Joint fatigue

In addition Section VIII, Division 2 contains data for smooth bar design fatigue curves and welded joint fatigue curves. The curves represent testing conducted in air, and can be adjusted for the effect of corrosive... 

Other mechanical characteristics, such as the moduli of the adherends, the geometry of the joint, fatigue and environmental exposure, must also be considered. 

The x-y CTE for standard FR-4 at 14 to 20 ppm/°C is higher than for ceramic or sihcon. The resulting thermal expansion mismatch between an ML-PWB and the attached devices can lead to solder joint fatigue failures when the system undergoes multiple heat cycles during power-up and power-down. Packages with comphant leads can accommodate the CTE mismatch and so can be used with standard ML-PWB material systems. 

The focus of the discussion is on surface-mount solder joints, which have been extensively researched however, many of the same principles also apply to through-hole solder joints. Through-hole joints are generally less prone to solder joint fatigue failures, so long as the through-hole barrel is full of solder. Ideally, complete fillets should be observed on both surfaces of the board. Reviews of this topic can be found in Refs. 18 and 19. 

Solder joint fatigue life may be increased by decreasing the strain Ae imposed on the solder joint by ... 

Ball grid arrays (BGAs) are a new style of leadless SMT component with an areal array of solder joints. The reliability of these components has come nnder intensive stndy. Plastic BGAs are less susceptible to solder joint fatigue failures than ceramic BGAs becanse the laminate and plastic body match the CTE of the PCB much better. At this time, it seems that there will be size and power limitations to ensure solder joint reliability. 

Identify probable failure modes (e.g., solder joint fatigue, conductive anodic filament growth). Accelerated rehabihty tests are based on the premise that the frequency and/or severity of the environmental exposure can be increased to accelerate the incidence of the failure that... 

The main drawback of thermal shock experiments lies in the fact that these types of tests tend to induce failure modes not observed in the field. Additionally, even if solder joint fatigue can be induced, it is very difficult to map the high ramp rates associated with thermal shock loading conditions to the more gradual thermal loading conditions that occur in the field. Consequently, this test technique is limited to relative comparisons between packages in which the failure modes have been shown to be comparable under the same thermal shock test conditions. 

Impact on First-Level Interconnect. In FCBGA-type packages using temperature-dependent material properties, failure resulting from solder joint fatigue may occur not... 

Berens, A., J. West, A. Trego (1998). Risk assessment of fatigue cracks in corroded lap joints. Fatigue in the Presence of Corrosion. 

A review was performed by Clech, which examined the state-of-the-art thermal fatigue data for Pb-free solders (Ref 45). Currently available data does not indicate a strong dependence of fatigue Ufe on either Sn-Ag-Cu composition or on circuit board surface finish. (The sensitivity of solder joint fatigue performance to circuit board finish and, in particular, those finishes that include a protective layer [e.g., Au, Pd, etc.] that dissolves into the solder, will become more significant as the size of the joint and thus, the volume of the solder per joint, becomes smaller.)... 

W. Lee, L. Nguyen, and G. Selvaduray, Solder Joint Fatigue Models Review and Applicability to Chip Scale Packages, Microelectronics Reliability, Vol 40,2000, p 231-244... 

Both integrated circuit vendors and their customers, therefore, are interested in the solder joint fatigue life of electronic packages when subjected to thermal loads. The character and... 

This chapter is focused on the numerical tools needed for lead-free solder joint fatigue predic-... 

In general, the calculation of solder joint fatigue has been based on a stress, strain, or energy value calculated for an intact solder joint. Most recently, these calculations have been performed through finite element analysis. The package is analyzed and the critical parameter, a stress, strain, or energy value, is incorporated into a failure theory. ITiese failure theories are, by and large, analytic expressions that are external to the finite element code. A few of these analytic failure theories that find extensive use with finite element analysis are briefly discussed. A detailed review of current failure theories is included in Ref 20. 

Originally developed for the low-cycle fatigue of traditional structural materials such as steel and nickel alloys, the Cofifln-Manson equation (Ref 21-23), has found application in the evaluation of solder joint fatigue fife ... 

Singularities. At multimaterial interfaces, as well as crack tips, elastic analysis will report singular stresses (Ref 46,47). The outer surface of the solder-to-pad interface where crack initiation is most likely, is an example of such a multimaterial interface. Except in the case of a perfectly plastic material, derived stresses, strains, and strain energies will he mesh dependent. Einer elements will return higher stress values than will larger elements. Since these derived quantities are most likely used to predict solder joint fatigue life, predictions become inextricably tied to the chosen mesh. 

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