Krishna Darbha

Impact of usage conditions on solder joint fatigue life

In flip chip microelectronic packages, solder bumps are used to connect the silicon die and package substrate for electrical functionality. However, due to the large mismatch of silicon and organic package in the coefficient of thermal expansion (CTE), the solder bumps undergo large viscoplastic deformation in temperature cycling test and in field operation. The viscoplastic damage accumulates cycle by cycle, which leads to the bump failure by fatigue cracking after hundreds or thousands of thermal cycles. Underfill plays a key role in solder joint fatigue lifetime. In addition to the effects of underfill modulus and CTE, the glass transition temperature, Tg, is also critical. Solder deformation is very sensitive to underfill modulus, the viscoplastic work is much larger when the temperature is above Tg than below Tg. The underfill is usually chosen such that Tg is compatible with the operation temperature of processors, since its overall integrity can be optimized in terms of solder joint fatigue, underfill delamination and copper/low-k cracking. However, in usage, the junction temperature Tj could potentially overshoot Tg several degrees for several minutes. This situation will compromise the load-sharing ability of the underfill to protect the solder joints. The fatigue life of solder could be significantly impacted. In this paper, the effects of overshoot duration and the ramp rate are studied by modeling/simulation for both temperature cycling and power cycling tests, and compared with normal setpoint condition. Then, the field lifetime of solder joints is projected based on the simulation and the temperature cycling qualification test.

Guidelines to select underfills for flip chip on board assemblies

The effect of thermo-mechanical properties of underfill materials, such as coefficient of thermal expansion (CTE) and Youngs modulus, on reliability of Flip Chip on Board (FCOB) under thermal cycling stresses is investigated in this study. Quasi two-dimensional finite element modeling (FEM) viscoplastic stress analysis is combined with an energy partitioning (EP) model for creep-fatigue damage accumulation, to predict the fatigue durability of the flip chip assembly for the given accelerated thermal cycle. Parametric FEM simulations are performed for five different CTEs and five different stiffnesses of the underfill material. The results show that the stiffness of the underfill material plays a crucial role in influencing the fatigue life of FCOB assemblies. The CTE does not have a significant effect on the fatigue life. The eventual goal is to define the optimum design and process parameters of the flip-chip-on-board assemblies in order to maximize the fatigue endurance of the solder joints under cyclic thermal loading environments.

Effect of glass transition slope of underfill on solder joint fatigue life

In flip chip microelectronic packages, solder bumps are used to connect the silicon die and package substrate for electrical functionality. However, due to the large mismatch of silicon and organic package in the coefficient of thermal expansion (CTE), the solder bumps undergo large viscoplastic deformation in temperature cycling test and in field operation. The viscoplastic damage accumulates cycle by cycle, which leads to the bump failure by fatigue cracking after hundreds or thousands of thermal cycles. Underfill plays a key role in solder joint fatigue lifetime. Besides the effects of underfill modulus, CTE, and the glass transition temperature Tg, the slope of temperature-dependent modulus curve in the range of glass transition is also critical. Since the solder deformation is very sensitive to underfill modulus, the viscoplastic work is much larger when the temperature is above Tg than below Tg.

Drop Simulation and Testing in Computer Peripheral Hardware Development

Drop is one of the most common user scenarios for computer peripherals like wired/wireless keyboards, mice, and webcams. Failures due to such impact events are typical examples of overstress types of failures that occur during the useful life of a product. The ability of computer peripheral devices to survive impact with minimal to no impact to product performance or aesthetics is a goal for the product development teams. Engineers have to choose the proper materials and use different simulation and test methods for creating product designs that will survive multiple drop events. Traditional product development process involves iterations of design-physical test-design modification cycles to meet drop requirements. To reduce iterations, expedite product development and reduce time-to-market, virtual drop simulations are conducted at an early stage in product development without the need for any physical samples. This paper presents case-studies on this approach to product development. Case-studies are presented for a keyboard and a mouse. The case-study shows how drop simulation results were used to inform design decisions and how design improvements in terms of material selection or actual feature changes were made early in the design cycle. The study illustrates the use of a high speed camera to capture movies of product performance in drop tests to further discover weaknesses and correlate actual results with simulations. Using a combination of simulation and capturing deformation information with a high speed camera during actual testing, the product development iterations were minimized in terms of cost and time.Copyright