By: Adam Larkin, Advanced Engineer – AEG, Future Electronics
When choosing the display type for your product, there are several variables to consider. Typically the selection is dictated by the content being displayed and the cost of the end product. Many other factors should be considered when selecting a display, including product availability, power consumption, ability to customize, the appearance or perception of your product, brightness, viewing angle, and product lifetime.
Mainstream users have become accustomed to beautiful color displays with an intuitive touch interface as the de facto standard. The appearance of a color display sets products apart from their counterparts using old monochrome technology.
The two color mainstream LCD technologies dominating the industrial marketplace are TFT and OLED.
TFT vs. OLED
There is no better way to compare technologies than a side-by-side comparison.
TFTs (thin-film transistors) have been available for many years and dominate the industrial color display market. Although this is the most popular color technology in use today, OLEDs (organic light-emitting diodes) are on the rise with new advancements every year.
TFTs and OLEDs can be seen in applications such as smartphones, laptops, tablets, and automobiles. Both technologies are capable of display- ing video and high-res photos.
Below are questions to ask yourself when choosing the display for your product.
What Temperature Range Do I Need the Display to Work in?
OLEDs operate at temperatures as low as -40oC (-40oF) and as high as +85oC (+185oF).
TFTs are not as strong below -20oC (-4oF) and above +70oC (+158oF); there are some manufacturers supporting as wide as -40oC to +85oC, but these products will come at a premium.
Which Technology Offers the Largest Variety of Sizes?
TFTs offer many more available sizes than the competing OLED technology. When choosing the size of your display, keep in mind two major factors: the popularity of the size and the number of suppliers that manufacture that size. Popular displays are LCDs that are manufactured in large quantities. The greater the popularity of the display, the less likely it will be discontinued in the near future, the lower the cost, and the easier to secure a second source.
Which Display Offers the Longest Life?
TFTs will typically have a longer life. Display lifetime is defined in hours of use before a display becomes half-as-bright as when it was first turned on. Products and applications have varying requirements for display ON time; seldom is a display on 24/7. Many portable products make use of the display for only a few hours a day, and many of these devices will be obsolete before the display reaches over 10,000 hours of use. It’s very important to evaluate the particular use-case of your product.
Which Offers the Faster Response Time?
Winner: OLEDs. OLED response times are estimated to be less than 0.01 milliseconds (mS). TFT response time is in the neighborhood of 15mS. That’s milliseconds. The human eye blinks every 300 to 400mS. There are 86,400,000mS in one day. The eye can barely consciously detect a difference of 15mS but the brain can tell there’s something up.
Which Technology is the “Best”?
OLEDs provide wider viewing angles than even the best TFT display.
OLEDs have a greater color saturation and color accuracy.
TFTs will typically have a sharper image, due to the more pixels per square inch.
OLEDs will use less power than their TFT counterpart, due to the need for a backlight with a TFT display.
OLEDs are thinner, and advancements in the technology have even brought flexible displays to the market.
Currently TFTs will, on average, cost less than a comparable OLED product.
Touch Panel Integration
The most common integration request we receive is to overlay a touch panel on top of an LCD module.
The Future Display Solutions team will help you select the right touch technology for your application. Four/five/eight-wire resistive, resistive multi-touch, surface capacitive, projected capacitive, surface acoustic waves (SAW), infrared (IR), acoustic pulse recognition (APR) and metal mesh are just some of the current touch technologies we offer and integrate.
Key factors in determining which touch technology to use are: the application environment, where the product will be used, durability, and whether it needs to work with a stylus or a glove.
Once the LCD and touch panel have been selected, we then narrow down the type of bonding required. The type of bonding required is determined by the mechanical, optical characteristics and how rugged the applications needs to be. We can recommend whether perimeter airgap bonding, optical bonding or any additional gasket or film is required.
The cover glass is normally adhered to the LCD glass by either tape, edge, optical or dry bonding.
Dry bonding uses a pressure sensitive adhesive layer (PSA) instead of a liquid or gel to adhere the layers of the display. By using a special process of either applying a pressure or vacuum to the PSA layers, they bond together. The advantages of dry bonding include:
• It is excellent for strengthening.
• There is less waste as the process does not use liquids and cleaning solvents are not required.
• The process is much faster to produce with no residuals to clean up; the tooling is ready for the next display.
• It can be scaled easily for any size of display.
Tape Bonding or Edge Bonding
In Tape or Edge bonding, the glass layers are bonded together only at the edges. This is generally a faster, lower cost option. Bonding at the edges creates a small air gap between the cover glass and the LCD glass. Depending on the environment, it is possible that moisture can enter between the layers, affecting the LCD image. This is known as fogging, for example in a marine environment. As there is no central support, this type of bonding might not be a strong as optically bonding the glass. Due to the air gap, ambient light may reflect off the internal surfaces, resulting in as much as 13.5% reflectance or a loss of luminance. Very high bond tape (VHB) is normally used for edge bonding and is available from our integrators.
Optical bonding of glass is when a special liquid adhesive is used for bonding the cover glass to the touch panel or the cover glass to the LCD glass. For bonding the entire glass layers, optically clear adhesive (OCA) is normally used.
This method seals the complete layer stackup and has no air gap. A main function of optically bonding glass is that it optically couples the glass, known as index matching. Compared to an edge bonded display, optical bonding also decreases the degradation of internal reflected light and improves the contrast ratio.
As well as the optical performance, the strength and longevity are also improved, as dust or moisture cannot enter between the layers. With as little as a 1mm increase in module thickness, a significant increase in impact strength of the LCD may be obtained.
Considerations When Sourcing Display Technologies
A quick reference for an OEM when deciding where to source their display; you have to decide what is most important to your customer (Table 1).
|Tier 1||Tier 2||Tier 3|
|Cost Driven||Willing to Pay for Quality||Good Quality and Good Price||Cheap and Quick*|
|Spec Driven||Size, Brightness, Resolution, Viewing Angle, Technology*||Open to some options in range||Best, Biggest, and Brightest for <$20...|
|Customization||Yes, Yes, Yes; Tooling/NRE Required*||Minimal||No Tooling/NRE|
|Product Uniqueness||Great Industrial Design*||Component Level (LCD + Touch)||I'll have what they're having...|
|Brand Name||Quality Suppliers*||-||Who?|
|Quality Expectation||Medical/Auto/ Industrial; High*||Industrial/Commercial; Medium to High||Commercial/Consumer; Low to Medium|
|Volume/EAU||500 - 10k EAU, $500+||1k - 10k EAU, Medium ASP/$200 - $500||1k - 100k EAU, Low ASP/<$200*|
|Product Lifecycle||5+ yr*||3-5 yr||1-3 yr/DNC|
|Level of Integration||Highly Integrated, Extreme Value-Added*||Semi-Custom||COTS|
|Reliability||High Quality, Proven Track Record, Accountable*||High||NC/NR|
|Warranty||Warranty Required, Service*||Yes, Standard||Warranty Doesn’t Matter|
|Lead Time||Long Lead, Customized, Engineered Product||-||Lots of Product - FAST*|
|Market||Medical, Military, Aerospace, Marine*||Industrial, Automotive, Gaming, Retail, Transportation, ATM||Entertainment, Retail, Kiosk, Vending, Wearables|
Table 1: Considerations When Sourcing Display Technologies
*Perceived value by OEM.