A-Z knowledge about tablet PC capacitive touchscreen: an in-depth analysis for touch panel variations of G+G, G+F, G+P and concave touchscreens

 

This report consists of the following 3 parts:

1. How does projected capactivie touchscreen work ?

2. Stack / Structure of Projected capactive touchscreen

3. What’s the difference bewteen G+G, G+F, G+P and concave touchscreens ?

 

How does projected capactivie touchscreen work ?

                                                Figure 1: How capacitive touchscreen works

The projected capacitive touch screen works on the principle of conventional capacitive touch screens with the addition of two sensing arrays (X and Y as shown in Figure 6) on two different layers that are perpendicular to each other along with a drive line. As X and Y arrays exist on different layers, the point where the two intercept becomes a capacitive node. When electrical currents run along one of the arrays through the drive line, the other array and the circuitry that detects change in capacitive values becomes connected. During actual operation, the controller sends electrical currents to the drive lines on different layers to create a specific electrical field with the nodes.
 
                                                   Figure 2: work flow of touch action of capacitive touchscreen

Since human body is a conductive organ, When the user places his finger or uses a touch medium to the touch screen, coupled capacitor is thus formed, the controller immediately picks up the change in capacitive value between the nodes and drive lines to pinpoint the exact location of the touch event.

Now we know what’s the projective capacitive touchscreen ? However, we need to dig deep to learn more about the structure and function process of every component that makes up of capacitive touchscreen.

Stack / Structure of Projected capactive touchscreen:
 
                                                      Figure 3: Structure of projected capacitive touchscreen

Generally, projected capacitive touchscreen consists of cover lens and sensor, but they may be of different constructions due to different requirements either on application or cost, the following chart shows the comparison of different contructions:

 

Structure	Characteristic	Transmissibility
G/G	High reliability	>90%
F/F/Lens	More thin	>90%
G/Lens	The same strucuture to iPhone	>90%
F(G) / Lens (Single ITO layer )	More thin, lower cost	>90%

 

 

Lens Materials	Thickness Spec. Recommended
Harden Glass ( Tempered glass )	0.5mm,  0.6mm,  0.7mm,  0.8mm,  1.0mm
PMMA / PC	0.65mm, 0.8mm
PET	0.188, 0.125mm

                                                                                 Figure 4: Different construction

Let’s look into G/G type of projected capacitive touchscreen, we need to know the ITO sensor construction.

 
                                                                 Figure 5: ITO sensor construction, source: 3M

 
                                                          Figure 6: Structure of sensor layer and electrode pattern

 
                                                                           Figure 7: Mutual Capacitance Rows and Columns

 
                                                           Figure 8: capacitive sensing

Sensor grid in diamond patterns are printed on the ITO sensor. As shown above.

Mutual capacitance is the intentional or unintentional capacitance between two "charge holding objects." Projected capacitance touchscreens intentionally create mutual capacitance between ele¬ments of columns and rows [see figure 8] in the vicinity where each intersect the other. This allows the system electronics to measure each node (intersection) individually to detect multiple touches on the screen during one screen scan.
When a finger touches near an intersection, some of the mutual capacitance between the row and column is coupled to the finger which reduce the capacitance at the intersection as measured by the consystem electronics. This reduced capacitance crosses the "touch threshold" set by the electronics indicating a touch has occurred.

What’s the difference bewteen G+G, G+F, G+P and concave touchscreens ?

Firstly, we need to know the first letter refers to sensor part,  while the latter refers to lens(cover).

Then, from the meaning of the words, we know G+G means glass type sensor part and glass type of lens (cover), for most of tablet PC now, glass type sensor is made with SITO, means two ITO sensor layers are housed in one single side of the sensor glass with an isolation layer in between.

G+F type of capacitive touchscreen employs single ITO layer glass sensor and printed film as lens( cover), likewise, G+P uses PET as lens (cover).

Concave touchscreen as one type of G+F, is the most popular low-cost solution due as it only needs a printed film on the sensor glass, in this way, the defective rate of this type of touchscreen is very high( no need other process with lens )

  

Figure 9: G+G capacitive TP example A7310                             Figure 10: concave touchscreen example:A7313