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Deep analysis of the principle of open short circuit test
The open short circuit test is widely used in various industries, particularly in electronics testing. It refers to the process of checking for open or short circuits in the bonding lines and integrated circuits (ICs). This fundamental technique is essential for test engineers, often referred to as continuity testing or open/short testing. The core principle behind this method relies on the forward voltage drop of ESD protection diodes embedded within the device.
Most device pins, especially those connected to ground or power supply, are equipped with ESD protection diodes. These diodes allow current to flow in one direction, which can be used to determine if a pin is properly connected or not. For example, when testing whether Pin1 is connected to ground, a small current (typically in the range of tens of microamps to several milliamps) is applied from Pin1, and the resulting voltage at V1 is measured.
If the pin is normally connected, a voltage drop of approximately 0.6V will be observed, and the measured voltage would be around -0.6V due to the polarity. If the pin is open, no current flows through the diode, and the resistance becomes very high, leading to a near-zero voltage or a value limited by the test source’s clamp voltage, such as -2V. In the case of a short circuit, the diode is bypassed, and the voltage at V1 will be close to 0V.
This same principle applies when testing between Pin1 and Vdd. By applying a known voltage from Vdd to ground, we can use the forward voltage drop of another ESD diode to detect continuity. Here, the current direction is opposite, so the measured voltage will be positive.
To check for shorts between all pins, you can ground all other pins and test each one individually. If any pin is shorted to ground, the measured voltage will be close to 0V. When dealing with multiple pins, it's efficient to perform parallel tests using multiple test sources. However, simultaneous testing may miss shorts between tested pins. To address this, some strategies involve cross-testing adjacent pins, such as grounding odd-numbered pins while testing even ones, then switching roles to cover all possibilities.
In cases where ESD protection diodes are absent, this method won’t work. Some devices also have dissipation pads that need to be checked for shorts. Normally, these should show an open circuit.
Once the open short circuit test is performed, the results must be compared against set specifications. Improper settings can lead to false readings. For instance, if the clamp voltage is set too low, an open circuit might be misclassified. Therefore, it’s crucial to define reasonable limits based on the device’s characteristics.
Additionally, some pins may have more complex ESD protection circuits, including series-connected diodes, resistors, or capacitors. While resistors have a simple effect, capacitors can complicate the test due to their charging behavior and influence on the measured voltage. Understanding these nuances is key to accurate and reliable testing.