Life prediction modeling of solder interconnects for electronic systems

Abstract

A microstructurally-based computational simulation is presented that predicts the behavior and lifetime of solder interconnects for electronic applications. This finite element simulation is based on an internal state variable constitutive model that captures both creep and plasticity, and accounts for microstructural evolution. The basis of the microstructural evolution is a simple model that captures the grain size and microstructural defects in the solder. The mechanical behavior of the solder is incorporated into the model in the form of time-dependent, viscoplastic equations derived from experimental creep tests. The unique aspect of this methodology is that the constants in the constitutive relations of the model are determined from experimental tests. This paper presents the constitutive relations and the experimental means by which the constants in the equations are determined. The fatigue lifetime of the solder interconnects is predicted using a damage paraneter (or grain size) that is an output of the computer simulation. This damage parameter methodology is discussed and experimentally validated. I NTRO DU CTlO Ndirectly related to the lifetime of the solder interconnects in the package. The solder interconnect is no longer simply an electrical conductor but is also the structural material that holds the package together. The importance of the reliability of solder interconnects increases as the trend in the electronics industry moves toward surface mount technology, smaller joints, and finer pitch. One of the serious challenges to solder joint reliability is failures that are a result of thermomechanical fatigue. In an electronic package, the solder is typically constrained between two materials with different coefficients of thermal expansion. Fatigue failures originate as cyclic strain is applied to solder joints when the package is exposed to thermal fluctuations caused by either ambient temperature changes or by heat dissipation from the integrated circuit devices in the package. A thermal fatigue failure of a solder interconnect is shown in Figure 1. The long term reliability of electronic systems is oftenThe solder typically used for electronic applications is the near-eutectic 60Sn40Pb alloy. The metallurgy and time-strain-temperature behavior of this solder alloy is Hli l!MSTf?lf3IJTION OF THIS DOCUMENT I S U N L I M m surprisingly complex and is n predicting the behavior and In the as-soldered condition, the Sn-Pb alloy has a two phase microstructure. This structure is metastable due to the high interfacial energy that results from the large number of phase boundaries present. Under thermomechanical fatigue conditions, this microstructure evolves into a heterogeneously coarsened band where all subsequently applied strain is concentrated. The evolution of this heterogeneous coarsening is shown in Figure 2 where the initial, as-solidified, microstructure can be compared to the heterogeneously coarsened structure in a 60Sn-40Pb solder joint. It is clearly important that the life predi...

Effect of Under Bump Metallurgy and Reflows on Shear Strength and Microstructure of Joint between Cu Substrate and Sn-36Pb-2Ag Solder Alloy

“…The Pb-rich islands are generally observed at off-eutectic solder compositions such as 50Sn-50Pb (high Pb-content solder). 14) This implies that some amount of Sn in the solder was consumed to form the Ag 3 Sn layer at the interface. The original solder composition was then changed to lower the Sn content and to increase Ag, thereby the cooling path changed as to precipitate the Pb-rich phase first during cooling after reflow.…”

“…13) They are called Pb-rich pockets or islands. 14) According to structure became homogeneous without Pb-rich pockets. After the third reflow, the Ni 3 Sn 4 layer was found and the isolated rod-type Ag 3 Sn particles formed inside the solder as shown in Fig.…”

Improvement of Board Level Reliability for μBGA Solder Joints Using Underfill

“…To decrease the testing time, an accelerated thermal cycling (ATC) test was used to evaluate the fatigue life. 2,16) The temperature ranged from 208 to 423 K and the cycle time was 30 minutes. Specimens without underfill were subjected to 200 to 800 cycles and specimens with underfill were exposed to 1000 to 6000 cycles.…”

Coarsening of the Sn-Pb solder microstructure in constitutive model-based predictions of solder joint thermal mechanical fatigue

“…The term B(&/k)E reflects the change to the flow stress caused by evolution of the microstructure. Therefore, equation (1) describing the deformation rate in the solder material is comprised of three principle terms: (1) the Arrhenius tempera&e dependence of the deformation mechanism, [exp(-Q/RT)]; a microstructure term, [&/k]P; and the stress term, [sinhm(z/~O)], which is also dependent on the solder microstructure through TO.…”

“…Predicting the service reliability of initiation and growth must be predicted since this metric determines, explicitly, the loss of solder joint functionality as it pertains to its mechanical fastening as well as electrical continuity roles. This paper will describe recent progress in a research effort to establish a microstructurally-based, constitutive model that predicts TMF deformation to 63Sn-37Pb solder in electronic solder joints up to the crack initiation step [ 1,2]. The model is implemented using a finite element setting; therefore, the effects of both global and local thermal expansion mismatch conditions in the joint that would arise from temperature cycling.…”