COG vs COB LCD Modules: What Changes in Your Design
COG and COB describe where the LCD driver chip sits. In a COB (chip-on-board) module, the bare driver die is wire-bonded to a printed circuit board behind the glass and sealed under black epoxy; the board, a metal bezel and a pin header make the module self-contained. In a COG (chip-on-glass) display, the driver IC is bonded straight onto a ledge of the LCD glass with anisotropic conductive film and an FPC carries the signals to your board. COG is thinner, smaller and has fewer parts; COB is sturdier, easier to mount and easier to connect while you are still developing the product.
The choice changes more than the thickness. With COG, the power components, the connector and the mechanical support move from the module onto your PCB and into your enclosure. This guide walks through what moves, with figures from our catalogue references.
What is physically different
| COB module | COG display | |
|---|---|---|
| Driver | Bare die wire-bonded to the module PCB, covered with epoxy | Driver IC bonded on the glass ledge with anisotropic conductive film (ACF) |
| Glass to driver | Zebra connector, pins or heat seal between glass and PCB | ITO traces on the glass run straight into the IC |
| Connection to the host | Pin header, often 2.54 mm pitch, or a cable | FPC tail into a ZIF connector or hot-bar soldered |
| Mechanics | Metal bezel and mounting holes on the PCB | Your enclosure holds the glass (frame, gasket or adhesive) |
| Power parts | On the module PCB | A few capacitors on the FPC or on your PCB |
| Rework | Glass, zebra and board are separate parts in the bezel | IC and glass are one part; a damaged IC or glass means a new display |
| Typical use | Character and graphic modules, development, products with room behind the display | Handheld and compact products, high volumes, thin housings |
Footprint: the board around the glass
The clearest way to see the difference is to compare drawings. Our 128 × 64 COB reference S12864D has a 72.00 × 40.00 mm viewing area, but its PCB is 93.00 × 70.00 mm, the bezel 77.20 × 50.20 mm, and four mounting holes sit on 88.00 × 65.00 mm centres. The host connects through a 20-pin header at 2.54 mm pitch. The board is more than twice the area of the viewing window, because it carries the driver, the power circuit, the header and the holes.
A COG display needs only the glass, a ledge for the IC and the FPC, plus a backlight if one is fitted. Our dot-matrix COG reference S2983D has a 34.00 × 19.00 mm minimum viewing area and connects by FPC, with no PCB of its own. That is why phones, meters and handheld instruments moved to COG: the display is little bigger than its viewing area.
The cost of that compactness is that your design has to provide what the COB board used to: a place for the connector, support for the glass and a path for the FPC.
Electrical: what moves onto your board with COG
A COG driver is powered and addressed through ITO traces on the glass, and ITO is far more resistive than copper. Controller datasheets therefore set limits for the trace resistance. The Sitronix ST7567, used in COG graphic displays such as our S2983D and S4417D (ST7567A), recommends less than 100 Ω on the VDD and VSS lines and less than 300 Ω on the booster and V0 lines. The glass maker handles this in the layout, but it explains two things you will see on your side:
- External capacitors. The on-chip booster and regulator generate the LCD voltage, but they need capacitors outside the chip. For the ST7567 that is two capacitors in the 0.1 to 1 µF range. They go on the FPC or on your PCB, close to the connector. S4417D runs from a 3.3 V logic supply and generates its own 9.0 V LCD drive voltage in this way.
- A sensitive reset line. The same datasheet raises the resistance of the reset trace on purpose so that an ESD event cannot reset the chip. On your board, keep the reset line short, filtered and away from noisy signals. Random resets or scrambled pixels when the product is touched usually trace back to reset, ground or the FPC path; see the LCD ESD protection guide.
COG controllers usually offer SPI or I2C as well as a parallel bus, which suits small MCUs. A COB module carries its own power parts and often a contrast adjustment, and it presents a plain header, which is why it is convenient on a development bench. Contrast on most COG parts is set in software through the controller’s electronic volume rather than with a potentiometer; see LCD contrast and V0 adjustment.
Mechanical and reliability trade-offs
On a COG display the ledge carrying the IC is the weak point. It is a thin strip of unsupported glass with a chip and an FPC bonded to it. In practice:
- Support the glass in the enclosure but keep pressure off the IC and the ledge.
- Bend the FPC away from the bond line, not at it, and stay within the FPC’s bend radius.
- Keep the IC covered. Most COG parts ship with black tape or resin over the chip because the bare silicon is sensitive to strong light; do not remove it, and do not let a light guide or LED shine on the IC area.
- Plan the ESD path. On COG the IC is directly behind the glass edge, so a discharge to the front of the product can reach it quickly.
A COB module is more forgiving. The die is sealed under epoxy on a PCB, the metal bezel clamps the glass and the connector, and the module bolts to the product. Its own weak point is the glass-to-board connection: a zebra strip needs the right compression to keep every segment in contact, as described in the LCD zebra connector guide.
Rework and field repair
In a COB module the glass, zebra strips, backlight and board are separate parts held by the bezel. A cracked glass or a poor zebra contact can be reworked by opening the bezel. On a COG display the driver is bonded to the glass, so damage to either one means replacing the whole display. For products that are serviced in the field this is part of the decision, not only the purchase price.
Segment displays can be COG too
COG is often associated with dot-matrix graphics, but segment glass uses it as well. Our S2947D is a segment COG display in FSTN, driven at 1/4 duty and 1/3 bias from 3.3 V, with an FPC and a backlight. At the other end, S3092D is a large DFSTN negative segment display with a 110.08 × 69.80 mm minimum viewing area. It uses a UC1601S controller at 1/32 duty and 1/7 bias, so a panel full of icons connects through a short FPC instead of dozens of pins. A chip-on-board segment option also exists, such as S4210D, a TN chip-on-board LCD with pins.
For a new segment design, a COG driver mainly changes the pin map. Instead of one pin per SEG line, the host sees a serial bus. The segment LCD design and pin map guide shows how the count works either way.
How we would choose
- Choose COB when there is room behind the display, the product is built in modest volumes, you want to bolt the display on and plug in a header, or you expect to rework displays in service. It is also the quickest route to a working prototype.
- Choose COG when thickness and area matter, volumes justify a tailored glass and FPC, and your PCB and enclosure can provide the connector, the capacitors and the support for the glass.
- Check the controller first in either case. The driver decides the interface, the voltages and the duty. The LCD controller and driver IC guide lists the common families.
Browse the COB LCD module, dot-matrix COG and segment COG references, or read the COG LCD display overview.
Video: COG vs COB LCD Modules
This SuccessLCD explainer compares where the driver sits in COG and COB construction, the board around the glass, what moves onto your PCB with COG, the mechanical weak points and how to choose, with real SuccessLCD COG samples.
COG vs COB FAQs
Is a COG LCD cheaper than a COB LCD module?
In volume a COG display usually has the lower part cost, because it has no PCB, bezel or header of its own. Some of that cost moves to your design: the connector, the booster capacitors and the mechanical support. For small quantities or a fast prototype, a standard COB module can still be the cheaper project overall.
Can a COG display replace a COB module without changing my PCB?
Rarely. The COG display connects by FPC instead of a pin header, needs its capacitors near the connector, and often uses a different logic voltage and interface. Plan a PCB revision and enclosure changes when you switch.
Why does my COG display reset or show random pixels when the product is touched?
The most common causes are ESD or noise coupling into the reset line, a weak ground through the FPC, or a long unshielded FPC run. Keep the reset line filtered, ground the FPC well, and give the discharge a path to chassis ground that does not cross the display connection.
Deciding between COG and COB? Send the viewing area, the space behind the display, the controller or interface you want, the expected volume and the environment. We will suggest a construction and the matching references. Ask for a construction review, or browse monochrome LCD modules. For a custom construction, see custom LCD modules.