April 10, 2019
How it's Made: iPhone Displays
How it’s Made: iPhone Display
Displays are the most common component to fail on an phone. They are the cutting edge of technology, the gateway to our world, and very fragile. To the layman it is one unit, to the technician it is many, and to the manufacturer it is hundreds. We are going to break some of these components down and learn more about what they do!
The first layer is the cover lens. A clear piece of glass that allows light, and electrical impulses through while protecting our expensive LCD panels underneath. It starts as a single block of glass the size of a container. Often made in Germany or in the United states these are shipped by ocean liner to cutting facilities in China where they are cut into large sheets and then stamped into their respective sizes. After they are sized properly the coating and polishing processes are started. what we are left with is the corner stone of our repair industry.
For Corning glass specifically it undergoes some proprietary chemical treatments to strengthen the glass. One of these commonly known as a ION Exchange bath uses a molten alkaline potassium salt.
You can learn more about this by visiting <http://www.corning.com/gorillaglass/worldwide/en/technology/how-it-s-made.html>
OCA is the adhesive used to bond the cover glass to the LCD panel. Optical bonding (vs No bond like the 3GS) improves the contrast ratio by reducing the amount of reflected light, thus improving view-ability It also improves the durability of the cover lens.
“Light is made of particles called photons, which travel through space like a wave, zig-zagging back and forth on their way to your eye. Natural, unpolarized light
Polarizing filters absorb light waves that vibrate along a certain axis. In the case of polarizing sunglasses, they absorb the horizontally-vibrating waves. That means that only vertically-vibrating waves get through the filter and reach your eye. This reduces the intensity of reflective glare without blocking too much of the surrounding light.” - Popular Science Magazine
Your polarizer on your phone works in a very similar way. Up until iPhone 6 this was a major issue. Newer models of iPhone starting with the 6 use a special “quarter plate” To send the light in a spiral like pattern eliminating issues with sunglasses. Those wave patterns look like this.
This is an iPhone 6 FPC Cable. FPC (flexible printed Cable) Connects our display to the logic board. It allows data to be sent and received between the two. It is often where any Display IC are housed. It’s important for this piece to be made of highly conductive material that is well insulated.
FFC/FPC: is made up of thin rectangular copper conductors laminated between two layers of polyester insulation. These copper conductors are left uncovered on each end and then tin plated to make electrical contact with the connector. A stiffener is bonded to the cable end which provides mechanical stability for the exposed copper conductors during mating and un-mating. This type of cable is used when a straight one-to-one connection is required.FPC: Flexible Printed Circuits (FPC) are similar in construction to the FFC except that copper film is chemically etched to produce a specific pattern. While these circuits are custom made for each application.
The backlight essentially takes light from LEDs located at the top of the assembly and distributes and filters that light so that it passes through evenly. This is largely done via the diffuser which is a layer of plastic with unevenly spaced bumps. These bumps change in density and are based on a diffusion equation. Some light passes through and the remaining light bounces back towards a reflective surface. If you have ever torn a back light apart this is the shiny piece. The advanced backlights in our iPhones are a bit more complex than this especially after the addition of force touch but this is the general functionality.
Frames and Force Touch:
We are going to combine the last two pieces together. They are the frame and force touch.
The frame is traditionally installed to the glass before the LCD is laminated and is used to attach the display assembly to the phone housing. It’s composed of metal and plastic. The Force touch is the most useless piece of over hyped hardware ever added to an iPhone.
Now that is all out of the way or you just skipped down because you hate to read; we are going to talk about the assembly process, quality, and why material composition is so important to long term quality.
Everything starts with your working environment. In order to ensure that there is not static on any of the components or contamination between layers bonding needs to be ISO 100 - ISO 10 rated room. Usually to do that the factories will have ESD mat floors similar to the ones used on work benches. Antistatic walls and a ceiling with air filtration vents (these units are used to both condition and filter the air). Because of the machines inside the clean room tend to get heated it is important to have cool fresh air blowing in. It is also important to have positive pressure and a high humidity. The humidity does two things; it actually reduces static and causes particles in the air to be heavier as they absorb moisture from the air. This increased humidity combined with positive air pressure causes the particles to fall to the ground.
Once all the individual components have passed initial QC then production would start; usually with the LCD panel receiving a layer of ACF tape. I put that in bold because it is probably one of the most critical and most often overlooked components in the display assembly. It's the weak link in most assemblies and one of the many factors that can lead to display malfunction. ACF tape stands for adhesive conductive film. Its purpose is to form a bond between the LCD panel and the FPC. Depending on materials, bonding temperature etc. it can have varying quality of bond. As a display goes through heat cycles poor ACF tape will break down or have a diminished connection.
Once the display has been bonded with a layer of ACF tape it's moved down the assembly line where microscopes and moulds line up the FPC and panel. Normally a good factory will stop here to confirm a good bond has been made by conducting an initial LCD inspection. It is then laminated with a polarizer if it was not done prior. It's important that the polarizer is earthed/grounded to prevent static build up on the panel over time. This is where things tend to fork out depending on which factory you are in. Some companies laminate the LCD with OCA and others Laminate the glass with OCA. The preferred method among our factories is to laminate to the cover lens. We then stage the assembly to have the backlight installed. The other area of variance is whether or not the frame is preinstalled to the cover lens. Once the cover lens has been initially set to the LCD and undergone an initial vacuum and press it will move to a much larger vacuum chamber to remove any bubbles from the adhesives and to form the final bond between the cover lens and LCD assembly. Besides removing air from in-between the LCD and the cover lens things boil/melt at lower temperatures when the pressure is lowered. By forming a negative pressurized space the materials in the OCA can change states and form a lasting bond. This is where a final factory inspection would occur and where some companies would install their frame.
Flaws like missing pixel would be downgraded and rejected from our stock then sold to the market. In the image below you can see how a pixel RBG cell can fail. This on is failing to turn on at the Red and Green.