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Equivalent Life of Temperature Cycle Testing

          

What is the equivalent life in the field of a Temperature Cycle Test (TCT)?

If a device passes TCT, what will its expected life be in the field?                      

      

There seems to be no single absolute answer to these questions.  The answer that we most frequently encounter for this question, however, is what will be discussed here.

   

The temperature cycle test is performed to determine the resistance of a device to alternating temperature extremes.  It consists of exposing the device to many cycles (usually 500 to 1000 cycles) of high and low temperatures, and primarily tests the device's resistance to fatigue failure ('fatigue' is defined as the failure due to cyclical loads). Needless to say, the higher the number of temperature cycles that a device passes, the longer is its equivalent life in the field.  The question, however, is how long is long. 

   

When finding the equivalence of temperature cycling to field lifetime, one has to consider two components of the equivalence:  1)  the equivalence of the number of TCT cycles performed on the device in the lab to the number of temperature cycles that the device will see in the field; and 2) the equivalence of the number of temp cycles that the device sees in the field to the device lifetime. 

   

The first step, which is correlating the number of 'lab test' temp cycles to 'field' temp cycles, is based on an acceleration factor (AF) equation.  One commonly used AF equation is given as follows:

   

AF = (Taccel/Tuse)m where:  Taccel = Tmin(accel) - Tneutral; Tuse = Tmin(use) - Tneutral; and Tneutral = zero stress temperature (approx. 175 deg C for plastic packages - the temperature at which the package is molded)). Tmin(accel) is the minimum temperature used for lab temperature cycling while Tmin(use) is the 'average' minimum temperature that the device will see in the field. The value of the fracture-property dependent constant m depends on the failure mechanism, and is usually set to 20 for plastic package cracking. 

   

AF is the ratio of the number of 'field' temp cycles to the number of 'lab test' temp cycles.  As an example, if the 'lab test' TCT employs a low temperature of -65 deg C and the device will see (on the average) a minimum temperature of 0 deg C in the field, then AF = (-240/-175)^20, or AF =  554.  This means that a single cycle of the lab TCT (where the minimum temperature is -65 deg C) is equivalent to the device experiencing 554 cycles in the field (where the minimum temperature is 0 deg C) as far as packaging cracking is concerned.

  

Another common AF equation used for temp cycling is as follows:

   

AF = Nuse/Ntest = C(Tuse)-n/C(Ttest)-n where Tuse is the difference between the maximum and minimum temperatures that the device will see in the field within one 'cycle', and ∆Ttest is the difference between the maximum and minimum temperatures used in temp cycling.

    

Once the equivalent number of 'field use' cycles is determined, one can proceed to the next step, which involves estimating how many 'field use' cycles are equivalent to every year of field use.  This is the more tricky (and more subjective) part of the process, since one has to make several assumptions in order to accomplish this.  For example, a device used under the hood of a car may be assumed to undergo X temp cycles a day if the car is expected to be parked and driven X times a day.  This is equivalent to 365 X  'field use' cycles in a year.

    

Finally, one has to combine: 1) the equivalence of 1 lab cycle to 1 field cycle, and 2) the estimated # of field cycles in a year, in order to obtain the estimated equivalence between 1 lab cycle and the expected life of the device.  To illustrate this, assume that the car above will be driven and parked 10 times a day, or 3,650 times a year.  Also assuming as above that each lab cycle equals 554 'field' use cycles, then a device that passes 1000 lab TCT cycles will pass 554,000 'field' cycles.  As such, it is not expected to fail due to 'field use' temp cycling-induced package cracking within 150 years!  Note that the numbers used in the example discussed here are purely hypothetical to simplify the discussion, and may not be reflective of what the actual figures would be.

 

   

See Also:  Temperature Cycle Test; Reliability Modeling

      

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