Abstract:
A method and apparatus for detecting and locating a circuit defect. In a preferred embodiment, the apparatus includes a resistance measuring device for measuring the resistance of the circuit, a heat source, and a recorder. The heat source is placed near the surface of the circuit but is not placed in physical contact with the surface. The heat source is moved about to a plurality of points in a plane parallel to and above the surface of the circuit while the resistance of the circuit is measured when the heat source is at each of the points. The recorder is functionally connected to the resistance measuring device and to the heat source, and determines and records the horizontal location of the heat source and records the resistance of the circuit at each of these locations.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to electric circuits and, more particularly, to a method and apparatus for detecting and locating circuit defects. 
     2. Description of Related Art 
     In a test process of the manufacturing processes of a large scale integrated (LSI) circuit, it is difficult to detect a short circuit point. Specifically, in a case of completion of an LSI circuit such as an application specific integrated circuit (ASIC) ordered by a user for delivery within a short period of time, a short circuit problem sometimes occurs. A short circuit is typically formed between a plurality of power supply wiring patterns because there are crosses between power supply wiring patterns and because bugs of wiring design data, error in setting the coordinates of wiring patterns, or overlapping wiring patterns are occasionally introduced in the computer aided design (CAD) process. Such a type of short circuit between the power supply wiring patterns is the cause of incomplete operation of logic circuits as a main functional circuit of the ASIC. 
     In highly integrated circuits, it is very difficult to detect a point where the short circuit is formed. Conventionally, there is a method of visually checking the wiring mask pattern as a method of detecting a short circuit point between wiring patterns. However, it is impossible to completely check the wiring mask pattern for a large scaled integrated (VLSI) circuit layout. 
     A more sophisticated method of detecting circuit defects uses a combination of heat and voltage injection. These methods allow detection of circuit defects that are not observable visually. These methods involve heating the entire circuit assembly and then cooling sections of the assembly with compressed gas or liquid to locate probable defective components. However, subjecting electronic components to this rapid temperature change introduces structural damage to the components and, ultimately, causes degradation or failure of the component under normal operating conditions. Furthermore, the machines needed to perform this method are very costly and complicated to use. Therefore, a method and apparatus that allows detection and location of defects within a circuit to ensure the highest confidence in the integrity of the component assembly being repaired which does not damage the circuitry and that maintains lower production costs is desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for detecting and locating a circuit defect. In a preferred embodiment, the apparatus includes a resistance measuring device for measuring the resistance of the circuit, a heat source, and a recorder. The heat source is placed near the surface of the circuit but is not placed in physical contact with the surface. The heat source is moved about to a plurality of points in a plane parallel to and above the surface of the circuit while the resistance of the circuit is measured when the heat source is at each of the points. The recorder is functionally connected to the resistance measuring device and to the heat source, and determines and records the horizontal location of the heat source and records the resistance of the circuit at each of these locations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts a pictorial diagram of a circuit defect location determining system in accordance with the present invention; 
     FIG. 2 depicts a pictorial diagram of a circuit defect isolation determining system for specifying specific defective circuits within the assembly in accordance with the present invention; 
     FIG. 3 depicts a block diagram of an automative system for testing defective circuits in accordance with the present invention; and 
     FIG. 4 depicts a flowchart of a preferred method of locating circuit defects in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the Figures and, in particular, with reference to FIG. 1, a pictorial diagram illustrating a circuit defect location determining system is depicted in accordance with the present invention. Circuit defect location system  100  includes a heat source  104 , such as, for example, a soldering iron or a forced air heat gun, and a resistance measuring device  106 , such as, for example, an ohmmeter. System  100  is used to determine the general area within a defective circuit  102  in which the defective component is contained. 
     Resistance measuring device  106  is connected to circuit  102  such that the resistance of the entire circuit can be measured. The Thermal Expansion Property of Thermodynamics says that as heat is applied to a consistent mass, the resistance of that mass changes in a consistent manner. Therefore, there is a high probability that a defect in the planar circuit assembly or semiconductor does not have the same mass as the circuits to which it is connected. Thus, while resistance measuring device  106  monitors the resistance of circuit  102 , heat source  104  is moved around the x-y plane at a constant distance d above the surface of circuit  102 . Heat source  104  is never placed into physical contact with any of the components of circuit  102  in this first stage of determining the general area in which the defective circuit is located. Heat from heat source  104  is transferred to circuit  102  inducing thermal ramping of the components within the general area beneath heat source  104 . Once heat source  104  is above the general area in which the defective component or circuit is contained within circuit  102 , the resistance as measured by device  106  will show a dramatic change. The change in resistance may be up or down depending on whether the defective mass is being compressed or elongated during thermal ramping. 
     The temperatures of heat source  104  depend on the requirements of the specific circuit  102  being tested for circuit defect isolation. However, traditionally, all of the components in typical circuit designs are able to withstand thermal ramping from ambient room temperature to +300 degrees Celsius when initial vapor soldering occurs. In a vapor soldering process, a solder paste is applied to the circuit board and the components placed in the paste. The entire assemble is then lowered into a chamber containing a “liquid” that has been heated into its vapor state. The thermal ramping that occurs to the circuit board once lowered into the chamber is virtually immediate, which then melts the solder/flux compound to solder the components into place. By using a radiant heat source, the possibility of thermal ramping that would exceed the specification for any given component of the circuit board can be dramatically reduced. 
     In practice, consistent results have been obtained when the heat transferred to the component is between 200 and 300 degrees Celsius. As little as 150 degrees Celsius may be all that is required to invoke a resistive change. However, the faster the heat transfer, the more dramatic the resulting change in the resistance of the circuit. 
     The distance d that heat source  104  is placed away from circuit  102  varies depending on the heat source utilized and the tolerances of the particular circuit  102  for which a circuit defect must be isolated. Heat source  104  must be close enough to the surface of circuit  102  to have a heat transfer from heat source  104  to circuit  102  without exceeding the material&#39;s heat tolerance. In practice, using soldering iron as the heat source, an appropriate distance d which produced good results was a value of one half inch. 
     The amount of time for the appropriate amount of thermal ramping to occur can be determined by placing the chosen heat source in proximity to a thermometer to gauge the time required for thermal ramping. This time frame will be determined based on the operator&#39;s available heat source and the minimum and maximum heat specifications of the individual components in the assembly being tested. 
     Referring now to FIG. 2, a pictorial diagram illustrating a circuit defect isolation determining system for specifying specific defective circuits within the assembly is depicted in accordance with the present invention. Once the general area  202  of circuit  102  in which the defective component resides is determined, heat sources  204  and  206  are used to determine the specific component within the circuit assembly  102  that is defective. Heat sources  204  and  206  are placed into physical contact with the leads of suspect components  220 - 226  within area  202 . The resistance of the entire circuit is measured by device  106  as the leads of each individual suspect component  220 - 226  are placed into physical contact with heat sources  204  and  206 . The resistance of circuit  102  will show a dramatic change as heat sources  204  and  206  are placed into physical contact with the leads  210  and  212  of the defective component  220 . 
     If the suspect component within area  202  is a multi-lead component with more than two leads, then more than two heat sources  204  and  206  will be required, since each lead must be in physical contact with a heat source. 
     The present invention may be implemented by a repair technician utilizing tools commonly available to a repair technician including a soldering iron or forced air heat gun for a heat source and an ohmmeter or digital multimeter. The processes of the present invention are relatively simple and would require a minimum amount of training to allow a technician to implement them. Furthermore, by using a localized heat source to minimize thermal ramping, the problem component within the circuit may be located without compromising the integrity of the component circuit assembly. 
     Referring now to FIG. 3, a block diagram of an automative system for testing defective circuits is depicted in accordance with the present invention. Defective circuit location system  300  includes a data processor  301 , an ohmmeter  320 , robotics system  324 , and heat sources  326 . 
     Data processing system  301  includes storage unit  312 , processor  302 , memory unit  310 , input/output unit  308 , and display  304 . Each of units  302 ,  304 ,  308 ,  310 , and  312  is connected to bus  306  which couples the various component units to each other. Processor  302  executes instructions that have been loaded into memory  310 , which controls the operations of robotics system  324 , determines the temperature of heat sources  326 , and correlates the resistance of a defective circuit as measured by ohmmeter  320  to the position of the heat sources  326 . Correlated data may be stored in storage unit  312  for further processing or viewing at a later time or may be displayed to a user via display  304 . 
     Connection from data processing system  301  to robotics system  324  and to ohmmeter  320  is provided by input/output unit  308 . Robotics system  324  is connected to heat sources  326 , which may comprise one or more heat sources such as, for example, heat sources  104 ,  204 , or  206 , and is also connected to the leads of ohmmeter  320 . Robotics system  324  includes mechanisms and electronics for controlling the movement and placement of heat sources  326  and the leads from ohmmeter  320 . 
     Robotics system  324  controls movement and/or placement of the two leads from ohmmeter  320  such that one lead is placed in physical contact with an input lead to a defective circuit and one lead is placed in physical contact with an output lead to the defective circuit such that the resistance of the circuit can be measured by ohmmeter  320  and such resistance measurement sent to data processing system  301 . 
     Robotics system  324 , under the control of data processing system  301 , moves heat sources  326  around in a plane above the surface of the defective circuit to determine the general area of the defective component within the defective circuit as described in more detail above. Once the general area of the defective component within the defective circuit has been found, robotics system  324  places various parts or ones of heat sources  326  into physical contact with the leads of each of several components within the general area in which the defective component is located successively while the resistance of the circuit is being monitored by data processing system  301 . Thus, the particular defective component within the circuit may be isolated. 
     Input/output unit  308  may also be coupled to a keyboard, mouse or other user input device to allow a user to control some or all of the process and to allow the user to selectively view the data in various formats. 
     Although the automated circuit defect isolation and location system  300  in FIG. 3 has been described primarily with reference to an ohmmeter, other devices for measuring the resistance of the defective circuit may be used as well. Furthermore, system  300  is an example of an automative circuit defect isolation and location system and is not meant to imply any architectural limitations to the present invention. 
     Referring now to FIG. 4, a flowchart illustrating a preferred method of locating circuit defects is depicted in accordance with the present invention. First, a heat source is placed over the surface of a circuit for which the location of a defect must be determined (step  402 ). The heat source is placed near the surface, but not in physical contact with the surface of the circuit. Preferably, the heat source is placed approximately one half inch away from the surface of the circuit. Next, the resistance of the circuit is measured with the heat source at the current point (step  404 ). Next, it is determined whether the heat source has been placed over all areas of the surface of the circuit and that the resistance of the circuit has been measured for the heat source in each location (step  406 ). If all areas of the surface of the circuit have not been covered, then the heat source is moved to be above a new area of the surface of the circuit (step  402 ) and the resistance of the circuit with the heat source over this new area of the surface of the circuit is determined (step  404 ). 
     This process continues with the heat source systematically moved around the surface of the circuit until the resistance of the circuit has been measured for all placements of the heat source. The process can be a continuous process with the resistance measured continuously while the heat source moves around the surface of the circuit. Alternatively, the process can be a discrete process in which the heat source is placed over a discrete set of points which are evenly distributed over the surface of the circuit and the resistance measured with the heat source located at each of the discrete set of points. Although described as taking individual resistance measurements, in practice, it is more likely that the resistance is constantly monitored for a dramatic change. 
     Once it is determined that the heat source has been placed generally over all areas of the circuit and the corresponding resistance of the circuit measured, then the general area of the circuit that contains the circuit defect is determined (step  408 ). The general area of the circuit in which the circuit defect is contained is the area of the circuit in which the measured resistance of the circuit, with the heat source placed over this area, varied significantly as compared to the measured resistances of the circuit with the heat source placed over other areas of the circuit. 
     Once the general area of the circuit in which the circuit defect is located is determined, then a heat source is placed in physical contact with the leads of a first suspect component within the defect area (step  410 ) and the resistance of the circuit measured (step  412 ). Next, it is determined whether all components within the suspect area have been tested (i.e. had their resistances measured when their leads are in physical contact with a heat source) (step  414 ). If all components within the suspect area have not been tested, then the heat source is placed in physical contact with the leads of the next untested component within the suspect area (step  410 ) and the resistance of the circuit measured (step  412 ). 
     Once all suspect components have been tested, then the component or components which are defective are determined (step  416 ). This determination is made by comparing the measured circuit resistances to each other. The component whose leads were physically in contact with the heat source for the resistance measurement that was significantly different from the other resistance measurements is the defective component. 
     It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media such a floppy disc, a hard disk drive, a RAM, and CD-ROMs and transmission-type media such as digital and analog communications links. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.