Hi David,
The difference between T-shock and T-cycling is the rate at which the 
temperature transition occurs.
In T-shock the rate of temperature change is more than about 30C/minute 
(typically much more) which causes loading conditions on the solder joints 
(SJs) significantly different than in actual product use;  (1) temperature 
gradients are large enough to cause significant warping of components and/or 
circuit boards (putting tensile stresses on SJs), and (2) strain loading 
rates large enough that the stresses increase in the solder more rapidly than 
the stresses cab be reduced by creep. T-shock is NOT a reliability test, 
because failures occur as the result of different failure mechanisms than in 
product use; that has of course not stopped people from using T-shock for 
this purpose because of the shorter test times to failures (the most infamous 
examples of misuse are the MIL-tests). T-shock is a good test method for 
environmental stress screening (ESS) to screen out defective product.
In T-cycling the rate of temperature change is less than about 30C/minute; 
this prevents (1) temperature gradients resulting in component/board warpage 
and (2) SJ stress buildup. This is what is recommended for accelerated 
reliability testing in IPC-SM-785. However, proper testing will take longer 
to cause failures. This has caused the setting of inadequate number of cycles 
to be passed for qualification of product leading to false claims of 
reliability for some of the more severe (severe environment——automotive, long 
life——satellites) applications.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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