A constitutive equation for a typical SnPb eutectic or near-eutectic solder joint is developed, based on empirical data in shear and generalized to three dimensions. Three strain components are considered: elastic, time independent plastic, and steady-state creep. A continuum mechanics rather than a metallurgical approach is taken with emphasis on the formulation of an equation useful for predicting solder behavior under a variety of conditions. Solutions of the constitutive equation predict the experimental hysteresis data for the loading histories available. Solutions of the equation for the test conditions of three independent sets of solder fatigue data show that the equation, together with the matrix creep failure indicator, can give an estimate of fatigue life. >

Constitutive relation and creep-fatigue life model for eutectic tin-lead solder

This paper describes the creep behavior of three tin-rich solders that have become candidates for use in lead-free solder joints: Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-0.7Cu. The three solders show the same general behavior when tested in thin joints between copper and Ni/Au metallized pads at temperatures between 60‡C and 130°C. Their steady-state creep rates are separated into two regimes with different stress exponents. The low-stress exponents range from ∼3–6, while the high-stress exponents are anomalously high (7–12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95°C to 60°C. The anomalous creep behavior of the solders appears to be due to the dominant tin constituent. Research on creep in bulk samples of pure tin suggests that the anomalous temperature dependence of the stress exponent may show a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating tin-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.

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The creep properties of lead-free solder joints

A lead-free solder which is comprised of three elements Sn-Cu-Ni. Cu and Ni are 0.1-2 wt % and 0.002-1 wt % respectively. Preferable weight percentage of Cu and Ni are 0.3 to 0.7 percent and 0.04 to 0.1 percent respectively. Both methods of additive Ni to a base alloy of Sn—Cu and additive Cu to a base alloy of Sn—Ni are applicable.

Lead-free solder alloy

Viscoplastic finite-element simulation methodologies were utilized to predict solder joint reliability for a same die size, stacked, chip scale, ball grid array package under accelerated temperature cycling conditions (-40C to +125C, 15 min ramps/15 min dwells). The effects of multiple die attach material configurations were investigated along with the thickness of the mold cap and spacer die. The solder structures accommodate the bulk of the plastic strain that is generated during accelerated temperature cycling due to the thermal expansion mismatch between the various materials that encompass the stacked die package. Since plastic strain is a dominant parameter that influences low-cycle fatigue, it was used as a basis for evaluation of solder joint structural integrity. The paper discusses the analysis methodologies as implemented in the ANSYS finite element simulation software tool and the corresponding results for the solder joint fatigue life.

Finite element based solder joint fatigue life predictions for a same die size-stacked-chip scale-ball grid array package

A tluee-dimensional finite elcincnt analysis methodolob?. which explores global. sub-structure_ and sub-inodeling solution options using both slice and syniinetry models has been developed and applied for the purpose of deriving life prediction equations that detenninc the fatibwe respoiise of inicmclectronic package structures using both eutectic (63Sn37Pb) and lead-free (95.SSnlAgO.5Cu) solder materials. The sub-structure and sub-modeling nictliodologies provide a means to iiiiprove siinulation run times for standard single die package structures alulc facilitating tlie development and execution of larger models rcquired for non-syimiietrical multi-die package structures. The study focuses on printed circuit board (PCB) level solder joint fatigue life under accelerated tcinpcrature ?cling couditions. Time-dependent plasticity (i.e. creep) and time-independent plasticity solder inaterid properties are simultaneously accounted for in Uie prcscnted finite element simulation methodolog. Thus, it is believed that the methodology and resulting life prediction equations can also be applied to board level bend cycling. power cycling, and drop shock solder fatigue. However. this point requires fuflhcr study and verification. The solder structures accoiiuuodate llie bulk of tlie total plastic strain that is generated during accelemted tempemiure cycling due to the thermal expansion mismatch behveen the wious iiiaterials that encompass the ovemll structures of microelectronic packages. Since the total plastic strain (i.e. crcep and timeindcpendcnt plasticity) is a donunant parameter that influences solder Fatigue, it was used as a basis for evaluation of solder joint structural integrity. The paper discusses the analysis methodolog? ‘aud results as implemented using the ANSYS finite element simulation sofhwre tool.

Solder joint fatigue life model methodology for 63Sn37Pb and 95.5Sn4Ag0.5Cu materials

Lifetime studies of a 16 I/O surface-mounted solder joint array undergoing isothermal cyclic fatigue in torsion shear under fixed plastic strain range show a strong correlation with creep fatigue and a creep-cracking mechanism. Experimental lifetime data follow an inverse dependence on matrix creep. Experimental measurement of the steady-state shear creep rate versus shear stress defines the creep characteristic that is sensitive to changes in metallurgical structure. The amounts of grain boundary and matrix creep taking place during a fatigue cycle are derived from experimental creep data combined with stress-strain hysteresis data obtained in steady-state cycling. Initially, thicker solder joints have a larger grain size than thinner solder joints, giving more matrix creep during fatigue and a faster failure rate. Fatigue increases the mean grain size of the solder joint as determined by the creep-rate-versus-stress characteristic and microstructure. Effects of grain size and joint thickness on lifetime are discussed. A maximum in the creep fatigue rate occurs at 333 K (60°C).

MC Shine

Papers

Fatigue of solder joints in surface mount devices