LCD Glass Panel Contacts

[Bob Scott]

This question has had me puzzled ever since I took apart an old Timex LCD digital watch. That was a loooong time ago.

Manufacturers of glass LCD panels like the ones used in Martel digital panel meters are still built the same way. I disassembled the watch a long time ago and I disassembled the panel meter just recently for curiosity. The LCD display panel is made of glass. The panel has no metal contacts. It has no visible contacts at all, but invisible ones must be present on the glass surface, making contact with what must be conductive rubber or flexible plastic pressure contacts in the meter assembly. In both cases, on reassembly, not all LCD segments work.

Is there a method or material used during the manufacture of these panels that I can use to get all of these LCD meter segments working properly again?

Bob

PS my brother recently disassembled an expensive back-lit Martel DPM3AS-BL panel meter and has the same problem: won't work on reassembly.

[positronicle]

--Edited by Positronicle--

[Gorgon]

Hi,
This is a problem with the contact rubber or zebrastrip. When mounted the first time it is compressed a bit, and when dismantled it stays more or less in the same state, and when moved and remounted the pressure is not kept for proper contact. There may also be a problem with alignment of the contacts on the glass and the pcb. The handling of the parts with bare hands will also add to the list of problems.

As a start, try to wash the parts(contact area) with a cleaning solvent. No handling with bare hands, and reassemble it again.

TOK

[Bob Scott]

Thanks for the replies guys, but:

Neither the contacts on these glass panels displays nor the "rubber" terminals were touched by any part of the body, so there is no contamination; no solvents needed to clean fingerprints etc. They were merely separated for a few seconds and failed to function correctly when brought back together.

So I'm still hunting for a credible solution, or does anyone have any technical information, like knowledge of how they assemble these things at the factory.

Bob

[positronicle]

--Edited by Positronicle--

[haklesup]

The "zebra stripe" elastomeric connector is not sensitive to ESD at all. It is also fairly robust with handling. It comes in a variety of types but the carbon conductor kind is most common in LCD displays but I have also seen the Silver and Gold wire stiips too.

In fact I use both the Gold and Silver Fujipoly material in test fixtures we make for our customers. Repeat contact relaibility is quite good on all materials I have tested except the gold wire material tesds to get deformed to the point you get opens. The silver is excellent even after touching the surfaces but it will oxidize over time while exposed to air.

http://www.fujipoly.com/
http://www.shinpoly.com/products/interconnectors/

The contacts on the LCD display's glass is a metal film which can often be seen when held in the light at an angle (about as faint as a fingerprint). It is very similar to an edge connector you might find on a PCI card in your PC only smaller. LCD displays without controllers are not particularly susceptable to ESD damage unless the charge is very high.

Alignment is critical! The pads of the LCD display must align perfectly over the matching array on the PCB. The orientation of the elastomer connector is less critical as long as it traverses the space between the contacts. Some contact pressure is required and is usually provided by the case or bezel for the display depending on the product.

Bottom line, you should be able to remove the display and replace it, I have done it in cell phones but not in a watch. Try aligning it more carefully and applying some pressure.

Current used by the panel is almost nil, any crappy resistive connection should get the digital signals through, you have opens if it is not working.

[Edd]

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.....OHHHH TAAAAAY …Sir Bob…here goes:


"PS my brother recently disassembled an expensive back-lit Martel DPM3AS-BL panel meter and has the same problem: won't work on reassembly."

....I can assure you……. that it's neither a fluke nor a familial genetically ingrained malady!

Since you made final reference to a panel meter and not an old digital watch, I am aware to that type of interfacing construction mode and some of its shortcomings, but just in that perspective, here are my findings, along with their corrective actions.
The most common and tried interfacing of the LCD element to its companion interfacing contacts is via the utilization of a conductive overlay pattern on the surface of the glass panel via plasma deposition of a metal film in a vacuum environment. This then leaves a multiple micron surface conductive layer to then be interfaced with.
If you will hold the naked LCD panel correctly in reference to a reflected light source, you can feebly see the reflection from the thin layering of metal deposition. Check the image below and you will vaguely see the 10 equi-spaced out rectangular contact lands /fingers on the bottom of the LCD panel shown .



The actual interconnect action is typically made with a conductive elastomeric multiple contact strip…..AKA… Zebra strip..(a la Gorgon)..... with multiple hundreds of electrically isolated conductive fine graphite strands oriented side by side. To save materiel they tend to precisely space out square or rectangular areas of conductive areas with their “black stripeâ€

[dacflyer]

WOW...talk about TMI, heehee Edd gets an A+ for that...
he must be a professor....