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Strain (Grid 1) Strain (Grid 2) Strain (Grid 3) Max. Principal Strain Min. Principal Strain Shear Strain
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FIGURE 59.11 Strain versus time plot of an actual ICT fixture as it is engaged, the test is performed, and the fixture is subsequently disengaged. The sharp increase in strain during engagement and disengagement is the source of the high strain rate. The fixture is engaged at high speed to improve electrical contact between the probe pins and the PWB test pads.
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high strain/strain rates is ICT. Once the ICT fixture is engaged, there is a sudden spike in strain (high strain rate), which is again repeated when the fixture is disengaged, as shown in Fig. 59.11. Based on experimentally generated data on the production floor and in controlled fourpoint bend tests described earlier, guidelines can be developed on the maximum range of strain for a given strain rate that would be acceptable for a package in different production operations. A typical example of such a guideline is given in Fig. 59.12. It is important to note that this is just a sample guideline. 59.3.1.2 Effect of Lead-Free Conversion. Because of the conversion to lead-free solder, the intermetallics formed at the package-joint and PWB-joint interface would be different from those in Pb/Sn assemblies. Consequently, the response to bending of a lead-free assembly would be significantly different from that of a Pb/Sn assembly. There are relatively little experimental data in the industry comparing the mechanical reliability of lead-free soldered assemblies with Pb/Sn assemblies. One data set showing a relative comparison between Pb/Sn and SnAgCu for two different surface finishes is shown in Fig. 59.13.37 The results in Fig. 59.13 show the following:
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For ENIG surface finish packages, the average force-to-failure value for the SnAgCu assemblies is almost three times that of the Pb/Sn assemblies. This is because the SnAgCu joints are stiffer than the Pb/Sn joints. Failure mode A is significantly more pervasive in the Pb/Sn assemblies but not as much in the SnAgCu assemblies. Thus, the propensity for brittle solder joint fracture is higher in the Pb/Sn assemblies with ENIG. The introduction of a small amount of copper in the Pb/Sn assemblies tends to increase the bending strength of the Pb/Sn assemblies by more than two times. The average force to failure with a solder-on-copper (SOC or SOP) surface finish is higher than that with ENIG, especially for the Pb/Sn solder joints. For the SnAgCu joints, there is a marginal increase in relative strength.
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Thus, mechanical bend strength is a function of the solder metallurgy and the surface finish used. These results indicate that SnAgCu solder joints are stiffer and have a higher average
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FIGURE 59.12 Typical example of strain/strain rate limits as a function of PWB thickness for BGA solder joint interconnect failure. Note that the acceptable strain limit decreases with increasing strain rate. Note also that for a given strain rate, the maximum acceptable strain limit decreases as the PWB thickness decreases. (Reprinted with permission from Sun Microsystems, Inc.)
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force-to-failure value compared to Pb/Sn joints with the same surface finish. Another important factor in the bend strength of solder joints is the time after reflow, as shown in Fig. 59.1437. The results in Fig. 59.14 show the following:
For ENIG surface finish packages, Pb/Sn joints are more sensitive to time after reflow than SnAgCu joints. The average force-to-failure value of Pb/Sn joints with ENIG increased by more than 50 percent, whereas the average force-to-failure value of SnAgCu with ENIG did not change noticeably over time. For solder pre-coat (SOP) packages, the strength of Pb/Sn joints increased by slightly more than 10 percent over three weeks, whereas the strength of SnAgCu joints with SOP does not change noticeably over time.
Thus, mechanical bend strength is a function of the time after reflow along with solder metallurgy and surface finish. The underlying failure mechanisms have not been understood clearly enough to explain the dependence on time after reflow.
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