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a = difference in thermal expansion coefficients of component and substrate l = distance between center of component and joint h = height of solder joint
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If the component has leads or if the substrate is flexible, there will be some compliance in the system that will reduce the strain imposed on the solder joints. The local mismatch between the solder and the component lead or the pad or via metallization on the substrate can also contribute to the strains imposed on the solder. Like plated-through-holes (PTHs), solder joints fail by a low-cycle-fatigue mechanism which can be crudely approximated by the Coffin-Manson relation Nf = 1 2 ef e
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where, again, Nf is the number of cycles to failure, ef is the fatigue ductility, and m is an empirical constant near 2. However, unlike the PTH case, the number of cycles to failure also depends on the frequency at which the cycles are imposed and the hold time at each temperature extreme. The reason for this dependence is that, for solders, the primary deformation mechanism causing thermal fatigue failures is creep. The phenomenon of creep and its connection to fatigue are fundamental to understanding thermal fatigue of solder. Creep is time-dependent deformation that occurs gradually in response to a fixed imposed stress or displacement (see Fig. 57.13). Creep occurs by a variety of thermally activated processes. These processes play an important role only when the temperature exceeds half the melting temperature (in degrees Kelvin) of the material and, even then, the rate of deformation increases strongly with increasing temperature. For electronic solders, even room temperature is well above half the melting temperature; consequently, creep is the most important deformation mechanism of solder. When a displacement is first imposed, the strain is a combination of elastic and plastic strain. The elastic deformation is reversible and damages the microstructure relatively little, while the plastic deformation is permanent and contributes more significantly to the initiation and propagation of fatigue cracks in the solder (see Fig. 57.14). Given time, the creep process relieves some or all of the elastic stress through further permanent deformation. This additional deformation does further microstructural damage and increases the amount of plastic strain imposed when the thermal cycle is reversed. Because there is less time for creep to occur, rapid thermal cycles are less damaging than slow cycles or cycles with long hold times at the temperature extremes, a fact which is important in designing accelerated reliability tests as well as in service. The importance of creep makes the fatigue behavior of solder different from structural metals, such as copper, aluminum, or steel. In summary, the effects of the thermal cycling profile on 25 C solder joint thermal fatigue life are as follows: Temperature extremes: Decreasing the size of the thermal excursion is the single most effective way to increase the life of the solder joints. Since creep occurs more rapidly at higher temperatures, decreasing the peak temperature of the thermal cycle further decreases the amount of creep deformation that occurs during the hold at high temperature. Frequency: The thermal fatigue damage per cycle is greater at lower cycling frequencies because there is more time for creep to occur, increasing the amount of permanent deformation. (Recall that most of the damage is
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FIGURE 57.13 Typical behavior of solder in response to a constant applied displacement (for example, due to thermal expansion). The initial stress is relaxed over time as the solder elongates.
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FIGURE 57.14 Thermal fatigue failure in eutectic Sn-Pb solder for a TSOP component. (Photo courtesy K. Gratalo.)
caused by the plastic deformation that occurs during each cycle, not by the cyclic stressing of the joint.) Hold time: As long as the stress on the solder joint remains nonzero, the thermal fatigue damage per cycle is increased if the hold time is lengthened, again because there is more time for creep to occur. Once the stress relaxation process has gone to completion, no further damage occurs and increasing the hold time further has no effect. Thermal shock: If the thermal cycle is extremely rapid, the components of the PCA may not be at the same temperature; consequently, the imposed strains may be larger or smaller than at slower rates.
Although the designer may be able to influence the peak temperature through cooling schemes, the thermal cycling profile and the frequency of thermal cycles in service are largely fixed by the application. Solder joint fatigue life may be increased by decreasing the strain e imposed on the solder joint by:
Choosing a package with a compliant attachment scheme. In this case, part of the strain is taken up by deflection of the lead, reducing the amount of strain in the solder. For these packages, the joint life can be further extended by decreasing the lead stiffness and increasing the joint area. Decreasing a, the difference in the package and substrate thermal expansion coefficients, by carefully selecting the CTE of the package and substrate (see Secs. 57.5.3.1 and 57.5.1.1, respectively). Decreasing the size of the package, decreasing l. Increasing the height h of the solder joint. Solder joint fatigue life may also be increased by:
Decreasing the local CTE mismatch that occurs at the interfaces between the solder and the component lead and substrate metallization.While the substrate metallization is usually
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