1. Field of the Invention
The present invention relates generally to automatic test equipment for testing electronic assemblies, and more particularly to a method for the testing of electronic assemblies such as printed circuits and the in-circuit testing of electronic components.
2. Description of the Prior Art
With the advent of larger and larger printed circuit boards (PCB) containing more and more signal paths and electronic components and the use of more and more complex electronic components on printed circuit boards, the need for automatic testing of such printed circuit boards has become a matter of economic necessity. Detection of failures in printed circuit boards at the earliest possible stages of manufacture prior to their being installed in the ultimate product or the investment of more and more labor and material cost is a common goal of most printed circuit board manufacturing processes. Automatic testing equipment has been introduced for the purpose of detecting faulty components and assembly errors at the printed circuit board level so that problems can be found at the printed circuit board level before a series of boards are assembled into a subassembly and subassemblies into systems.
In a typical manufacturing process, a system containing printed circuit boards may undergo one or more of the following tests or inspections. The printed circuit itself will be ring-out tested before electronic components are mounted on the board to assure that signal paths (etches) connect all required points and that there are no shorts between signal paths. After the electronic components are mounted and soldered to the printed circuit, a second ring-out test is performed to again verify signal path continuity and that there are no shorts between signal paths. Next the printed circuit boards with the electronic components mounted may then be placed in a thermal chamber and/or subjected to a burn-in period. Burn-in is performed by applying power to the printed circuit board and exercising the printed circuit board logic. This may be done while the PCB is in the thermal chamber and cycled through a range of temperatures. The purpose of the thermal chamber and/or burn-in periods is to make marginal connections and electronic components fail before subsequent testing. The heat and exercising stimulus cause marginal connections and electronic components to fail in a much shorter period that would otherwise be required. These failures can then be identified by subsequent testing prior to installing the printed circuit board in the ultimate product where their eventual failure may be very costly both in repair time and down time.
Following the terminal chamber and/or burn-in period, the printed circuit boards are then subject to a second set of tests which are usually performed by mounting the printed circuit board on an automatic test equipment device and cycling through a set of tests. Tests in this second set include: testing for shorts, analog testing, testing component orientation, and digital testing. The shorts test checks for shorts between signal paths by introducing a low impedance signal on one path and testing for its presense on other signal paths. The component orientation test checks for components being properly mounted on the PCB, i.e., that integrated circuits have been properly oriented without lead reversal. The digital testing checks are performed on integrated circuits by using truth tables to apply known input signals so that the outputs can be checked against expected results. For example, a known pattern can be placed in a shift register and the output monitored as the register is shifted (clocked). Analog testing is used on discrete components such as resistors, diodes, transistors and capacitors to verify that they are of the proper value (ohms, etc.) and that they perform as expected.
Some of these test are performed without power on the electronic assembly and others are performed with power. For those performed with power, the various voltage and ground paths on the printed circuit board are connected to the various voltage and ground potentials that will be applied to the PCB when it is in use in the ultimate product, and the tests are run with the electronic components operating under power. For example, if a PCB containing TTL electronic components will have ground, +5 volts DC, and +12 volts DC applied to it in the ultimate product, the ground lead will be attached to ground, the 5 volt DC lead will be supplied with +5 volts DC and the 12 volt DC lead will be supplied with +12 volts DC when the component orientation and digital tests are performed. The digital test may also be performed with other marginal voltages applied to test for marginal components.
Other tests such as the shorts and analog testing are performed without power applied to the PCB. These tests are usually performed before the other tests so that power is applied only after faults that could cause other components to fail when power is applied have been identified and corrected. It should be noted that the earilier described ring-out testing (without and with components mounted) is performed without power applied to the PCB.
As printed circuit boards become larger and more complex the test points, to which automatic test equipment must have access, are no longer accessible by signal paths brought out to edge connectors on a printed circuit board. The automatic testing equipment must have access to points within the board to do ring-out testing to check that an etch (signal) path does in fact connect all the points that it is supposed to without discontinuities and that it is not shorted with other parallel paths. To gain access to these internal test points, automatic test equipment has been designated such that the PCB to be tested is placed in a fixture containing a matrix of spring loaded test probes which are placed to contact the PCB at the desired test points on the PCB. In addition to being spring loaded, test probes may be gold plated to provide good conductivity by reducing corrosion and resistance. To further aid in providing positive contact, a variety of test probe head designs are available to probe test points such as integrated circuit pins, PCB lands (an exposed area of copper etch on the PCB surface), extended leads, thin etch traces and others. One such test fixture is the Thinline system manufactured by Fairchild Test System Group, Subassembly Test System Division, Latham, N.Y. 12110. Thinline is a registered trademark of Fairchild. This test fixture system holds the PCB in the fixture by use of a vacuum.
Unfortunately, as good as fixturing systems are on present automatic test equipment, the failure of a test probe in the fixture to make contact with the test point on the PCB will greatly effect the test results and consequently the use that can be made of the ATE results. Test probe contact failure can be caused by: misalignment of the PCB on the fixture, individual misaligned test probes, bent or missing probe pins, dirt or other foreign matter on the PCB at the test point, a discontinuity in the signal path between the test probe and the sensing circuit in the ATE (e.g., broken wire, nonclosing relay), etc. This problem can be ameliorated in some cases by using multiple test probes for each test point on the PCB. However, this is not always possible due to space limitations on the PCB itself, or limitations on the total number of test probes in a fixture or connectable to (testable by) the ATE.
This failure of one or more test probes to make contact becomes more likely as the size of the PCB increases and as more components are mounted on a PCB. For example, in a present day minicomputer, a typical PCB containing the central processing unit logic or peripheral controller logic may be approximately 15 by 13 inches in size, have approximately 300 electronic components mounted on it, and have approximately 1100 test points. Some of these test points on particularly long or common signal (etch) pulses may go to as many as 30 places on the PCB and therefore be involved in testing many electronic components on the PCB.
The results of testing the PCB by the ATE may be presented to the user as a printout which indicates which component (e.g., the resistor at location B3) on the PCB failed a particular type (analog, digital, etc.) of test. As a matter of economy and production efficiency, it has been found in some cases that the more automated the test is and the lower the skill level of the ATE operator, the more reliable the test results. This results from the fact that more highly skilled operators tend to want to improve the test results by trying various things to overcome poor test results. This improving process often slows down test production and/or leads to decreased reliability in the results as the skilled operator tries various fixes to cure the problem.
The use that is made of the test results varies widely depending on testing and repair philosophy, the confidence level placed in the test results, and the skill level of the person who interprets the test results, the complexity of the PCB, the skill level of the person who is to remove and replace identified faulty components, component and labor costs, etc. For example, at one extreme the test results can be put aside and used only as an aid to repairing the PCB after the PCB fails in a subsystem or system test. The test results may be used to guide a visual inspection in which the inspector simply verifies that components that failed the tests are the proper components and properly oriented. Alternatively at the other extreme, all components identified by the ATE tests can be systematically removed from the PCB and replaced by new components. Unfortunately, with large complex printed circuit boards, this remove and replace philosophy may not be economical in terms of labor and component costs if the ATE results are not 100 percent reliable. Further, with unreliable test results, this remove and replace philosophy may not be a converging process in that removal of the identified faulty component from a wave soldered PCB with fine etch signal paths may result in damage to the PCB itself which can lead to the introduction of more failures into the PCB than are removed at any one pass of the PCB through the inspection process.
Therefore, what is needed is a method for automatic testing of electronic assemblies which improve the reliability and useability of ATE test results.