Solder time-dependent deformation

Time-independent or plastic deformation refers to a material performance that results from relatively fast loading rates. In the laboratory, time-independent deformation is typically generated by the stress-strain experiments. The tests are carried out under either strain-rate control or stress-rate control, but most often, the experiments are performed under strain-rate control. An approximate boundary between time-independent deformation and time-dependent deformation for solders are strain rates of s . The test sample dimensions are typically large, relative to the microstructural features of the material. However, there is a growing need to understand size or length-scale effects on these properties as solder interconnections become increasingly smaller, particularly solder joint dimensions less than 100 pm. 

Elevated temperature, time-dependent deformation, or creep, is a critical parameter that affects the performance of solder interconnections. This signihcant contribution is a result of the low solidus temperature of solder alloys. Even... 

The ASTM method for creep testing is provided in Ref 53. Any one of the test methods described in Ref 4 to 7 can also be adapted for creep testing. Novel test procedures can be used to assess the creep behavior of soldered joints. Although time-dependent deformation is, in general, less sensitive to joint geometry that is the faster stress-strain test, it is not completely immune to the effects of joint dimensions, as well as possible size or length-scale effects that may occur for very small interconnections (e.g., flip chip joints). 

The presence or even predominance of one of the three creep stages depends upon the following factors (a) the material properties and microstructure (b) the temperature and (c) the applied stress. In the case of Sn-Ag-Cu lead-free solders exposed to the accelerated aging conditions of - 55 and 125 °C (- 67 and 257 °F) and hold times of 15 min., time-dependent deformation will be largely primary creep. On the other hand, service conditions that expose the Sn-Ag-Cu solder to several hours at temperatures exceeding 125 °C, or 257 °F (e.g., automotive underhood applications) will potentially place (depending on the applied stress) the material into the steady-state regime. At this time, there is no data that correlate the final failure of a solder interconnection to laboratory tertiary creep data. This lack of correlation is likely due... 

The relatively high sinh term exponent value in Eq 16, 5.0 0.8, indicates that the Sn-Ag-Cu(-Sb) solders behaved more like a simple metal than an alloy. Like the bulk tensile creep data of the Sn-Ag alloy, the AggSn and Cu Snj phases did not have a significant role in the creep deformation. Rather, the Pb-free solders exhibited the creep behavior of a simple metal because the microstructure of the Sn-rich matrix was largely responsible for time-dependent deformation. Lastly, the apparent activation energy of 59 8 kJ/mol for creep in the Sn-Ag-Cu(-Sb) alloys was similar to that of the binary Sn-Ag solder, indicating that a fast-diffusion mechanism was likely responsible for creep. 

J. Stephens and D. Frear, Time-Dependent Deformation Behavior of Near-Eutectic 60Sn-40Pb Solder, Metall. Mater. Trans. A, Vol 30A, 1999, p 1301-1313... 

Failures will occur preferentially in the Pb n solder under slow strain rate load conditions. As such, an intermetallic compound layer has minimal impact on failures that occur due to time-dependent deformation (creep) and low-cycle fatigue (e.g., thermal mechanical fatigue). The failure path occurs in the solder, near to one of the intermetallic compound layers. Cracking in the solder is preempted when one of the interfaces associated with the intermetallic compound layer is inherently weak, such as in the case of the AuSr /Au structure (Fig. 14). 

The use of Pb n solders at high temperatures (relative to their solidus temperature) causes time-dependent deformation, or creep, to be a significant contributor to mechanical degradation in electronic interconnections. Creep studies have been performed on bulk eutectic and neareutectic Pb-Sn solder in tension, torsion, compression, and by indentation [69-73]. Minimum compression creep rate (de/dt in) data can be represented by a sinh law expression over the temperature range of —55° to 125°C [72] ... 

The critical task is the development of an appropriate constitutive equation for the Pb-Sn solder that is of interest. Current efforts in constitutive equation development have assumed the solder to be a continuum. Deformation is represented by a unified viscoplastic (or creep plasticity) constitutive equation [90,91]. The advantage of the unified viscoplastic approach is that both time-dependent deformation (creep) and time-independent deformation (plasticity from the stress-strain curve) are included in a single equation, thereby greatly facilitating the subsequent numerical computations. 

Stephens, J. Frear, D. Time-dependent deformation behavior of near-eutectic 60Sn-40Pb solder. Metall. Mater. Trans., A 1999, 30, 1301-1311. 

Stress relaxation describes the time-dependent deformation behavior of a material under constant displacement, which is evaluated by measuring the stress reduction with time. The evaluation of the stress relaxation process is necessary in order to assess the eflect of hold periods in the thermomechanical fatigue behavior of solder joints. Understanding of the stress relaxation mechanisms is important because the deformation processes involved in stress relaxation are fairly representative of the stress history experienced by solder joints at the temperature extremes of thermal cycling experienced in service. Thus the stress relaxation behavior may be a better... 

One of the challenges with both Pb/Sn and lead-free solders is that they nndergo viscoplastic deformation (creep) as a function of time, temperature, strain rate, and applied stress. A variety of creep deformation models have been used to model the viscoplastic behavior of lead-free solders. The Anand model has been successfully used to model the viscoplastic behavior of Pb/Sn solders. The model allows for the simultaneous incorporation of time-independent plastic deformation as well as time-dependent creep deformation. 

On the other hand, relatively low stresses (e.g., below the yield stress) that are applied at reduced loading rates cause the deformation to shift into the solder material. These conditions characterize the service life environment of most solder interconnections as well as accelerated aging tests. As a consequence, the solder deformation will include time-dependent or creep deformation. In the case of cyclic loading environments, the solder deforms by a combination of creep and fatigue responses. Temperature vari-... 

The dwell time. Reducing the dwell time is an easy way to save test time. Assuming that a device has a duty cycle of 8 h on, 16 h off, applying dwell times of 30 min. already gives an acceleration factor of 24. During heat-up/cool-down, the PCB and the components are elastically deformed due to their mismatch in the coefficients of thermal expansion (CTE). During the dwell, the elastic stress is relaxed by the creep of the solder. However, as shown previously, the creep rate depends on the stress and the temperature. This results in an asymptotic relaxation where, theoretically, the difference of the thermal expansion will never be fully equalized, especially at temperatures below 0 ""C (32 (Fig. 8). 

In actual applications, solder interconnects are often under complex loading conditions (Ref 25). For example, ball grid array (EGA) solder joints may be simultaneously under cyclic shear loading and static tensile (or compressive) loading often with vibration. The deformation behavior and reliability prediction under complex loading conditions warrant further examination. Time and path-dependent creep models are needed for the solder joints under different and often complex loading conditions (Ref 2). 

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