chops off the unneeded pixels) then shrink
it down to 50% to cut down bytes, 30% is minimum for smallest size
without losing too much details.

Cheers,

Wizard

 

[Jim Adney]

It's not a zebra elastomer connection. The copper traces of the flex
cable bear directly against the copper traces of the PCB. The two are
retained with some kind of glue. It's a very peculiar and IMO very
poor connection method. In fact, I've seen more adhesive on a postage
stamp.

I once had a basket case pocket calculator that someone else had taken
apart and destroyed. The damage they did consisted of breaking apart a
cable to board connection that was exactly like you are describing. I
was never able to come up with a repair method, but the one thing that
I thought might be worth trying was some sort of heat & pressure, like
layng a rag over it and then ironing it into place.

It's just possible that the adhesive is temperature sensitive.

I finally gave up and threw the calculator parts away, because I was
just never going to find the time to try to fix it.

-

 

[Franc Zabkar]

Yes, those "bumps" are for the bed of nail test points.

Looks like the ribbon and the pads on the circuit board are good, Why
not do a low-temp soldering?

My patience and eyesight aren't good enough for this. In any case an
email from a kind gent has cleared up the mystery for me. The
connection method is 3M Scotch 9703 conductive tape:

http://products3.3m.com/catalog/us/...e_us_oc_all_3_0/command_OCHandler/output_html

"Tape 9703 is an isotropic, electrically conductive, pressure
sensitive adhesive (PSA) transfer tape. It is a permanently tacky
system consisting of a PSA matrix with aligned conductive particles.
These particles allow electrical conduction through the adhesive
thickness ("Z-axis"), but not in the plane of the tape."

Next time use a good older program like photo shop pro 5.x (ebay
hopefully) to crop it (this chops off the unneeded pixels) then shrink
it down to 50% to cut down bytes, 30% is minimum for smallest size
without losing too much details.

Sorry for the abysmal quality. I should have made better use of Paint
Shop Pro, and I should have taken more care with the photography.

Here are the same photos with better processing:
http://members.optusnet.com.au/~fzabkar/pcb_trace.jpg (33KB)
http://members.optusnet.com.au/~fzabkar/flex_lead.jpg (19KB)
http://members.optusnet.com.au/~fzabkar/flex_lead2.jpg (17KB)
http://members.optusnet.com.au/~fzabkar/label.jpg (13KB)
http://members.optusnet.com.au/~fzabkar/instruments.jpg (13KB)

Cheers,

Wizard

I wonder if Scotch 9703 conductive tape may be a viable solution for
repairing broken flex leads?


- Franc Zabkar

 

[NSM]

| My patience and eyesight aren't good enough for this. In any case an
| email from a kind gent has cleared up the mystery for me. The
| connection method is 3M Scotch 9703 conductive tape:

Most interesting. I wonder if this is obtainable in service quantities?

N

 

[James Sweet]

Yes, those "bumps" are for the bed of nail test points.

Looks like the ribbon and the pads on the circuit board are good, Why
not do a low-temp soldering?

Next time use a good older program like photo shop pro 5.x (ebay
hopefully) to crop it (this chops off the unneeded pixels) then shrink
it down to 50% to cut down bytes, 30% is minimum for smallest size
without losing too much details.

You don't even need that, you can get IrfanView for free, it'll resize,
convert formats, colors, etc not to mention view just about anything.

 

[James Sweet]

| My patience and eyesight aren't good enough for this. In any case an
| email from a kind gent has cleared up the mystery for me. The
| connection method is 3M Scotch 9703 conductive tape:

Most interesting. I wonder if this is obtainable in service quantities?

Perhaps if you contact them they'd be willing to send you a sample of it?

 

[Gerard Bok]

My patience and eyesight aren't good enough for this. In any case an
email from a kind gent has cleared up the mystery for me. The
connection method is 3M Scotch 9703 conductive tape:

http://products3.3m.com/catalog/us/...e_us_oc_all_3_0/command_OCHandler/output_html

"Tape 9703 is an isotropic, electrically conductive, pressure
sensitive adhesive (PSA) transfer tape. It is a permanently tacky
system consisting of a PSA matrix with aligned conductive particles.
These particles allow electrical conduction through the adhesive
thickness ("Z-axis"), but not in the plane of the tape."

In other words: self adhesive zebra strip

 

[Jason D.]

Here are the same photos with better processing:

http://members.optusnet.com.au/~fzabkar/pcb_trace.jpg (33KB)
http://members.optusnet.com.au/~fzabkar/flex_lead.jpg (19KB)
http://members.optusnet.com.au/~fzabkar/flex_lead2.jpg (17KB)
http://members.optusnet.com.au/~fzabkar/label.jpg (13KB)
http://members.optusnet.com.au/~fzabkar/instruments.jpg (13KB)


I wonder if Scotch 9703 conductive tape may be a viable solution for
repairing broken flex leads?


- Franc Zabkar

That special tape will work! Key is clean both real good. Careful on
that flex cable, it is made of conductive paint printed on clear
plastic sheet. Many chemicals will attack that paint. This is same
kind of flex circuit with this printed on conductive paint.

Otherwise, there is room on PCB to drill two holes and use two thick
(say .050" to .100") strips of aluminum and screws & nuts, one thin
rubber strip from your junk stuff to pad the flex so both flex and
circuit board is squeezed together. Much better design IHMO!!

What type of display panel? LCD?

Cheers,

Wizard