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Deep analysis of the principle of open short circuit test
The open short circuit test is widely used across various industries. Specifically, it refers to two main types: the open/short test of the test bonding line and the open/short test of the integrated circuit (IC). As a fundamental skill for test engineers, this method is commonly known as continuity testing or open/short testing. The core principle behind this test relies on the forward voltage drop of the ESD (electrostatic discharge) protection diodes built into the device.
Most pins on a device that can be tested for open/short conditions are connected to ground or power supply through ESD protection diodes. By applying a small current and measuring the voltage, we can determine whether the pin is properly connected or has an open or short condition.
For example, when testing Pin1 to ground, a small current I1 (typically in the range of tens of microamps to several milliamps) is sourced from Pin1, and the voltage V1 is measured. If the pin is normally connected, there will be a voltage drop across the ESD diode, usually around 0.6V. Depending on the direction of measurement, V1 may show approximately -0.6V.
If there's an open circuit at Pin1, the ESD diode is not conducting, so the resistance between Pin1 and ground becomes very high. In this case, the voltage V1 would be close to the clamp voltage of the test source, which might be limited to something like -2V. On the other hand, if there’s a short between Pin1 and ground, the ESD diode is bypassed, resulting in nearly zero voltage across V1.
The same principle applies when testing between Pin1 and Vdd. A current I2 is applied, and the forward voltage of the diode is used to determine the connection status. Here, the voltage would appear positive, as the current direction is opposite to the previous case.
To check for shorts between all pins, you can ground all other pins while testing one at a time. If any pin is shorted to another, the measured voltage will be close to zero. When dealing with a large number of pins, parallel testing is often used. However, if multiple pins are tested simultaneously, a short between them may go undetected. To improve efficiency, some strategies involve cross-testing adjacent pins by grounding alternate groups and testing others in sequence.
It's important to note that this method only works if the device has ESD protection diodes. In cases where no such diodes are present, alternative methods must be used. For example, some devices have a dissipation pad that must also be tested to ensure it is not shorted to other pins. Normally, this should show an open circuit.
Once the open/short test results are obtained, they must be compared against a defined test specification. Setting appropriate limits is crucial to avoid false readings. For instance, if the clamp voltage is set to -1V but the test specification allows values down to -1.2V, an open circuit might be incorrectly identified as a pass. Adjusting the lower limit to something higher, like -0.9V, can help prevent such errors.
Additionally, some pins may have more complex ESD protection circuits, such as multiple diodes in series or additional components like resistors and capacitors. While resistors are straightforward, capacitors can introduce more complexity and affect the test results significantly. Understanding these variations is essential for accurate diagnosis and testing.