PLS Tests Descriptions
4066 TEST DESCRIPTION
The 4066 test is intended to allow the user to perform method 4066 of MIL-STD-750. Please refer to the official specification for details and to ensure compliance. The following describes the way the FEC tester works.
When you have selected the 4066 test, you will be asked to also choose between -1, and -2 options.
The -1 option provides I/O bias to the diode in the form of 8.3mS half sine pulses with an average value of I/O. On alternate half cycles, current limited constant voltage half sines are provided in the "Reverse" direction.
The -2 option provides I/O bias as DC with no reverse bias.
PARAMETERS
Note: All timing for 4066 tests is measured and programmed in units of "Cycles" which refers to power line cycles. The line frequency is assumed to be 60 Hertz although the output of the tester is independent of the actual power line frequency. One cycle therefore is assumed to be 1/60 second or 16.7mS.
DUTY 1 in n
This refers to the ratio of "bias" cycles to SURGE pulses. For example, if "Rest" is 0 and Duty is 3599, then each repetition of the test takes 3600 cycles or 1 minute.
Rest cycles
If you don't want the bias to be applied continuously between SURGE pulses, you can program a "rest" period of no bias. For example, if "Duty" is 3 and "Rest" is 3596, then the total time is still one minute, but the bias is on for only 3 cycles.
# of repetitions
You can specify the number of times that the above test will be performed. For example, you could set this to 5. The program above would be repeated 5 times for a total time of 5 minutes, during which 5 SURGES would occur.
Average I/O
This is the "Forward" bias current. For -2 it is DC. For -1 it is the average value of the 8.3mS half sine pulses.
VRM voltage
For -1 only. This is the peak voltage of the applied 8.3mS reverse voltage pulses applied alternately with the I/O pulses.
SURGE Current
This is the peak current of the single half cycle Forward current pulse applied at the end of each repetition. On the final SURGE of any 4066 test, the voltage corresponding to the peak current is measured and displayed. It may also be compared with minimum and maximum limits. If the reading is reported as "Invalid" that means that the tester was unable to provide the current programmed.
VRSM voltage
For -1 option only. This is a single (usually higher) final Reverse voltage pulse, provided at the end of each repetition. This will be the final pulse (after the SURGE).
Notes: The Reverse power supply is current limited with a large series resistor. It will not be damaged, by working into a shorted load. The maximum current supplied to a failing diode under test will be in the order of 100 to 20 mA.
The Forward supply since it is expected to supply a very high peak SURGE current is sensitive to the effective series resistance of the wiring to the test socket and the socket itself. It is also, affected by the amount and duration of the bias current flowing before the SURGE. The compliance of the supply is checked during the SURGE and if the current fails to follow the program, the final VF reading will be marked "Invalid".
4066 TESTING (Details)
In the 4066 mode there are the following choices to be made. This can be done from the keypad or with the companion software.
- Mode 1 or 2
Mode 1 IO is alternate half cycles of Average IO and Peak reverse voltage (VRM).
Mode 2 IO is DC amps (no reverse voltage).
- Average IO
Average IO is the average background forward current. - Surge Current
Surge Current is peak surge pulse current (once per rep.) - VRM
Volts
VRM is peak reverse voltage - VRSM Volts
VRSM is peak reverse voltage surge (also once per rep) (mode 1 only). - DUTY and REST Cycles
The time between surge pulses is the total of DUTY plus REST. Each of these is programmed in units of 1 power line cycle (1/60th of a second). For example, if DUTY is 3600 and REST is 0, there will be 1 SURGE per minute. Or if REST is 1800 and DUTY is 1800, there will still be 1 pulse per minute but the IO will be on for just the last 30 seconds. - REPS
The above may be repeated as many times as you wish. If REPS is 5 and DUTY is 3600 there will be 1 SURGE per minute for 5 minutes. - LIMITS
The "Test limits" in the menu refer to the peak VF during the FINAL SURGE. This reading will be displayed also.
THERMAL RESPONSE TESTS
There are two tests in the PLS series testers for THERMAL RESPONSE. They are DVF and THETA. The TR test (THERMAL RESISTANCE) is described elsewhere.
DVF and THETA are substantially the same as far as the hardware measurement is concerned. DVF reads out in mV and THETA in °C/Watt.
Both tests measure DVF, but THETA uses some additional data supplied by the operator as well as an additional VF test, to calculate the ºC/W value.
PARAMETERS
- IM
IM is a small Forward current used to measure VF both before and after a heating pulse at a much higher current. - DVF
This is the difference between the VF readings before and after the heating pulse. - IH (programmed as "Forward current")
This is the current used for the heating pulse. - TH (programmed as "Pulse width")
This is the pulse width of the heating pulse. - TMD
TMD is a small delay after the end of the heating pulse to allow time for the diode and the power supplies to settle back to IM before reading the second VF. - 1/K
This is expressed in mV and is the change in VF vs junction temperature. The unit is mV per ºC. You must know and enter this value in order for the tester to calculate the correct value of THETA.
Notes:
VF at IH is measured and remembered during the THETA test in order to calculate the heating power (W) which is VF * IH.
∆T is DVF(MV) * 1 /K.
THETA is ∆T/W
PROGRAMMING HINTS
In some cases the registered specifications for a part will dictate the test conditions. The following hints would then be used only to confirm that the test is working properly.
If you are able to choose your own test conditions, these hints will be most useful. The parameters that you should choose carefully are IH, TH, IM, and TMD
The combination of IH and TH will determine the maximum junction temperature. If the temperature is too low, the resolution of the test is poor with poor repeatability. If the temperature gets too high (even below the point of damage to the part) the readings will be inaccurate due to changes in various device parameters. We recommend a ∆T of no more than 100ºC (40-50 would be better).
TH should be high enough for the heat to pass from the junction through any internal bond that you wish to check. If it is too narrow the result, will not be affected by a bad bond. If it is too wide the heat will be flowing into the package and at best this will dilute the effect on the reading of a bad bond. At worst a long pulse will make the reading dependent on the usually uncertain heat-sinking of the part to its environment.
TMD is a very critical parameter. Ideally TMD would be zero so that the part has no time to cool after the heating pulse. This is not practical, since both the diode and the tester require time to make the transition from IH to IM.
The software program VFS2 contains a function called "Cooling Plot" which plots DVF or THETA vs TMD. This allows you to observe the response of the current setup and find the shortest TMD that will produce reliable readings. You could do this plot manually from the control panel, but that would be tedious.
If you observe how DVF or THETA increases smoothly with decreasing TMD it will be easy to see where the response is effected by the switching transients. You can then choose a TMD that is as low as possible but safely longer than the switching time.
IM:
Many specifications call for an IM of 1mA. If the cooling plot, shows good response at the required IM and TMD that is good. However if the response is poor or marginal you should consider using a higher value of IM. If you know the value of 1/K for the higher TMD, you will get a correct result and you will also find the switching time to be lower and the repeatability better.
THERMAL RESISTANCE TEST
The THERMAL RESISTANCE (TR) test measures the steady state Thermal Resistance from junction to heat sink. The test is similar in most ways to the THERMAL RESPONSE tests (which see). This sheet deals with parameters and issues specific to THERMAL RESISTANCE.
TR always reads out in ºC/W
Since this test measures all the way out to the heat sink, the mounting of the part to the sink is a very critical part of the test. In most cases the readings will be significantly effected not only by the heat sink itself but by the torque used to fasten the part and by any heat sink compound or other fluid used to fill any voids.
Heating of the heat sink itself may effect the test significantly, so this is a parameter that should be monitored.
The time duration of this test is determined in one of two ways for valid tests and a third parameter may terminated the test prematurely with an "Invalid" output.
You can program the time in seconds with the "Max. time" parameter. The user can also stop the test by pressing any key on the keypad.
If the calculated ∆T is too high (greater than 100ºC) the test will terminate with an "Invalid" readout.
VF and SURGE TESTS
Both of these tests read VF at a programmed peak current. The VF test uses a rectangular pulse with programmable pulse width. The SURGE test provides an 8.3mS half sine pulse.
The VF test measures voltage at the end of the rectangular pulse and the SURGE test measures voltage at the time of maximum current.
Both tests check compliance to be sure that the programmed current is flowing. Since this current may be very high, high resistance test leads or test socket could result in non-compliance. If this occurs the readout will be marked "Invalid".
The VF test may be programmed over a wide range of current and time. The software and firmware will enforce a derating of current for wide pulses.
Since both of these are single pulse tests, you will be able to use the maximum peak current rating of the tester for SURGE or for narrow VF pulses.