LCD Zebra Connectors: Compression, Pad Design and Fixing Missing Segments
A zebra connector, or elastomeric connector, is a silicone strip built from alternating conductive and insulating layers. Squeezed between the ITO contacts on an LCD glass ledge and matching pads on a PCB, it connects every contact to the pad below it without solder. It works when three things are right: the strip is compressed by about 10 to 15 %, every pad is covered by at least three conductive layers, and the pads are flat, clean and gold-plated. When a segment LCD with a zebra connector shows missing or flickering segments, one of those three is usually the cause, not the glass or the driver.
What the strip is made of
The conductive layers in a standard strip are silicone loaded with carbon, separated by layers of plain silicone. The layer pitch is the conductor plus the insulator: a 0.1 mm pitch strip, for example, is 0.05 mm of conductor and 0.05 mm of insulator. Common pitches are 0.05, 0.1 and 0.18 mm. The conductive core is usually about 0.6 mm wide, and many strips have solid or sponge silicone support walls on both sides that keep them upright and spread the load. Silver-filled versions exist where a lower resistance is needed.
Carbon layers are far more resistive than a metal pin, and that is fine for an LCD. Segment and common lines charge a small capacitance and draw very little current. Current ratings are modest (one manufacturer quotes 50 mA for a 1 mm × 1 mm pad), so a backlight is normally connected by its own pins or wires rather than through the zebra.
In our catalogue, S4101D (HTN, 1/6 duty, 3.0 V, 74.50 × 74.50 mm viewing area) and S4056D (HTN, 1/4 duty, 5.0 V, 152.20 × 43.90 mm viewing area) are both specified with elastomeric connectors. The second shows why zebras suit wide glass: one strip carries a long contact row with no soldering.
Getting the compression right
Compression is set by the gap h between the contact surface of the glass ledge and the PCB pads. The bezel or holder fixes that gap. The strip’s free height H then follows from the compression p you want:
H = h ÷ (1 − p)
Aim for about 10 to 15 %. Below about 10 %, contact resistance becomes unstable and segments drop out, especially in the cold. Above about 15 %, the strip bows sideways and loads the glass ledge and the PCB harder than they should be. Taller strips are usually run at the lower end of the range (around 10 % for strips 10 mm or taller) and short strips nearer 15 %.
Worked example. The holder gives a nominal gap of h = 3.40 mm. For 12.5 % compression, H = 3.40 ÷ 0.875 = 3.89 mm, so a 3.9 mm strip is the natural choice. With that strip, 10 % compression happens at h = 3.51 mm and 15 % at h = 3.32 mm. The gap must stay within about ± 0.1 mm of nominal.
That is a tight budget. On our segment references each glass layer is 1.10 ± 0.10 mm, so glass thickness alone can use most of the window if it sits in the stack. Add bezel height and PCB flatness, and it is clear why zebra assemblies need a holder that controls the gap, and why the compression should be checked at both ends of the tolerance stack, not only at nominal.
Two practical points follow. A sponge-supported strip tolerates gap variation better than a solid one. And the load must reach the strip, not be carried by something else in the stack: if the rear glass or the backlight bottoms out on the PCB first, the zebra is never properly compressed.
Pads on the glass and on the PCB
- Layers per pad. Two conductive layers on a pad often work; three are the reliable minimum, and four or five are better with a 0.05 mm pitch strip. For a 1.2 mm contact pitch with 0.6 mm pads, a 0.18 mm pitch strip puts about 0.6 ÷ 0.18 ≈ 3.3 layers on each pad.
- Gaps between pads. Make every gap wider than one conductive layer so no single layer can bridge two pads. With the pitches above this is easy; it matters on very fine glass.
- Plating. Use ENIG or hard gold on the PCB pads. HASL leaves an uneven surface that the strip cannot follow.
- Solder mask. Leave one continuous mask opening over the whole contact row. Mask ridges between pads hold the strip off the copper.
- Vias. Keep vias out of the contact area. One connector maker recommends at least 0.35 mm from a pad edge to the via centre.
- Length and width. Let the strip run at least 0.5 mm past the first and last pad, and keep it no wider than the glass ledge it sits under.
- Alignment. Locate the strip in a slot in the holder or between bezel features, so it cannot shift sideways during assembly.
On the glass side, the ITO contact pads and their pitch are part of the glass layout. If you are designing a new segment LCD, define the pad row together with the pin map; the segment LCD design and pin map guide covers the ledge and margins.
Holding it together
A zebra connection is only as good as the part that clamps it. Metal bezels with twist or solder tabs are common on modules; plastic holders with a slot for the strip are common in products that mount the glass directly. Whatever the method, the clamping force should be even along the whole strip. A long strip in a bezel that is held only at the corners will lose contact in the middle when the PCB bows. Add clamping points or stiffen the board under long strips, and give the holder room for the strip to bulge sideways as it is compressed.
Keep the clamp off the viewing area. Pressure on the active area of the glass changes the cell gap and shows as dark patches.
Fixing missing or flickering segments
Before blaming the glass or the driver, check the connection. A quick test: with the display showing all segments, press gently on the bezel or holder directly above the strip. If missing segments come back, the problem is contact pressure.
| Symptom | Likely cause | What to check |
|---|---|---|
| A group of neighbouring segments missing at one end of the strip | Low compression at that end: loose tab, bowed PCB or strip shifted | Press above that end; check tabs, PCB flatness and strip position |
| Segments flicker when the product is tapped, or drop out in the cold | Compression near or below 10 %, or compression set after time at temperature | Measure the gap against the strip height; consider a taller strip or a holder with a stop |
| One segment missing, neighbours fine | Dirt or damage on one pad, or too few conductive layers on a narrow pad | Inspect and clean that pad on glass and PCB; compare pad width with the layer pitch |
| Two segments switching together, or extra faint segments | Debris or flux bridging pads; for faint ghosts, see ITO routing in the design guide | Clean pads and strip; check pad gaps; check drive voltage for the duty |
| Faults that appear after humidity or condensation | Contamination or film on the contacts | Clean with isopropyl alcohol and a lint-free swab; review enclosure sealing |
| Whole display blank or uniformly faint | A common (COM) contact lost, or the strip fitted over the wrong row | Check the COM pads first; check strip orientation and position |
Handle strips by their sides and keep fingers off the glass contacts. A strip that has been compressed for a long time keeps some of that set; after rework, fit a new strip rather than reusing a deformed one. For other display symptoms, see common LCD display problems.
When to use something else
Zebras need no soldering, are easy to rework and suit long contact rows, but they depend on a controlled clamp and clean contacts. The alternatives trade those points differently:
- Metal pins clip onto the glass ledge and solder into the PCB, which suits vibration. Our pin-type references S04805D and S04778D use 2.00 mm and 1.80 mm pitch.
- Heat seal connectors bond a printed flexible film to the glass and the PCB and let the board sit away from the display, as on S3966D.
- FPC or chip-on-glass handles fine pitches and puts the driver on the glass; see COG vs COB LCD modules.
The LCD connection types guide compares all of them on real parts.
Video: LCD Zebra Connectors, Compression and Missing Segments
This SuccessLCD explainer shows how a zebra connector is built, how to set the strip height for about 10 to 15 % compression, the PCB pad rules, and how to trace missing or flickering segments, with real SuccessLCD zebra-connected LCD samples.
LCD Zebra Connector FAQs
How much should an LCD zebra connector be compressed?
About 10 to 15 % of its free height. Calculate the free height as H = h ÷ (1 − p), where h is the gap set by the holder and p the compression. Below about 10 % contact becomes unstable; above about 15 % the strip bows and overstresses the glass ledge and the PCB.
How many conductive layers should cover each pad?
At least three. Two often work, but three gives a margin against misalignment and dirt. With a 0.05 mm pitch strip, four or five layers per pad are better.
Can a zebra strip be reused after the display is taken apart?
Only if it has no visible permanent set and the layers are clean. In production rework, fit a new strip; a deformed strip is a common cause of intermittent segments after repair.
Why do segments disappear in the cold on a zebra-connected LCD?
Silicone stiffens at low temperature and the assembly shrinks, so a connection that was only just compressed enough at room temperature can lose contact. Check the compression at the worst-case end of the tolerance stack and keep it above about 10 %.
Designing a zebra-connected display? Send the glass or module drawing, your PCB stack-up and holder design, and the temperature range. We will check the strip height, compression and pad layout. Ask for a connection review, or browse monochrome LCD references. For a custom connection, see custom LCD module options.