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FIGURE 59.7 Weibull plots for a CBGA package cycled under two different loading conditions. Solid triangles and the dotted line are employed to indicate failures and associated failure distribution of a CBGA package cycled from 20 to 80 C (the least severe case presented in this figure). Solid squares and the dashed line indicate the failures and associated distribution for the identical package cycled from 0 to 100 C. Notice that an increase in the severity of the thermal cycle loading conditions (from 20 to 80 C to 0 to 100 C) creates an associated decrease in fatigue life. (Reprinted with permission from J. Lau, Ball Grid Array Technology, McGraw-Hill, 1995, p. 162.)
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59.2.4.4 Modified Norris-Landzberg Acceleration Transform for Lead-Free Solder. Based on experimental data generated on a select variety of packages, Pan et al. proposed an updated form of the Norris-Landzberg acceleration transform (see Eq. 59.15).23 AF = where NfF N fL T = L TF
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AF = acceleration factor tF = dwell time at high temperature in the field tL = dwell time at high temperature in the lab TF = maximum temperature of the thermal cycle in the field (K) TL = maximum temperature of the thermal cycle in the lab (K) TF = difference between maximum and minimum temperature of the thermal cycle in the field TL = difference between maximum and minimum temperature of the thermal cycle in the lab
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The primary differences between Eq. 59.14 and Eq. 59.15 are that the frequency term is replaced by the dwell time at high temperature, and the constants have been revised. This equation has been shown by Pan et al. to predict well within the temperature range of 0 to 100 C. However, predictions are less accurate for temperatures outside of this range.23 Although the aforementioned proposed acceleration transform is a first-order estimate, much more data would be needed to validate or improve this estimate. Several additional variables must be evaluated to study their impact on the acceleration transform. Some of the variables that impact the acceleration transform include the following:
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Effect of different package types The differential strain imparted on solder joints in a ceramic flip-chip ball grid array (FCBGA) is quite different from that imparted on a thin small outline package (TSOP) package, for example. As a result, the acceleration factors from one may not necessarily apply to the other. The construction and effective CTE mismatch of the packages to be qualified must be as close as possible to the packages tested.23 PWB thickness effect A thick board is likely to constrain a package more rigidly than a thin board. Similarly, the number of layers in the board could significantly alter its overall stiffness. Tests are frequently done on thinner boards because they are less costly and easier to fabricate. However, if the end-use condition entails the use of a thicker board, IPC9701 recommends performing the test on at least two different board thicknesses so that the data can be extrapolated to a range of board thicknesses. The effect of board stiffness on lead-free packages appears to be similar to that on Pb/Sn packages, as shown in Sec. 58.3.3 of Chap. 58. Cycling frequency As evidenced in the study performed by Pan et al. 23 and by Clech,24 the temperature cycling frequency could have a significant effect on the reliability of the packages being tested. Both the ramp rate and the dwell time affect the results. Most of the damage accumulation during temperature cycling occurs in the hot dwell part of the cycle. A reduction in dwell time reduces the amount of damage accumulated per cycle, which in turn means longer solder fatigue life. A long hot dwell would result in almost complete creep stress relaxation in the solder joints. Consequently, some studies have indicated that a longer dwell time could result in eventual creep saturation.25,26,27 Increasing the dwell time has shown some reduction in fatigue life in some studies, but the reduction was not very significant in some of those.28 In addition to hot dwell time, ramp rate of heating and cooling also impacts the effective fatigue life of lead-free solder joints. The amount of reduction in leadfree solder fatigue life due to ramp rate is a function of the package type. Packages with higher effective CTE mismatched substrates (e.g., CBGAs) are more sensitive to the ramp rate than packages with a less differential strain (e.g., plastic ball grid arrays [PBGAs]). Temp cycling range Clearly, the higher the temperature cycling range, the lower the fatigue life of the solder joints. Some studies have also indicated that an increased average temperature could also result in a reduction in the fatigue life of lead-free solder joints. In other words, both factors high maximum temperature and high average temperature could contribute to a reduction in solder fatigue life.24 The Distance from neutral point (DNP) effect This is the effective distance along the diagonal of the package, from the center to its outermost corner. Some studies have indicated that the DNP plays a significant role in altering the acceleration transforms of a leadfree package.24 Size and Orientation of tin (Sn) grains29 Depending on the cooling rate, Pb free solders containing a high percentage of Sn tend to have very large Sn grains with varied orientation. Sn has very anisotropic material properties, and consequently, the bulk properties of the Pb free solder joints could change significantly depending on the size and orientation of the Sn grains in any given solder joint. Consequently, the strength of the solder joints within a single BGA could vary significantly. In other words, the joints at the high stress regions may not be the first to fail. This random orientation of Sn grains within a solder joint poses a significant challenge in determining the fatigue life and acceleration factors of Pb free solder joints.29
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It has also been observed that while the ductility of Pb/Sn increases with aging, the ductility of lead-free solders tends to decrease with aging.30 In other words, the strain/strain rate characteristics of lead-free solders could change with aging. Further studies are also needed to quantify fully the aging effect and incorporate it into long-term field life prediction models. 59.2.4.5 Backward Compatibility. On small PWBs with relatively few components, it is possible to convert all the components to lead-free assemblies at once. However, in the transition to lead-free assemblies, there will be instances where not all the components on a complex
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