1. Cross-section of LCD panel
2. LCD MODULE ASSEMBLY DEFINITIONS
 
2-1. LCD Interconnection
This section primarily pertains to the interconnection methods of Liquid Crystal Display Glass to the PCB. These methods are used in standard LCD applications, as well as LCD Module assembly processes. We usually use three primary methods.
 
  a. Zebra connector    
 

Zebra connector is used silicon strips of alternating conductors and insulators. These materials are generally soft and compliant and can be easily compressed between the Liquid Crystal Display and PCB. Zebra connector requires a bezel to squeeze the display and PCB together. This method will yield a higher conductor interconnection than pins, potentially less costly than pins, but requires a specialized compression bezel.

 


  b. Pins    
   

Pins are attached to the display to allow the user to either mount the display in a socket or solder it directly into a PCB. From an end user standpoint, pins are the easiest to use since there is no requirement for a compression bezel or expensive heat seal bonding equipment. The pins are attached to the glass with a structural epoxy on the back. On the top, we apply an electrically conductive epoxy with an RTV overcoating.

Pins are the most reliable connection method, they are also the easiest to deal with for prototyping and smaller production runs. However, of all three methods, they have the lowest number of interconnects per inch

 

  c. Heat seal    
   Heat seals are similar to flexible circuit boards with the difference being that the interface tabs are made of a conductive hot melt adhesive. Generally, particles such as carbon, gold, or silver are added to the adhesive to make it conductive.

The pads of the heat seal are aligned with the pads of the display and a hot bar is brought down under pressure and the conductive adhesive is melted and bonded to the display. The adhesive is allowed to cool and an electrical bond is made with the display.

This method is the most cost effective for the higher volume applications, but due to the expensive setup and equipment required in this process, Heat Seals are typically not used for lower volume / low interconnection density requirements

 


2-2. Back light
LCDs create their display with the manipulation if visible ambient light. In the absence of this light, we must add backlighting to make these LCD displays visible. There are many choices to consider when backlighting an LCD. Once again the choice comes down to appearance vs. cost vs. features. Each approach has its advantages and disadvantages, and no one method is right for all applications.

The data below will only give the highlights of each technology with general comments. For our LCD modules we integrate most of these types of backlights into our displays.

We usually use three four type back light. Below are their information.

  a. LED back light  
 

LED back light is the most popular backlighting for small and medium LCDs. The advantages of LED back light are its low cost, long life, immunity to vibration, low operational voltage, and precise control over its intensity. The main drawback is it does require more power that most of the other methods, and this is a major drawback if the LCD size is large enough.

LED backlights come in a variety of colors: yellow-green, green, red, blue, amber, etc. And yellow-green being the most common, and now white is becoming cost effective and very popular.

LED backlights offer a longer operating life - 50,000 hours minimum - and are brighter than EL. Being a solid state device, they are configured to operate with typically a +4.2VDC power, so they do not require an inverter.

The LED backlight has two basic configurations; Array and edge lit. In both types the LEDs are the light source that are focused into a diffuser that distributes the light evenly behind the viewing area. In Array lit configuration there are many LEDs mounted uniformly behind the display, it offers more uniform and brighter lighting and consumes more power. In Edge lit configuration, the LEDs are mounted to on side (typically the top) focused edge on into the diffuser, it offers a thinner package and consumes less power.

 
   

  b. EL back light  
 

EL (Electroluminescence) is a solid state phenomenon which uses colored phosphors, not heat, to generate light.

EL backlights are very thin, lightweight and provide an even light. They are available in a variety of colors, with white being the most popular for use with LCD.

While their power consumption is fairly low, they require voltages of 100 VAC @ 400Hz. This is supplied by an inverter that converts a 5, 12 or 24 VDC input to the AC output. EL also has a limited life of 3,000 to 5,000 hours to half brightness.

The biggest drawback to an EL panel is that it requires an inverter to generate the 100VAC, consistent brightness, and limited life.

 
   

  c. CCFL Backlighting  
 

CCFL (Cold Cathode Fluorescent Lamp) backlights offer low power consumption and a very bright white light. The primary CCFL configuration used in LCD backlighting is edge lighting.

A cold cathode fluorescent lamp is the light source with a diffuser distributing the light evenly across the viewing area. CCFL require an inverter to supply the 270 to 300 VAC @ 35KHz used by the CCFL tube.

They are used primarily in graphic LCD and have a longer life (10,000 to 20,000 hours) than EL back light do. Their biggest drawbacks are: cold weather will reduce the light output by as much as 60% (see graph below), they require an inverter to generate the 350VAC (please note that the inverters do not function well at low temperatures), the light intensity cannot be varied (it is either on or off), and vibration can reduce the life expectancy of up to 50%.