Solder joint fatigue models: review and applicability to chip scale packages

Fine-pitch ball grid array (BGA) and underfills have been used in benign office environments and wireless applications for a number of years, however their reliability in automotive underhood environment is not well understood. In this work, the reliability of fine-pitch plastic ball grid array (PBGA) packages has been evaluated in the automotive underhood environment. Experimental studies indicate that the coefficient of thermal expansion (CTE) as measured by thermomechanical analyzer (TMA) typically starts to change at 10-15/spl deg/C lower temperature than the T/sub g/ specified by differential scanning calorimetry (DSC) potentially extending the change in CTE well into the accelerated test envelope in the neighborhood of 125/spl deg/C. High T/sub g/ substrates with glass-transition temperatures much higher than the 125/spl deg/C high temperature limit, are therefore not subject to the effect of high coefficient of thermal expansion close to the high temperature of the accelerated test. Darveaux's damage relationships were derived on ceramic ball grid array (CBGA) assemblies, with predominantly solder mask defined (SMD) pads and 62Sn36Pb2Ag solder. In addition to significant differences in the crack propagation paths for the two pad constructions, SMD pads fail significantly faster than the non solder mask defined (NSMD) pads in thermal fatigue. The thermal mismatch on CBGAs is much larger than PBGA assemblies. Crack propagation in CBGAs is often observed predominantly on the package side as opposed to both package and board side for PBGAs. In the present study, crack propagation data has been acquired on assemblies with 15, 17, and 23mm size plastic BGAs with NSMD pads and 63Sn37Pb on high-T/sub g/ printed circuit boards. The data has been benchmarked against Darveaux's data on CBGA assemblies. Experimental matrix also encompasses the effect of bis-maleimide triazine (BT) substrate thickness on reliability. Damage constants have been developed and compared against existing Darveaux Constants. Prediction error has been quantified for both sets of constants.

Model for BGA and CSP reliability in automotive underhood applications

A solder fatigue model for the 63Sn/Pb solder alloy has been previously introduced which characterizes the creep fatigue phenomena of the solder by combining nonlinear finite element modelling with experimental thermal fatigue lives of various flip chip assemblies The model correlates the amount of creep strain energy dissipated per thermal cycle with the characteristic Weibull life of the critical flip chip solder joint The model has been validated for various die sizes, bump geometries, board materials and thermal profiles Furthermore, the model has accurately predicted fatigue life for flip chip assemblies with and without underfill The solder fatigue model has been previously employed to study the effect of design parameters on the reliability of flip chip on ceramic assemblies subjected to thermal cycling (ie, Part I) In particular, the parametric study showed the effect of die size, die thickness, solder joint geometry and underfill properties on predicted solder fatigue lives A similar study has now been performed for flip chips assembled to organic printed circuit boards (Part II) This study investigates the parameters listed above, as well as specific solder joint parameters such as conductor thickness, total stand-off and net solder joint height (stand-off minus conductor thickness) A comparison of fatigue lives for ceramic and organic assembled flip chips is also made Results show that the coefficient of thermal expansion of the underfill is the most significant parameter affecting fatigue life, with no significant difference seen between fatigue lives of underfilled ceramic and organic assembled devices The effect of solder joint geometry is shown to be solely dependent on the ratio of net solder joint height to chip stand-off

A Parametric Study of Flip Chip Reliability Based on Solder Fatigue Modelling: Part II - Flip Chip on Organic

Increased use of sensors and controls in automotive applications has resulted in significant emphasis on the deployment of electronics directly mounted on the engine and transmission. Increased shock, vibration, and higher temperatures necessitate the fundamental understanding of damage mechanisms which will be active in these environments. Electronics typical of office benign environments use FR-4 printed circuit boards (PCBs). Automotive applications typically use high glass-transition temperature laminates such as FR4-06 glass/epoxy laminate material (T/sub g/=164.9/spl deg/C). In automotive underhood application environments, metal-backing of PCBs is being targeted for thermal dissipation, mechanical stability, and interconnections reliability. In this study, the effect of metal-backed boards on the interconnect reliability has been evaluated. Previous studies on electronic reliability for automotive environments have addressed the damage mechanics of solder joints in plastic ball-grid arrays (BGAs) on nonmetal backed substrates and ceramic BGAs on nonmetal backed substrates. Other failure mechanisms investigated include delamination of PCB from metal backing. The test vehicle is a metal backed FR4-06 laminate. Metal backings investigated include aluminum and beryllium copper. Three adhesives have been investigated for metal backing including arlon, pressure sensitive adhesive, and pre-preg. The use of conformal coating for reliability improvement has also been investigated. Component architectures tested include plastic BGA devices, C2BGA devices, quad flat no-lead (QFN), and discrete resistors. Reliability of the component architectures has been evaluated for hot air solder level and electroless Ni/Au finishes. Crack propagation and intermetallic thickness data has been acquired as a function of cycle count. Reliability data has been acquired on all these architectures. Material constitutive behavior of arlon and pressure sensitive adhesive has been measured using uni-axial test samples. The measured material constitutive behavior has been incorporated into nonlinear finite element simulations. Predictive models have been developed for the dominant failure mechanisms for all the component architectures tested.

Damage mechanics of electronics on metal-backed substrates in harsh environments

A solder fatigue model for 63Sn/Pb solder has been developed by combining nonlinear finite element modelling with thermal fatigue data of assorted flip chip assemblies. The model characterizes the creep fatigue phenomena of the solder alloy by correlating the amount of creep strain energy dissipated per thermal cycle with the characteristic Weibull life of the critical solder joint. It has been validated for various die sizes, bump geometries, board materials and thermal profiles. Furthermore, the model has accurately predicted fatigue lives for flip chip assemblies with and without underfill. The solder fatigue model has been utilized to investigate the reliability of flip chip joints subjected to thermal cycling. In particular, a parametric study had been performed which shows how various flip chip design parameters will affect solder joint fatigue. Finite element models have been developed to analyze the effect of die size, die thickness, solder joint height, cap diameter and underfill properties on solder fatigue. For this investigation, all analyses have been carried out for parts on ceramic substrates. The results for underfilled parts show that while die size does not influence solder joint reliability, the effects of underfill CTE are very important. Non-underfilled parts are significantly influenced by die size, cap size and joint height.

A parametric study of flip chip reliability based on solder fatigue modelling

A combined design of experiment and numerical analysis approach is used to determine the effect of four design parameters on the thermal fatigue life of solder joints. The four parameters considered were: substrate thickness, array configuration, ball pitch, acid pad size. A full factorial experiment was designed which was conducted numerically. A validated life prediction model was then used to determine the fatigue lives for each combination. Up to a factor of five improvement in fatigue life is predicted when these parameters were changed from one level to another.

http://ieeexplore.ieee.org/iel3/3763/10995/00550889.pdf

A.R. Syed

Papers

Thermal fatigue reliability enhancement of plastic ball grid array (PBGA) packages