A small clarification: The most common term used to discuss random vibration environments in the electronic industry is power spectral density [P] [or mean squared acceleration density]. Power spectral density is expressed in squared acceleration units per hertz [G^2/Hz]. Here, "G" is the acceleration expressed in gravity units. Gravity units are without dimensions [dimensionless???] because, G equals the measured acceleration [a] divided by the acceleration of gravity [g]. In this parlance, an acceleration level of 10g is 10X the acceleration of gravity. Typically, random
vibration measurements of acceleration, velocity, and displacement are in terms of root mean squared [RMS].
Dave Fish
"Ingemar Hernefjord (EMW)" wrote:
> Of course other gurus are welcome to comment following question, but Werner seems to be one of those most familiar with life prediction of solder joints and specs.
>
> We have mounted superBGAs, 3 peripherical rows, 600 I/Os, chip down, copper top plate, on SMD multilayer FR-4. We are afraid that somewhere among the produced boards we may have some boards with weak solder joints, probably classical ultrathin weak interface layer between Sn3Ni4 and bulk Ni. PWB is ENIGd. Balls are 95/5, solder is NoClean Sn60. Process is stencil and oven is conveyor N2 heat oven. Cleaning is Citriclean. In IPC-D-279 app. A you can read about the SMD concerns with starts of cracking, epoxy outbleeding caused by hot solder agains solder mask etc. Now the question.
>
> Q: In A-7.2 Random vibration screening of weak bonds should be done for 10-20 minutes and loaded with 6-10 grams, whole run at -40C. Does not affect good joints, but reveals weak ones. Not easily done, as I can see.
>
> a) what is 6-10 grams loading? Is that momentary top load in grams per BGA solder joint or do you instead mean 6-10 g's?
> b) is this for dummie testing daisy chains, or really meant for produced boards at 10,000 each cost?
> c) general acceleration input on whole board or measured on individual superBGA?
>
> Mebbe a question for Lockheed, Sandia, Rome or like instead, but let's see...
> Thanks in advance
>
> Ingemar Hernefjord
> Ericsson Microwave Systems
>
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