Introduction
In the semiconductor production test world, the maxim “Throughput is King” is as true today as when the term was coined decades ago. The underlying assumption is that the test “adequately” evaluates the operation and functionality of the UUT. Special emphasis is given to the word "Adequate" because the typical practice is to perform a robust, comprehensive functional test and characterization for design verification, and then perform a subset of those tests on the production line. Once a design has been verified, it is usually not necessary to functionally test every aspect of each device, or fully characterize the AC and/or DC parameters of each device during the production test cycle. An adequate subset of the functional and parametric tests are selected such that, if the UUT passes these tests, you have high confidence that the component will function correctly in its intended application.
Limit Testing
By limiting the number and scope of the tests performed, test time can be significantly reduced. And, as “time is money”, reducing test time can save money, improve production yields, and directly impact the profitability of a device in the market. But limiting the number and scope of tests performed during production is not the only way to reduce test time. Limit testing can also be an effective time saver that will improve yield without sacrificing quality. Limit testing is the practice of testing only to a minimum or maximum value to validate device parameters. For example, if a device is specified to provide a minimum 2.5V output under a specified load, it is not necessary to test the device to 2.6V. Nor is it necessary to actually measure the output voltage. So long as the device meets the minimum output threshold (or limit), it is considered within specification.
Testing to a specific limit or threshold, rather than measuring an actual value, can improve test time by utilizing comparators for the limit test, rather than making a measurement. The output of a comparator provides an instantaneous pass/fail indication of whether the parameter meets the minimum (or maximum) allowable value for the parameter being measured. A measurement is not only slower than using a comparator, but requires that the measured value be evaluated against the pass/fail criteria. Typically this evaluation is performed in software, which is slower than a hardware comparison.
GX5295 Limit Testing
The GX5295, from Geotest - Marvin Test Systems, Inc., supports both methodologies. It has a full-function Parametric Measurement Unit (PMU) per pin, capable of forcing voltage or current, and measuring current or voltage, respectively. It also provides a dual-threshold comparator for each pin with minimum and maximum thresholds that can be used to validate whether or not a DC parameter meets an acceptable limit. By providing a dual-threshold comparator per pin, the GX5295 can be programmed with a unique minimum AND maximum limit for EACH pin, and then simply select for each pin which comparator output to use for a limit test. The output of the comparator is binary, “0” or “1”, “pass” or “fail”, “good” or “bad”. No measurement is performed, no evaluation of the measured value is needed. Compliance for the parameter is known immediately. Further, comparators can run at the maximum rate of the GX5295 and has minimal impact on throughput.
A block diagram for one GX5295 channel input is shown below. Each channel can be independently configured to operate in Digital I/O mode (DIO) or Parametric Measurement Unit mode (PMU). The DIO mode is useful for performing standard digital functional test; supporting both Digital Stimulus/Response and Digital Real-Time compare. The PMU mode is useful for making DC measurements or validating DC limits.
In PMU mode, the GX5295 can force a voltage or a current on each channel, and then measure the respective current or voltage. The measured property can also be passed to the PMU comparators to be used in limit testing. Since each channel has fully independent control over the high and low limit values for it's comparator, as well as the force property and value, a complete DC validation of all input pin thresholds and output pin levels can be performed in a single measurement cycle. For bidirectional channels, two cycles would be required. Features and specifications for the GX5295 PMU are provided in the Table below:
Parametric Measurement Unit (PMU) Specifications:
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| Number of PMUs | 32, one per data channel | | 4, one per auxiliar channel (for timing / control functions) | | Modes: | Force voltage, measure current | | Forcecurrent, measure voltage | | Force Voltage Range: | -1 volts to +6 volts | | Force Accuracy: | +/- 25 mV | | Force Voltage Resolution: | 16 bits | | Force Current Range: | +/- 32 mA FS, +/- 8 mA, +/- 2 mA, +/- 512 uA, +/- 128 uA, +/- 32 uA, +/- 8 uA, +/- 2 uA FS | | Force / Measure Current Accuracy: | +/ 160 uA, 32 mA range | | +/- 40 uA, 8 mA range | | +/- 10 uA , 2 mA range | | +/- 2.56 uA, 512 uA range | | +/- 640 nA, 128 uA range | | +/- 160 nA 32 uA range | | +/- 40 nA, 8 uA range | | +/- 10 nA, 2 uA range | | Measure Voltage Range: | -2 volts to +7 volts | | Measure Voltage Accuracy: | +/- 5 mV | | High & Low Voltage Clamps: | VCLo: -2 to +5 V | | VCHi: 0 to +7 V | | Voltage Clamp Accuracy: | +/- 100 mV |
The DLL API functions for utilizing the GX5295 PMU functions are:
DioGetChannelMode()
DioGetInputLoadCurrent()
DioGetInputLoadCommutatingVoltage()
DioGetInputLoadState()
DioMeasure()
DioPmuGetComparatorsSource()
DioPmuGetComparatorsStates()
DioPmuGetComparatorsValues()
DioPmuGetComparisonBooleanResult()
DioPmuGetComparisonResults()
DioPmuGetForcedCurrent()
DioPmuGetForcedCurrentCommutatingVoltage()
DioPmuGetForcedVoltage()
DioPmuSetupComparatorsSource()
DioPmuSetupComparatorsValues()
DioPmuSetupForcedCurrent()
DioPmuSetupForcedCurrentCommutatingVoltage()
DioPmuSetupForcedVoltage()
DioSetupChannelMode()
DioSetupInputLoadCurrent()
DioSetupInputLoadCommutatingVoltage()
DioSetupInputLoadState()
Example
A example test setup and code for validating the output drive levels of a UUT are provided below. The UUT is a 10-bit full adder. There are 20 input signals mapped to GX5295 channels 0:19; Ain[0:9] and Bin[10:19]. There are 11 output signals, 10 bits for the sum and one bit for the carry. these signals are mapped to GX5295 channels 20:30.
Example ATEasy Code for limit testing UUT outputs:
! Define DIO output as force voltage (drive UUT inputs)
Dio Setup Channels Mode PmuForceVoltageMode RangeOfChannels(nHandle,0,19)
! Set default voltage to 0V for DIO outputs - logic '0' for UUT Ain and Bin inputs
Dio Setup Channels PMU ForceVoltage RangeOfChannels(nHandle,0,19,0,1,0,1)
! Define DIO inputs as force current mode (load UUT outputs)
Dio Setup Channels Mode PmuForceCurrentMode RangeOfChannels(nHandle,20,30)
! Set force current to 1mA load using the 2mA range
Dio Setup Channels PMU ForceCurrent RangeOfChannels(nHandle,20,30,1,2)
! Set high/low commutating voltage to +3.5V and -1V, respectively
Dio Setup Channels PMU ForcedCurrentCommutatingVoltage RangeOfChannels(nHandle,20,30,3.5,-1)
! Set channels 20 - 30 for comparator mode (limit testing)
Dio Setup Channels PMU ComparatorsSource RangeOfChannels(nHandle,20,30,aPmuComparatorsSourceIoVoltage)
! Define High/Low input comparator limits to 1.5V/0.5V, respectively
Dio Setup Channels PMU ComparatorsValues RangeOfChannels(nHandle,20,30,1.5,0.5)
! Apply logic '0' (0V) to UUT Ain inputs (Bin is also '0') - force UUT sum outputs to '0'
Dio Setup Channels PMU ForceVoltage RangeOfChannels(nHandle,0,9,0,1,0,1)
! Return high/low comparator results in arrays adwHighStates and adwLowStates
Dio Get Channels PMU ComparatorsStates(nHandle,False,adwHighStates,adwLowStates)
! Apply logic '1' (3.0V) to UUT Ain inputs (Bin inputs are '0') - force UUT sum outputs to '1'
Dio Setup Channels PMU ForceVoltage RangeOfChannels(nHandle,0,9,3,1,0,1)
! Return high/low comparator results in arrays adwHighStates and adwLowStates
Dio Get Channels PMU ComparatorsStates(nHandle,False,adwHighStates,adwLowStates)
Summary
Using comparator-based limit testing improves test throughput for DC parametric tests by streamlining the procedure for validating DC parameters of the UUT, without sacrificing quality or coverage.
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