FAQ: HBM Low Parasitic Testers

What are tester parasitics?


Tester wiring from the pulse generator to the DUT has inductance and capacitance as do all wires and PCB traces. The unspecified and unwanted wiring inductance and the capacitance between wires are often termed parasitics. The fast rising HBM pulse edges are affected by these parasitic circuit elements and result in waveform distortions that can vary the stress applied to the DUT. Tester parasitics also refer to the capacitance of relays that form the switching matrix and the wiring of test fixture boards and their sockets.




Are parasitics bad?


Parasitics can distort the waveform and therefore the proper stress may not be applied to the DUT. This can increase or decrease the peak stress so parts are not accurately characterized. When the change in stress causes a DUT to fail when it would otherwise have passed, the result is a false failure. False failures can be verified when the part is retested on a low parasitic tester and it passes at a higher HBM classification level.




Why don’t I see parasitic distortions when checking tester waveforms?


When testing short circuit waveforms to verify proper tester operation, the test load is a two pin DUT. The parasitics in this configuration will not produce out-of-specification waveforms. The parasitics play a role when the DUT has more than two pins. And an even larger role when the DUT has pins that are internally tied together. Larger DUTs typically have groups of tied pins, such as bussed power pins or multiple Vss pins. When such a DUT is placed in the tester socket, the parasitics of several tester channels are paralleled and pulse ringing can occur. Waveforms are checked using a DUT with 2 pins, but real testing often connects a group of pins together causing significant distortions. Another way to view the tester-DUT interaction is when the DUT is inserted into the tester, the tester connections are modified. Connecting tester channels together at the DUT allows pulses to echo up and down unterminated wires that act like small transmission lines. The parasitic inductance and capacitance forms these transmission lines with electrical length close to the HBM rise time so these pulse echoes can change the pulse peak current.




Why is this a new issue?


Parasitics are not new but are becoming a bigger problem as technology drives smaller device dimensions. Documented cases of false failures have been published during the last several years.




What is a low parasitic tester?


The ANSI/ESDA/JEDEC HBM Standard JS-001-2012 Annex B4 explains how to determine if a tester is Low Parasitic. Testers that can have many channels tied together and still maintain the required HBM pulse waveform are low parasitic.




Why is a 2-Pin pin tester a low parasitic tester?


2-pin testers only contact two DUT pins a time, so each pulse has the same current path from the pulse generator to the DUT. This makes the ideal connection to the DUT like is shown in Figure 1 of the JS-001-2012 HBM Standard. All testers have parasitics, but by their nature 2-pin testers aren’t affected by devices that have pins connected together. 2-pin testers maintain the prescribed HBM pulse waveform for all device types and pin combinations so they are low parasitic.




Are all parts effected by tester parasitics?


No, but the number is growing as HBM targets are lowered and dimensions shrink. Small pin count parts without multiple power and ground pins may not be effected by parasitic issues. Parts with connected groups of pins, like multiple power pins, will tie tester channels together can cause pulse distortions. High frequency parts are sometimes more affected by tester parasitics. A relay matrix tester with its test fixture board effectively ties a one meter long wire to every pin, and the parasitics of those wires affect the high frequency measurement.




What are my choices in testers?


The commonly used relay matrix testers have significant wiring parasitics and are not low parasitic testers. Matrix tester manufacturers are recognizing that this is becoming an ever more significant problem and some are now offering an upgrade to partly fix their problem. Newer technology HBM testers, like 2-pin or socketless testers, provide a simpler solution using mechanical switching rather than relay matrix switching and are low parasitic testers by their basic design. Due to their simplicity, a complete 2-pin tester can cost less than upgrading a relay matrix tester to be low parasitic.





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