How to mount a 3.18 inch 128x64 COG LCD on a PCB?

How to Mount a 3.18 Inch 128x64 COG LCD on a PCB

To mount a 3.18 inch 128x64 cog lcd display on a PCB, you need to align the display’s 18-pin flex tail with the PCB’s matching pad pattern, then apply heat and pressure using a hot bar soldering system or a precise soldering iron with a temperature-controlled tip set between 180°C and 220°C. The COG (Chip-on-Glass) design means the driver IC is bonded directly to the glass, so you must avoid excessive force or heat that could crack the substrate. Start by securing the PCB in a vise or fixture, clean the pads with isopropyl alcohol, and pre-tin them with a thin layer of solder. Place the display’s flex tail over the pads, ensuring the alignment marks on both sides match within 0.1 mm tolerance. Use a hot bar press with a silicone rubber tip, applying 2 to 4 Newtons of force per pad for 3 to 5 seconds. After soldering, inspect the joints under a microscope for shorts or cold joints; the typical pitch is 0.5 mm, so even a slight misalignment can cause bridging. For prototypes, you can use a fine-tip soldering iron with a 0.2 mm conical tip, but this requires steady hands and a flux pen to prevent solder bridges. The display’s operating temperature range is -20°C to +70°C, so the soldering process must stay within that limit to avoid thermal shock to the glass. The 3.18 inch 128x64 cog lcd display uses a ST7565R or similar controller, which communicates via SPI, so you also need to route the 6 signal lines (CS, A0, RST, SCK, MOSI, LEDA) to your microcontroller with proper impedance matching—keep trace lengths under 50 mm to reduce noise. The display’s backlight draws 20 mA typical at 3.3V, so include a 100-ohm series resistor to limit current, and add a 10 µF capacitor near the power pin to filter ripple. The overall thickness of the COG assembly is 1.2 mm, so you must account for this in your enclosure design, ensuring no pressure is applied to the glass surface. For production, use a stencil to apply solder paste, then reflow at 200°C peak for 10 seconds, with a ramp rate of 2°C per second to prevent glass cracking. The flex tail’s material is polyimide, which can withstand 260°C for brief periods, but prolonged exposure above 240°C will degrade it. The display’s viewing angle is 6 o’clock, meaning the best contrast is when looking from below, so mount it so the user’s eyes are slightly below the display plane. The pinout on the flex tail is: 1: VSS, 2: VDD, 3: A0, 4: RST, 5: CS, 6: SCK, 7: MOSI, 8: LEDA, 9: LEDK, 10: NC, 11: NC, 12: NC, 13: NC, 14: NC, 15: NC, 16: NC, 17: NC, 18: NC. The NC pins are for future use or can be left floating, but grounding them can reduce noise in high-EMI environments. The display’s active area is 73.4 mm x 38.8 mm, with a dot pitch of 0.57 mm, so the pixel density is 44 PPI, which is fine for text but not for graphics with fine details. The contrast ratio is typically 10:1, and the response time is 150 ms at 25°C, so it’s not suitable for fast-moving images. The COG process eliminates the need for a separate PCB for the driver, but it makes the display more fragile—bending the flex tail more than 10 degrees from flat can crack the glass. To mount it securely, use double-sided adhesive tape (3M 467MP or similar) on the back of the display, but avoid covering the driver IC area, which is 10 mm x 6 mm and protrudes 0.3 mm from the glass. The tape should be 0.1 mm thick to maintain flatness, and you should apply it in a cleanroom environment to prevent dust from creating bubbles. The display’s weight is 8 grams, so the tape must hold at least 20 grams of shear force to withstand vibration. For soldering, use lead-free solder (SAC305) with a flux core, but if you use leaded solder (63/37), the lower melting point (183°C) reduces thermal stress. The flex tail’s gold-plated pads are 0.3 mm x 0.5 mm, with a thickness of 0.1 µm, so they are delicate—scratching them with a probe will cause open circuits. The display’s SPI clock speed is up to 10 MHz, so you need to keep the SCK trace short and avoid running it near high-current lines like the backlight LEDA. The backlight is a white LED edge-lit type, with a brightness of 200 cd/m² typical, but it can be dimmed via PWM at 1 kHz to avoid flicker. The display’s power consumption is 0.8 mA typical with the backlight off, and 20 mA with it on at full brightness, so it’s suitable for battery-powered devices if you use a sleep mode. The driver IC supports inverse display, vertical scrolling, and partial display updates, which can reduce power by updating only changed pixels. The display’s contrast is adjusted via a software command to the internal voltage regulator, which outputs a negative voltage from -5V to -10V relative to VDD, so you don’t need an external negative supply. The mounting process must consider the display’s operating humidity range of 10% to 90% non-condensing, so if you’re using it in a humid environment, you need a conformal coating on the PCB, but avoid coating the flex tail or the glass. The display’s storage temperature is -30°C to +80°C, so it can survive shipping in most climates, but rapid temperature changes can cause condensation, which will short the pads. To prevent this, include a desiccant pack in the enclosure if the device is used in high-humidity areas. The display’s viewing direction is 6 o’clock, but you can rotate it 180 degrees via software command, which changes the scan direction, but this reduces the contrast by about 10% due to the liquid crystal alignment. The pixel matrix is 128 columns by 64 rows, with each pixel being 0.54 mm x 0.54 mm, so the total active area is 69.12 mm x 34.56 mm, but the bezel adds 2.2 mm on each side, making the total glass size 77.5 mm x 43.0 mm. The flex tail extends 20 mm from the bottom of the glass, so you need to leave that much space on the PCB for the tail to bend without stress. The tail’s bend radius should be at least 1 mm, but 3 mm is safer to prevent the copper traces from cracking. The display’s connector is a ZIF socket if you use a separate board, but for direct mounting, you solder the tail directly to the PCB, which saves space but makes replacement impossible. The typical failure mode is a cracked glass due to uneven pressure during mounting, so use a soft silicone pad between the clamp and the glass when pressing the tail. The display’s driver IC has a built-in oscillator that runs at 1.5 MHz typical, but you can synchronize it with an external clock if needed, though this is not common. The SPI interface requires four wires: CS, A0, SCK, and MOSI, plus two for power and two for backlight, totaling eight connections. The A0 pin (also called D/C) selects between command and data mode, so you must set it low for commands and high for data. The RST pin is active low, and you need to hold it low for at least 1 µs after power-up to reset the driver. The CS pin is active low, and you can share it with other SPI devices if you use a separate chip select line. The display’s maximum SPI clock frequency is 10 MHz, but for reliable operation, use 4 MHz to avoid signal integrity issues if the traces are longer than 30 mm. The display’s power supply is 3.3V typical, but it can operate from 2.8V to 5.5V, though the contrast will vary with voltage. If you use 5V, you need to level-shift the SPI signals to 3.3V, as the driver IC is not 5V tolerant on the logic pins. The backlight LED has a forward voltage of 3.2V typical, so at 5V, you need a 90-ohm resistor to limit current to 20 mA. The display’s built-in voltage regulator generates the negative voltage for the LCD drive, which requires a 1 µF capacitor between VOUT and VSS, and a 1 µF capacitor between VDD and VSS, placed as close to the display as possible. The display’s contrast is set by writing a command byte 0x81 followed by a value from 0x00 to 0x3F, where 0x20 is the default. The typical value for good contrast at 3.3V is 0x28, but you may need to adjust it based on the viewing angle and temperature. The display’s temperature compensation is built-in, but it only works within the operating range, so if you use it outside that range, the contrast will drift. The display’s response time increases at low temperatures, so at 0°C, the response time is 300 ms, which makes it unusable for fast updates. The display’s pixel refresh rate is 60 Hz, but you can update individual pixels via the SPI bus, which takes about 1 ms per byte. The entire screen of 128x64 pixels requires 1024 bytes, so at 4 MHz SPI, it takes about 2 ms to refresh the whole display, which is fast enough for text and simple graphics. The display’s memory is organized as 8 pages of 128 bytes each, where each page is 8 rows tall. To update a pixel, you set the page address, column address, then write the data byte. The display supports hardware vertical scrolling with a scroll range of 0 to 64 rows, which is useful for moving text without rewriting the entire buffer. The display’s built-in charge pump generates the negative voltage, so you don’t need an external DC-DC converter, but the charge pump can cause noise on the power line, so add a 10 µF capacitor and a 100 nF capacitor in parallel near the display’s power pins. The display’s ground plane should be solid under the flex tail to reduce EMI, and the SPI traces should be routed on the same layer with a ground return path. The display’s mounting holes are not present on the glass, so you must use the PCB or an enclosure to hold it in place. The typical method is to use a plastic bezel that clamps the glass edges, with a foam gasket to distribute pressure. The display’s thickness is 1.2 mm, so the bezel must have a recess of 1.5 mm to allow for the adhesive tape. The display’s glass is 0.55 mm thick, and the polarizer adds 0.2 mm, so it’s fragile—dropping the device from 1 meter can crack it. The display’s recommended storage is in an anti-static bag, as the driver IC is ESD-sensitive. The flex tail’s gold fingers are ESD-sensitive too, so handle them with grounded tweezers. The display’s soldering process must be done in an ESD-safe workstation, with a grounded iron and a mat. The display’s typical lifespan is 50,000 hours at 25°C, but it drops to 20,000 hours at 60°C due to the liquid crystal degradation. The backlight LED has a lifespan of 100,000 hours, so the LCD is the limiting factor. The display’s viewing angle is 6 o’clock, but the contrast is best at a 30-degree angle from the normal, so design the enclosure so the user’s line of sight is at that angle. The display’s reflectivity is 5% due to the polarizer, so in bright sunlight, you may need a higher backlight brightness or a transflective polarizer, but this display is transmissive only. The display’s pinout is standard for 128x64 COG displays, but always verify with the datasheet, as some manufacturers swap the A0 and RST pins. The display’s driver IC is the ST7565R, which is compatible with the popular u8g2 library for Arduino, so you can use example code to test it. The display’s SPI mode is mode 0 (CPOL=0, CPHA=0), meaning the data is clocked on the rising edge of SCK. The display’s initialization sequence is: power up, wait 10 ms, set RST low for 1 µs, then set it high, wait 10 ms, then send commands to set the bias, contrast, and display on. The typical bias is 1/9 duty, which is set by command 0xA2. The display’s segment direction is set by command 0xA0 for normal, or 0xA1 for reverse. The display’s common direction is set by command 0xC0 for normal, or 0xC8 for reverse. The display’s power control is set by command 0x2F to enable the internal voltage regulator and charge pump. The display’s display on command is 0xAF. The display’s sleep mode is entered by command 0xAE, which reduces power to 0.1 µA. The display’s backlight can be controlled by a separate transistor if you want to dim it via PWM, but the simplest method is to use a resistor. The display’s mounting on a PCB is a one-time process, as the flex tail cannot be re-soldered more than three times without damaging the pads. The display’s typical application is in handheld meters, medical devices, or industrial controls where low power and simple graphics are needed. The display’s data sheet provides a recommended footprint for the PCB pads, which is a 0.5 mm pitch with 0.3 mm pad width and 1.5 mm pad length. The pads should be gold-plated to match the flex tail’s gold fingers, and the solder mask should be removed from the pads. The display’s alignment marks are two small circles on the glass and two on the PCB, and they should be aligned within 0.1 mm using a microscope or a camera system. The display’s soldering temperature profile for a hot bar system is: preheat at 150°C for 30 seconds, then ramp to 200°C at 2°C per second, hold for 5 seconds, then cool at 4°C per second. The display’s soldering iron temperature should be set to 200°C for leaded solder, and 220°C for lead-free, but never exceed 240°C. The display’s flux should be no-clean type to avoid residue that can cause corrosion. The display’s cleaning after soldering is not recommended, as solvents can damage the polarizer. The display’s inspection after mounting should check for solder bridges using a multimeter on continuity mode, and for open circuits by measuring the resistance between the flex tail pads and the PCB pads. The display’s typical resistance per pad is less than 1 ohm after soldering. The display’s capacitance between power and ground should be around 10 µF due to the decoupling capacitors. The display’s current draw during operation should be 0.8 mA with backlight off, and 20 mA with backlight on at full brightness. The display’s contrast should be set to a value that gives a clear distinction between on and off pixels, typically a ratio of 10:1. The display’s viewing angle test should be done by rotating the display in front of a light source, and the contrast should remain consistent within the specified angle. The display’s temperature test should be done in a thermal chamber, and the contrast should be adjusted via software if it drifts. The display’s vibration test should be done at 10 Hz to 500 Hz with 1 g acceleration, and the display should not show any flickering or pixel dropout. The display’s drop test should be done from 1 meter onto a concrete floor, and the display should not crack or delaminate. The display’s ESD test should be done at 2 kV contact discharge, and the display should not reset or show artifacts. The display’s lifetime test should be done at 60°C and 90% humidity for 1000 hours, and the display should not show any significant degradation in contrast or brightness. The display’s storage test should be done at -20°C for 24 hours, then at 80°C for 24 hours, and the display should not show any physical damage. The display’s mounting process is critical for reliability, and any deviation from the recommended parameters can cause failures. The display’s typical failure modes are: cracked glass due to uneven pressure, solder bridges due to excess solder, open circuits due to insufficient heat, and ESD damage due to improper handling. The display’s repair is not possible if the glass is cracked, but if a solder joint is bad, you can reflow it with a hot air gun at 200°C for 5 seconds. The display’s replacement requires desoldering the flex tail, which is difficult because the polyimide material can melt. The display’s cost is typically $5 to $10 per unit in small quantities, but the mounting cost adds $1 to $2 per unit for manual soldering. The display’s mounting in production is done with a hot bar machine that costs $10,000 to $20,000, but for prototypes, you can use a soldering iron and a steady hand. The display’s mounting yield is typically 95% for manual soldering, and 99% for machine soldering. The display’s mounting time is 2 minutes per unit for manual soldering, and 10 seconds per unit for machine soldering. The display’s mounting process should be documented in a work instruction that includes the alignment, soldering, and inspection steps. The display’s mounting tools include: a hot bar machine or soldering iron