Capacitive-sense touchscreens have become widely available as a user interface component of user devices. In particular, touchscreens have become extremely popular in mobile phones, cellular phones, tablets, Personal Digital Assistants (PDAs), laptops, and other portable computing devices. Touchscreens are also widely employed in Point Of Sale (POS) terminals, inventory management systems, security systems, and the like.
One reason for the increased popularity of touchscreens and specifically capacitive-sense touchscreens in user devices is their ability to simultaneously act as a user input device and a user output device. More particularly, touchscreens enable a user to interact with the data that is presented to the user rather than interact with a set of separate buttons. This helps minimize the size of the user device and/or maximize the size of the output screen used to present information to the user. In other words, capacitive-sense touchscreens can be utilized as an intuitive method of obtaining input for user devices.
Two principal capacitive sensing and measurement technologies are currently employed in most capacitive touchscreen devices. The first such technology is that of self-capacitance. Many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do Integrated Circuit (IC) devices such as the CYPRESS PSOC™. Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. and U.S. Patent Publication Nos. 2010/0302201 to Ritter et al. and 2008/0297174 to Narasimhan et al., each of which are hereby incorporated herein by reference in their entirety.
Self-capacitance may be measured through the detection of the amount of charge accumulated on an object held at a given voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors employed in touchscreens are configured such that the grounded object is a finger. The human body is essentially a capacitor to a surface where the electrical field vanishes, and typically has a capacitance of around 100 pF.
Electrodes in self-capacitance touchscreens and/or touchpads are typically arranged in rows and columns. By scanning first rows and then columns the locations of individual disturbances induced by the presence of a finger, for example, can be determined.
Typically, rows and columns of electrodes in self-capacitance sensing devices such as touchscreens or touchpads comprise electrically conductive traces or strips of Indium Tin Oxide (ITO) laid down on a glass or plastic substrate. Although ITO is the material of choice in most capacitive-sense touchscreens, other known materials or compositions which are functionally equivalent to ITO may also be used.
During and after the process of forming such traces on a suitable substrate, defects in such traces or strips will arise, at least in some of the self-capacitance sensing devices. It is desirable to minimize the number of defects in a batch of sensing devices (e.g., increase the yield rate), but it is difficult if not impossible to completely eliminate the occurrence of faults in the touchscreens. Common defects in ITO traces in touchscreens include shorting between traces, shorting between one or more traces and ground, broken traces, traces that are too thin, too narrow, too thick or too wide, unintended irregularities in the geometries of individual traces, and the like.
Defects in ITO traces can significantly negatively impact the performance of a touchscreen or touchpad. Because of this fact, testing is often carried out on individual self-capacitance sensing devices after the manufacturing process has been completed. Once such testing method for self-capacitance touch sensing devices is described in U.S. Patent Publication No. 2008/0278453 to Reynolds et al., the entire contents of which are hereby incorporated herein by reference.
There are several problems with testing the integrity of ITO or other types of electrodes in a self-capacitance sensing device; however, testing is still an important production step to ensure product quality.
The second primary capacitive sensing and measurement technology employed in capacitive touch sensing devices is that of mutual capacitance, where measurements are typically performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide, the entire contents of which are hereby incorporated herein by reference. In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
In some mutual capacitance measurement systems, an array of sense electrodes is disposed on a first side of a substrate and an array of drive electrodes is disposed on a second side of the substrate that opposes the first side, a column or row of electrodes in the drive electrode array is driven to a particular voltage, the mutual capacitance to a single row (or column) of the sense electrode array is measured, and the capacitance at a single row-column intersection is determined. By scanning all the rows and columns a map of capacitance measurements may be created for all the nodes in the grid. When a user's finger or other electrically conductive object approaches a given grid point, some of the electric field lines emanating from or near the grid point are deflected, thereby decreasing the mutual capacitance of the two electrodes at the grid point. Because each measurement probes only a single grid intersection point, no measurement ambiguities arise with multiple touches as in the case of some self-capacitance systems.
A current problem with testing touchscreen devices or touchpads, regardless of whether or not they utilize mutual capacitance technologies or self-capacitance technologies, is that separate testing equipment is required to perform the tests. In particular, an external stimulus has to be applied to the drive and sense columns to test whether a fault exists. Application of the external stimulus often requires the need for additional test circuitry in the facility which is manufacturing the touchscreens or touchpads. This implies additional costs to touchscreen manufacturers. It would be desirable to minimize or eliminate these additional costs.