PCB Technical - PCB fault detection technology ASA (VI) curve test

Analog Signature Analysis is a fault detection technology widely used in electronic PCBs. Has the following characteristics:

1. It does not involve the circuit principle and does not need the circuit to be in working condition, so it can be used for fault detection of the circuit board without drawings and data, which is out of the equipment (without online testing);

2. There is no need to power up the circuit board during testing, which is relatively safer;

3. It does not involve the functions of the components on the circuit board, so no matter what type of components the circuit consists of, including digital, analog, mixed digital and analog, known functions, unknown (such as dedicated, programmable), etc., it can be test;

4. It is tested by circuit nodes (device pins), and is basically not limited by the packaging of components on the circuit board.

Since a single device can be regarded as the simplest circuit board circuit, ASA technology can also be used to detect the quality of electronic components. In particular, it does not involve device functions and is not restricted by device packaging. It has become the only means for many users to detect the quality of large-scale, complex or unknown integrated devices.

ASA is also convenient and intuitive when used to detect the function of discrete components.

One, the basic principles of ASA

As far as the basic detection principle is concerned, the ASA test can be regarded as a natural extension of the multimeter detection method.

For the circuit board without circuit principle drawings and separated from the equipment, the most commonly used multimeter fault detection method is as follows: first measure the resistance of the device pins on the PCB board (actually the circuit node) to ground; then contact the faulty board Compare the resistance to ground of the corresponding device pins, and judge whether there is a fault on the node according to the difference. From node to specific components need to be manually determined. Many people have used this method to repair complex and expensive circuit boards.

In addition to the high requirements for users and low efficiency, this method is mainly due to the fact that the multimeter can only detect the impedance value under 1.5V (the battery voltage of the multimeter), and the pin of the semiconductor device The impedance changes with the change of the test voltage-the impedance under different test voltages may not be the same. For example, a TTL device pin has a soft breakdown at 2.5V, resulting in a large leakage current. Such failures cannot be detected.

In addition to the high failure detection rate, the other two reasons why the ASA test is popular in maintenance testing are:

1. The test efficiency is very high. Take Huineng tester as an example. For a 40-pin device, 128 voltage points are measured on each pin, and the test time is less than 1 second;

2. The test data extracted from the good board can be stored in the computer (that is, the board library is established), as a reference standard for future testing, and used repeatedly.

2. The realization method of basic ASA test

Use the tester to generate a varying voltage signal and add it to the object under test, while recording the current under different voltages. The current that changes with the voltage is expressed on the voltage-current coordinate system, and a (impedance) curve is obtained. The user judges the fault based on the difference in the shape of the curve of the corresponding node of the relatively good and bad circuit boards.

In view of the popularity of microcomputers, in order to reduce development difficulty and product cost, most of the current tester products of this type are used in conjunction with microcomputers. The tester generates a sine wave test signal; under the control of the special test software, the microcomputer accepts user instructions, implements the test algorithm, and controls the tester to apply test signals, display test results, and store test data according to user requirements.

Starting from actual use requirements, we will discuss how to obtain efficient and practical ASA test functions.

Three, about connecting to a computer

From the current microcomputer technology and development, the software and hardware of the tester should:

1. Test software should support mainstream operating system version

Since Win98, the Windows operating system has made great changes to the management machine of external devices. The test program running on Win98 cannot be automatically upgraded to run on subsequent system versions. In view of the fact that the operating system of Win98 and below will be completely withdrawn from use soon, if the test software cannot support the mainstream operating system version, such as Windows XP, it will cause trouble for users in the future.

2. The tester preferably supports USB port

The early tester used the way of inserting a card in the computer to realize the connection with the computer. Due to the many disadvantages of this method, the parallel port (print port) is used instead. But in recent years, the faster and safer (allowing hot plugging) USB ports have been rapidly popularized

Four, about the test signal

For any electronic instrument, the test signal is the basis of the entire test function. Its quality basically determines the test quality of the tester.

1. About the main test signal-sine wave:

In order to ensure the test effect without damaging the device under test, the sine amplitude should be greater than the actual working voltage of the pin of the device under test and less than its limit voltage. Since different components require different voltage values, this requires that the amplitude of the sine wave output by the tester is adjustable.

B. Maximum output (short circuit) current

The maximum current that can flow out after the sine wave is short-circuited is called the maximum output current:

Maximum output current=equal to sine peak value/output resistance

C. Frequency Range

The wider the frequency range, the better it can adapt to the test of capacitive and inductive nodes. For example, the effective ASA curve of a capacitance of 10,000 to 20,000 microfarads can be measured with the Huineng tester-the curve will not degenerate into a short-circuit line, and it can be clearly seen from the curve whether the leakage and the capacity are sufficient.

D. Fidelity (or distortion)

refers to the shape difference between the actual sine wave and the ideal sine wave. The shape of the ASA curve of the non-DC node is not only related to the frequency, but also related to the shape of the waveform. For example, the ASA curve of a capacitor is only an ellipse under a sine wave.

E. About the problems in the generation of sine waves

Since the ASA test judges failures by the shape of the curve, the consistency and repeatability of the test results are very important. The consistency and repeatability of the test results are guaranteed by the stability of the test signal-frequency accuracy and waveform fidelity. .

Huineng tester does not have this problem at all. After recent military inspections, the frequency accuracy and waveform distortion of its test signals are both less than 2%, which does not change with external conditions. Of course, the hardware of the Huineng tester is relatively more complicated.

2. About auxiliary test signal-pulse:

The introduction of pulse auxiliary test signal is to make the ASA test better used for the test of three-terminal devices. Thyristors, MOS transistors, even relays, voltage regulators, etc. can all be regarded as three-terminal devices.

Different three-terminal devices require different control methods-sine wave and pulse matching (synchronization) form.

5. About the number of test channels

The quality of the test signal determines the quality of the test, and the number of test channels mainly affects the test efficiency. The use requirements are different, and the requirements for the number of channels are also different. There are three main types:

1. Online test: At present, for devices with more than 80 pins, there is basically no test clip that can be used in conjunction, so 80 channels basically meet the requirements of use;

2. Circuit board port test: lead the test channel to each pin of the circuit board through the adapter board and the corresponding circuit board socket on the board, and then perform single/multi-port test. Usually 160 channels can meet most usage requirements.