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Strain gage testing: the delta effect of thermal cycle testing
in compression. There is another signifi- Y-X over temperature points this a silent killer over time in an acceler-
cant finding - it is that the degree of slope and this led to higher degree ated thermal cycle test and also equate to a
on Location A was far greater than B and microstrain rate changes higher degree of risk in a fielded state.
the time duration of the stress being ap- between temperature points
plied was greater on A than B. This graphi- (degree slope change)
cal representation of forces being applied 3. Time duration at a higher acknowledgements
allowed one to capture the silent killer over strain rate The findings of this study could not have
time of applied stress and its duration on been accomplished without the support of
Slope analysis:
adjacent component locations. This subtle the STI Analytical Lab. The author would
Temperature fall from 105 to -45
change in location and the applied forces like to acknowledge the efforts of Marietta
Loc. A: + 80.59 microstrains per
can make all the difference between meet- Lemieux, Aaron Olson, and Jonnie John-
minute
ing warranty expectations and excessive son for support of the testing and analysis
Loc. B: +57.59 microstrains per
field failures over time. When designing a of the assemblies.
minute
system, it is sometimes the smallest details
Slope Differential: 22.64
such as part type and location that can
microstrains
have the greatest impact on reliability and references
Temperature fall from -45 to 105
meeting warranty and life design objec- 1. IPC/JDEC—9704 Guideline
Loc. A: – 54.43 microstrains per
tives. Location A has three distinct differ- Document
minute
ences over Location B which led to this 2. IPC—TM-650 Number 2.6.9
Loc. B: – 39.85 microstrains per
failing the thermal cycling test: Vibrational Test Guideline
minute
1. Maximum difference between Slope Differential: 14.58
maximum compression and microstrains
tension of 1445 microstrains
These subtle differences combine to make
2. Higher delta strains between
Chart A. Chart of raw data spread by Temperature vs. Microstrain by location in the X- and Y-axis (continues onto next page).
Time Chamber LOC-A LOC-A Y- LOC A LOC B LOC B Y- LOC B New LOC New LOC New LOC
Elapsed Temp (ºC) X-Axis (µε) Axis (µε) Y-X (µε) X-Axis (µε) Axis (µε) Y-X (µε) A X-Axis A Y-Axis A Y-X (µε)
(µε) (µε)
0:00:00 105 -112 -774 -662 -271 -841 -570 33 -350 -383
0:02:00 105 -116 -775 -659 -272 -845 -573 35 -358 -393
0:04:00 105 -118 -767 -655 -254 -849 -595 45 -367 -412
0:07:00 105 -110 -781 -701 -232 -851 -619 74 -379 -453
0:09:00 103 -104 -786 -682 -240 -844 -604 64 -373 -437
0:11:00 88 -100 -778 -654 -245 -843 -598 51 -369 -420
0:13:00 71 -97 -770 -673 -244 -839 -595 38 -353 -391
0:15:00 67 -110 -770 -660 -267 -830 -563 31 -333 -364
0:17:00 53 -105 -753 -648 -271 -816 -545 20 -308 -328
0:19:00 43 -90 -746 -656 -262 -788 -526 12 -278 -290
0:21:00 25 -69 -732 -663 -258 -744 -486 1 -246 -247
0:23:00 20 -51 -706 -655 -260 -698 -438 1 -227 -228
0:25:00 6 -46 -656 -610 -271 -635 -364 -20 -205 -185
0:27:00 0 -34 -584 -550 -282 -566 -284 -46 -200 -154
0:29:00 -17 -22 -468 -446 -289 -487 -198 -72 -195 -123
0:31:00 -32 -15 -325 -310 -304 -399 -95 -107 -199 -92
0:33:00 -39 -16 -177 -161 -325 -311 14 -144 -201 -57
0:35:00 -45 -26 -25 1 -350 -229 121 -185 -205 -20
0:37:00 -46 -43 125 168 -384 -144 240 -233 -213 20
0:40:00 -46 -63 297 360 -428 -44 384 -296 -226 70
0:43:00 -43 -89 413 502 -464 25 489 -352 -239 113
0:46:00 -45 -114 495 609 -494 75 569 -393 -246 147
16 – Global SMT & Packaging – February 2009 www.globalsmt.net
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