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Custom LCD Glass Design

Since 1970, Densitron has been providing clever solutions to it's customers in markets as diverse as industrial, automotive, telecommunications, medical and process control, to name a few. Utilising our expertise allows the engineer the maximum amount of time to concentrate on their key competancy safe in the knowledge that the user input/output interfacing is taken care of by a single, well qualified Design manufacturing partner. We set out to provide the designer with details on the many varied ways in which Densitron can utilise its expertise to your advantage. This could be as simple as integrating a few keyswitches to a standard module providing warranty and supplier reduction benefits, to a full customised display solution providing the exact mechanical, electrical and optimum characteristics required for any application.


LCD Glass-Introduction

This short guide explains the basic design criteria when considering a custom LCD panel for a particular application. It is important to understand and adhere to these guidelines in order to generate not only a feasible LCD design, but also a cost-effective engineering solution.

In order to assure that the application and technical requirements for the custom LCD are fully understood, the Custom LCD Design Form (Appendix 1 of this guide) should be completed and submitted to Densitron along with other relevant drawings and specifications. This information will then be reviewed by our factory engineers and a quotation will be generated.

The custom LCD design and production process involves the following steps:

1. Upon review and acceptance of the quotation for a custom LCD panel, the customer places a purchase order for the production quantity, pending approval ofprototypes.

2. A detailed drawing of the custom LCD panel is generated by Densitron and submitted to the customer for approval.

3. Upon approval of this drawing, the LCD artwork/tooling is generated and 10 prototypes are produced and delivered to the customer.

4. Upon approval of the prototypes, the production is carried out in accordance with the customer's purchase order.

Densitron has established minimum order quantity guidelines for custom LCD panels. In general, Densitron will only consider those applications with minimum production releases of 1000 pcs or more. Below this production level, yield rates are difficult to predict and such designs are not considered to be cost-effective for either the customer or Densitron.


General LCD Usage
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The use of LCD's has increased dramatically since their introduction about 25 years ago. Currently LCD's are used in a wide range of applications from watches, calculators and games to industrial and medical instrumentation to laptop computers.

One of the main advantages of LCD's over other display technologies is the relative ease in which the display content can be customised to satisfy the specific requirements of an application. in this way, the custom LCD panel can present specific user-interface information which will enhance the performance and value of a product.

Custom LCD panels can display a combination of numeric and alphanumeric digits, on/off indicators, messages, annunciators, graphic icons and symbols, bar graphs, pie charts, etc. The content of a custom LCD panel is limited only the imagination of the product designer and by the design guidelines explained in this brochure. Some examples of custom LCD panels are illustrated below.


LCD Glass-Design Criteria
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Several factors must be considered in the design of a custom LCD panel. As with most product designs, there will be cost/performance trade-offs which the customer must judge. The information presented below is intended to assist the customer in making these decisions. Because many of these design trade-offs are somewhat subtle and may affect cost, the customer should identify those factors which are critical as well as those which are flexible.

The following design information is needed by Densitron in order to prepare a quotation for a custom LCD panel:

1.Display Size
2.Display Content
3.Viewing Mode
4.Operating Environment
5.Drive Method
6.Connection Method
  
Each of these factors is discussed in detail below.


LCD Glass-Display Size
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The size of the LCD panel is the main factor affecting cost. An understanding of LCD panel construction will provide a better understanding of those factors which determine LCD panel size. The size of a custom LCD panel can be determined using the drawing in Appendix 1 and the basic dimensional design guidelines listed in Appendix 2.

An LCD consists of two plates of glass which are sealed together around their perimeter with a thin layer of liquid crystal (LC) fluid in between. Polarizer films are laminated to the outer surfaces of the glass plates. Electrical contacts are present on the inner surface of one of the glass plates, typically coming off both the top and bottom edges of the LCD panel.



The total LCD panel size is determined by adding together the desired viewing area, the seal width and the contact ledge (if any) dimensions. For the case of an LCD panel with contact ledges along both top and bottom edges:

LCD width = Viewing area + (2 x seal width)
LCD height = Viewing area + (2 x seal width) + (2 x contact ledge)

Densitron LCD panels are fabricated on a 300mm x 300mm substrate with a usable area of 290mm x 290mm. So, the most efficient use of the glass substrate is obtained when the LCD panel dimensions are exact integral multiples of 290mm. For example, 50 LCDs of size 58mm x 29mm can be fitted into the 290mm x 290mm usable area on the glass substrate, while only 36 LCD's of size 60mm x 30mm can fit.

Other design guidelines relating to display size are that the total number of LCD's on yhr 300mm x 300mm substrate should not exceed 200, and the total number of LCD's in any one direction on the substrate should not exceed 20.


LCD Glass-Display Content
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The digits, annunciators and other images which may be displayed on an LCD are created as overlapping areas of the conductive layers on the inner surfaces of the glass plates. These transparent conductive layers are patterned using a photo lithographic process to generate the desired display characters and segments.

The layout of the artwork which forms the conductive patterns on the front and rear glass plates is complicated by the need to avoid overlap in those regions where active segments are not desired. For many designs, the type of controller or driver IC to be used will dictate the best way to layout the LCD artwork, thus it is necessary to know this information when preparing the quotation.

Many different fonts and character styles are available for designing annunciators on the custom LCD panel. These should be specified by the customer at the time of quotation. The user should also refrain from specifying annunciator heights below 2.5mm since these become difficult to read.




LCD Glass-Viewing Mode
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Densitron custom LCD panels can be designed to operate in a variety of viewing modes to allow operation in any lighting condition, from direct sunlight to total darkness. The polarizers used and the LCD panel construction determine the viewing mode of the LCD.

There are three elements which contribute to the LCD viewing mode. These are the polarizer type, the display image and the preferred viewing direction. The customer needs to define these elements and indicate them on the Custom LCD Design Form. Below is a discussion of these elements.



LCD's have the ability either to reflect ambient light, to transmit a backlight, or to do both. These operating modes are determined by the type of polarizer film applied to the rear glass plate of the LCD panel. Reflective LCD's have a full reflector attached to the rear polarizer and cannot be backlit. They are often used in battery-powered application and provide the best contrast in high ambient light conditions. Transflective LCD's have a partially reflecting film attached to the rear polarizer and can either reflect ambient light or transmit EL, LED, fluorescent or incandescent backlights. They provide the most versatile viewing mode with only a slight reduction in contrast. Transmissive LCD's do not have any reflective properties and must be used with a backlight. Depending on the backlight brightness, they can be used in well-lit indoor environments as well as dark conditions, however, this mode is not recommended for daylight usage. Transmissive LCDs offer a different appearance for typical LCD's, bringing a light emitting look to the product.



LCD's can be constructed to operate in either a "positive image" mode with dark characters on a light background or "negative image" mode with light characters on a dark background. Reflective and transflective LCD's provide the best contrast and viewing angle as positive image displays, while transmissive LCD's exhibit best viewing characteristics as negative image displays. LCD viewing characteristics are the result of complicated electro-optic effects resulting from the twisting of the LC molecules inside the display. LCD's have a particular viewing direction in which the contrast is optimum. The user must determine whether, for his particular application, the custom LCD will be viewed primarily from above or below the LCD panel. This direction is referred to as the "preferred viewing direction" and specified as 12:00 (top view) or 6:00(bottom view).

Operating Environment
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Densitron custom LCD panels can be designed to operate in a variety of environmental conditions. Two grades of custom LCD's are available as standard options with the following performance specifications.

  Commercial Grade Industrial Grade
Operating Temperature Range  0 to +50C -20 to +70C 
Storage Temperature Range -10 to +60C -30 to +85C

In addition to these standard grades, other environmental requirements will be considered on a case-by-case basis.


LCD Glass-Drive Method
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All Densitron custom LCD panels operate by the field effect mode using either twisted nematic (TN) or supertwisted nematic (STN) LC fluids. Application of an electric field across the conductive patterns on the LCD glass surfaces causes the LC molecules to "untwist" and change the optical properties of the LCD in that region.

There are two types of drive methods for Densitron custom LCD panels:

The static drive method continually applies an Ac voltage cross each addressable segment in the LCD, either in a full "on" or full "off" state. This results in optimum contrast and viewing angle characteristics for the LCD. The disadvantage of the static drive method is that one LC driver circuit is required for each addressable segment.

The multiplex drive method was developed to reduce the number of driver circuits, the number of connections between the LCD and the driver PCB, and the cost when driving many display segments. In this approach, a display segment experiences a full "on" voltage for only a fraction of time, which is referred to as the duty cycle. This results in reduced contrast versus a static-driven LCD but allows for a greater number of segments in the same size display. Many commercially available CMOS LCD driver ICs support duty cycles of 1/2. 1/3 and 1/4 which are commonly used on Densitron custom LCD panels.

The other factor affecting display contrast and viewing angle is the operating voltage. This is the DC voltage which the driver IC converts into the driving waveforms for both static and multiplex operation. In general, a higher operating voltage will allow improved contrast to be achieved. Since the operating voltage, duty cycle, drive bias and temperature range combine to determine the selection of the LC fluid for a particular custom application, it is important that this information is provided to Densitron for quotation preparation.


LCD Glass-Connection Method
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Densitron custom LCD's can be connected to the user's PC board containing the driver IC(s) in two different ways, namely by elastomeric connectors or by pins on the LCD panel. The illustration below shows typical examples of these two connection methods.


The choice of connection method will depend on the pitch of the contact pads along the edge of the LCD panel. Pins can be used on LCD panels with contact pitches of 1.8, 2.9, and 2.54mm. Elastomeric connectors are suitable for all LCD panels with contact pitch of 0.4mm or greater.


Elastomeric Connection
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Elastomeric connectors are constructed with alternating layers of non-conductive and conductive material, usually silicone rubber and carbon-filled silicone rubber.
  

   
The following design guidelines should be observed to ensure reliable electrical connections when using elastomeric connectors with Densitron LCD panels:

1.Elastomeric connectors should be compressed by 5% to 15% of their relaxed height. This must be considered when designing the mounting bezel and choosing the appropriate elastomeric connector height.

2.The elastomeric connector width should be no greater than 85% of the LCD panel contact ledge dimension.

3.The pitch of the elastomeric connector should be at least 4 to 5 times finer than the LCD panel contact pitch to provide redundant electrical contacts between the LCD and the PC board.

4.There are several manufacturers of elastomeric connectors, each with several versions of connectors. Some trial-and-error experimentation is probably needed to determine the most appropriate type and dimensions for a particular application.

Elastomeric connectors have several advantages over pin connectors including: higher LCD yield, high contact density, smaller contact ledge (typically 2.9mm), lower cost per connection and solderless final assembly for easy rework and repair.

Disadvantages of elastomeric connectors versus pins include: difficulty in testing the LCD panel, reliability (if designed improperly), and a more complicated mounting arrangement.


Pin Connection
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Densitron custom LCD's are available with stamped metal pins attached to one or both edges of the LCD panel. A small drop of conductive epoxy is applied at each pin contact on the LCD and a bead of epoxy is applied on top of the pins along the LCD panel edge to ensure reliability and connection.

Pin pitches of 1.8, 2.0 and 2.54mm are available, although 2.54mm is the most common and least costly. The pin length can be specified up to a maximum of 25mm.


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