HMM testing

The ESD Association Human Metal Model SP5.6-2009 (HMM) [1] describes the stress testing of ICs with the waveform defined in IEC 61000-4-2. The setup and test can be done in several ways using discharge guns and pulsers that deliver pulses by coaxial cables. GTS manufactures an HMM tester [2] (PurePulse HMM) that produces the stress current with the cable delivery method. This note describes the GTS PurePulse HMM tester which is a member of the PurePulse ESD testing suite of products.

Why HMM

Many product required testing to the IEC 61000-4-2 Standard method in order to meet the requirements of CE Marking or similar regulations. This IEC standard can only be properly applied to completed products. The testing of ICs, discrete semiconductors and passive devices is not covered by 61000-4-2. The desire to apply the stress of 61000-4-2 in a repeatable manner to semiconductor products was the motivation for the ESDA to develop the HMM Standard Practice.

HMM Quality

The IEC 61000-4-2 defines a discharge gun that can be used in air discharge and/or contact stressing of the outer surfaces of a product or equipment under test. HMM defines method to extend the 61000-4-2 test to components. The use of test fixture boards and/or test benches is specified along with the 61000-4-2 pulse generators to delivery repeatable discharges in a direct contact mode [3].

Inter-laboratory testing was conducted by the ESD Association Working Group on HMM with results published in a Technical Report [4]. Its findings didn’t show that the discharge from 61000-4-2 discharge guns was repeatable enough to become a Standard Test Method. There are several reasons that repeatability and reproducibility of discharge guns have been questioned, which include:

Guns produce large electromagnetic pulses which introduce additional stress currents
Electromagnetic pulses are unshielded and their reflections from operators and work bench are uncontrolled
Non-reproducible positioning of discharge gun (unless robotically held)
±30% waveform current variations are allowed in the waveform specification for discharge guns
Gun impedance to test fixture board mismatch causing reflections of significant energy
Gun ground return path is not controlled

The HMM Standard Practice also includes a “cable discharge” test system which is an alternative to the discharge guns. This test method uses a pulse generator similar to a TLP system [5] which can be highly repeatable and the pulses are delivered through 50 Ω coaxial cables. The cable delivery of a repeatable pulser addresses all of the problems found in discharge gun testing.

GTS provides only the cable delivery HMM systems as they are the only reliable way to delivery and verify the stress pulse actually meets the current waveform of the IEC 61000-4-2 standard.

HMM engineering information

The PurePulse HMM pulser is capable of producing a current pulse meeting all the criteria of the IEC 61000-4-2 standard waveform. In the IEC specification the test load is 2 ohms, and the GTS system can be verified with a resistance of 2 ohms or a 1 ohm impedance current probe. The PurePulse HMM pulser also monitors and records the current from every pulse; low impedance ICs usually produce “in-spec” current waveforms.

DC leakage and curve tracing is available to determine the stress levels that induce permanent DUT characteristic changes. Very flexible definitions of failure criteria are user selectable as part of the automatic failure level detection.

Engineering studies are also aided with the DUT voltage measurements possible in all PurePulse systems. The DUT Clamping Voltage information is often quite useful in determining failure mechanisms.

PurePulse HMM Specifications

1 kV to 16 kV stress levels are settable to the resolution of 1 V. This is roughly equivalent to gun contact stress of the same voltages. Gun air discharges have an even larger variation of stress due to effects of humidity and air temperature. Air discharges are of twice the contact voltages are typically used.

While the IEC specification allows ±30% tolerance on current amplitudes GTS maintains a calibration with tolerance of better than 10%.

PurePulse test systems are compatible with wafer, bare die, chip scale packages, standard IC packages with leads, land and ball grid arrays, and multi-chip carriers. Testing without sockets using mechanical switching eliminates test fixture boards and relay matrices with their parasitics which distort pulses and have been shown to produce false failures.

PurePulse systems (offering HBM, MM, TLP, VF-TLP, UF-TLP and HMM) can be manual, probe station based or fully automated test stations. Smaller systems can be upgrade and pulse sources added so PurePulse can expanded to meet your testing needs.

Summary

PurePulse HMM has these advantages

Meets the ESDA HMM Standard Practice
Built-in current monitor verifies every pulse
Recordings document the stress current passed through the DUT
DUT voltage recorded during stress
Testing up to 16 kV (equivalent to IEC 61000-4-2 contact discharge at 16 kV)
Simple set up and operation
Extendable testing to include HBM, MM and TLP

[1] The Standard Practice can be purchase from the ESD Association, see https://www.esda.org/standards/esda-documents/view/1571

[2] US Patent 8,278,936

[3] Air discharge is not used in HMM.

[4] https://www.esda.org/standards/esda-documents/view/1746

[5] Transmission Line Pulser, see https://www.esda.org/standards/esda-documents/view/1591