tekHPoynt

by: Allan B. Dizon

The continuity test (also called contact test) verifies that, during a device test, electrical contact is made to all signal pins on the DUT and that no signal pin is shorted to another signal pin or power/ground. Device cost is directly related to how long it takes to test each device. One of the best ways to reduce average test time per device is to reject bad devices as soon as possible. The continuity test determines very quickly whether a device has shorted pins, or missing bond wires, a pin damaged from static electricity, a manufacturing defect, etc. It can also point to test system related problems such as wafer probe card or a device test socket which is not making good contact.

Test conditions for a continuity test are not normally defined in the device specification or data sheet, but there are standard values which apply to most standard device types. These standards values are presented in the guidelines here:

• To test for continuity , first ground all device pins including power and ground pins. Next connect the Parametric Measurement Unit (PMU) to a single device pin and force a current which will forward bias one of the protection diodes (see figure 1). A negative current will forward bias the the diode to ground; a positive current will forward bias the diode to VDD. A current in the range of 100uA and 500uA should be adequate. Once forward biased, the voltage drop across the protection structure can be sensed (typically 0.65v). The sensed voltage may change between various technologies and designs, but 0.65v is good for all silicon based devices.

Figure 1

• Since the PMU will be programmed to force current, a voltage clamp must be programmed to limit the voltage produced when an open pin is tested. A typical clamp value for the continuity test 3v. When an open pin is tested, the measured result is will be the clamp voltage(3v).
• To test the upper diode, which is connected to VDD, use the PMU to force a positive current of approximately +100uA. Set the upper test limit to fail if the measured result is less than 0.2v to detect short. This test method is used to test signal pins(input and output) but not power pins such as VDD or VSS.
• To test the lower diode, which is connected to VSS or ground, use the PMU to force a positive current of approximately -100uA. Set the lower limit to fail if the measured result is less than -1.5v to detect an open.
• Power and ground pins may also be tested for an open condition, but their structure is different from that of a signal pin. Test the power pins, observe the measured value on a good device, and set the test limits accordingly.