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Design Rules for Flip Chip Packaging Systems
ITT Power Solutions develops mission-critical power supplies and circuit card assemblies for military and commercial applications. It is critical that the designs meet their life-cycle requirements while satisfying manufacturability and cost guidelines. The life of flip chips used in these applications is limited in large part by solder joint fatigue from thermal cycling.
The traditional approach for design of these flip chip assemblies is to base them based on past experience and conduct lengthy life-cycle testing to verify that the design is acceptable. This is a time-consuming and costly process which must be repeated multiple times for each design variation, if life requirements are not met the first time around.
To help reduce testing costs, speed up the design cycle and meet schedule deadlines, ITT Power Solutions asked CAE Associates to help develop design rules that would produce flip chip designs which exceed life requirements for the power supplies of their Night Vision System products.
As a first step toward developing the design rules, CAE Associates created a two-dimensional parametric flip chip finite element model using the ANSYS® finite element software. Ten key parameters, representing geometric dimensions such as lengths, thicknesses, relative positioning, and radii of the various chip components were defined as likely to have significant impact on solder ball fatigue life.
The FEA model of the flip chip assembly was subjected to all of the mechanical and thermal loads experienced during the complex manufacturing process. This process initially consisted of 225 individual loading conditions and included the addition of layers of underfill and encapsulant, bending of the board into its container, and multiple thermal cycles. Initial studies indicated that only 21 of these steps had an appreciable effect on the fatigue life of the solder, greatly simplifying the subsequent ANSYS Design of Experiments (DOE) parametric study.
The solder creep behavior, which is a nonlinear function of temperature, stress, and modulus of elasticity, was implemented via a user-defined material model in ANSYS. Accumulated creep strain was used as a basis for the life prediction calculations.
Hundreds of different analyses were run, varying all 10 parameters. The results were compiled into a series of design rules which ITT Power Solutions engineers can use as a predictive look-up tool to quantitatively assess proposed designs without conducting specific finite element analysis or testing.
The result is products that exceed life requirements while saving cost, as stated in this testimonial: "CAE Associates developed a set of design rules that K and M Electronics applies when using Chip Scale Packages (CSP) in power supplies to insure the thermal cycle life of Night Vision System products. "Predicted and measured life was within 5%. Establishing design rules was a highly non-linear problem and CAE was very insightful in setting up the solution concepts. At this point, we just use the design rules for other configurations thus saving us the cost of FEA analysis and destructive testing!"
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