Hi Bob,
>With respect to creep-strain relaxation fatigue of solder joints,
>physically what does the effect of low temperature dwells (under -20C)
>have on this mechanism? I visualize thermal cycling as puting stresses
>on the solder joint that alternatingly pull or push on the joint, thus
>causing creep strain to occur in one direction followed by creep strain
>in the other direction. However, when subjected to a dwell at cold,
>creep is negligible, so basically creep will only occur in one
>direction. (I understand that other than fatigue failure mechanisms come
>into play when very cold extremes are applied, but my question mainly
>deals with creep-stress relaxation fatique.)
Do NOT visualize the effect of T-cycling in terms of stress--wrong mental
picture; rather, visualize it in terms of displacement. It is the
visco-plastic strain energy per cycle (=size of the stress-strain hysteresis
loop) that determines the fatigue damage. At temperatures above about -20C
solder is not strong enough to resist displacement unless the attachment is
very soft (compliant leads or soft internal component structure [microBGA]).
For most leadless components the yield strength of the solder is the upper
stress-limiting parameter (remember, yield strength for solder is highly
temperature-dependent) and therefore the strain range is a variable and the
stress range is not, and the fatigue damage can be successfully estimated by
the strain range alone. For most of the microBGAs, this is no longer true
because the stresses in the solder joints are too low to be bound by the
yield strength. Thus, both the strain range and the stress range become
independent variables; just like with compliant leads.
It is incorrect to assume that the dwell at -20C is unimportant--the
stress-strain hysteresis loop has a small stress-range at the higher
temperatures, but a rather large one at lower temperature; thus, even small
amounts of plastic strains (either from yielding or creep) multiplied by the
large stress-range represent a large amont of visco-plastic strain energy.
>I understand the intent of what IPC-SM-785 equation 17 provides, but I
>do not understand how it was derived. Can you give any guidance on that?
>Using equation 17, does it make sense for leaded case of n =32, B = 2,
>with using the consumer category: td(use) = 720, td(test) = 15, etc.
>Thus c(use) = -0.4574 and c(test) = -0.4235. Resulting in N(test,3%) =
>1.157 N(test, 50%). So test time is 16% longer than mean time.
>In equation 17, as x gets larger, the equivalent test time gets smaller.
It's been too long I have not been able to locate my notes on this--you need
to go to basic Weibull and multiple sample statistics.
However, you are Eq. 17 incorrectly. As I have said before, Eq. 17 determines
the number of failure-free cycles you need to pass to assure an acceptable
failure probability of no more than x% given the number of samples, n, on
test, based on the the design life given as N(use, 50%). You first convert
N(use, 50%) to N(test, 50%). Than you determine the number of failure-free
cycles you need to pass to assure the acceptable failure probability of no
more than x% (much less than 50%) given the number of samples, n, on test.
That means, as the number of test samples, n, increases, the number of cycles
to be passed failure-free, N(test,x%), decreases. As the acceptable failure
probability, x%, increases, the number of cycles to be passed failure-free,
N(test,x%), decreases.
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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