Surface Mount International Conference Proceedings


Author: Hans-Jurgen Albrecht
Company: Siemens AG
Date Published: 4/28/1997   Conference: Surface Mount International

Abstract: The input for materials analysis of BGA packages is given by serial accelerated life time studies of BGA loaded assemblies. Under test conditions, such as temperature cycles (water to water and air to air), high temperature storage, high temperature storage plus electrical powerment, mechanical shock test, corrosive ageing and power cycles significant failure mechanisms are obtained. The following packages were considered: Full-array BGA 225, 289, 400; Perimeter-array BGA 240, 256, 342, 416, 576; Staggered-array BGA 313. Special designed test boards were used to analyze the fatigue behavior of the BGA solder joints by destructive and non-destructive methods. The package and solder joint interfaces can only be analyzed by microsections. The damage features can be summarized as follows: T-Cycle (air/air) - Cracks pad (Substrate) / ball-material; Mechanical Shock - Cracks pad / ball, electrical interruptions; T-Shock (air/air) - Laminate cracks / die edge, cracks pad / ball, electrical opens; T-Shock (water/water) -Corrosion interface pad (Cu-Ni) / ball, corrosion ball-material, electrical interruptions; High Temperature Storage -High amount of intermetallics, Pb-rich interface, cracks in intermetallic compounds; High Temperature plus Electrical Powering - Non-uniform intermetallics (Heat transfer), local cracks pad / ball. Responsible for strain and stress development in the modular structure is the power dissipation inside of the package. The level of ambient conditions depends on the application and the quality of the internal package interfaces. The lateral and vertical stability of the package will be influenced by the materials used (organic boarchme, mold compound, adhesive for chip-mounting, symmetrical or asymmetrical constitution etc.). It should be noted that plastic materials have been extensively used in modular microelectronic packages. However, not only “homogeneous materials” such as organic boardframes or mold compounds are responsible for dimensional instabilities, but also the package interfaces chip / adhesive, chip / mold, mold / solder mask, solder mask / board, interfaces in the board frame (ratio glass / epoxy, adhesive strength to prevent delamination). To optimize interface properties in BGA packages different boardframes, metallizations of used boardframes, solder masks and mold compounds were compared. For this reason surface and interracial tensions were measured depending on the process flow of the BGA manufacture (chip-bonding, wire-bonding, cure of solder mask, molding, ball attach). In the same process flow warpage and shrinkage of the boardframe were obtained. On the basis of well known materials data CP,CL,TCE, Tg, E-moduli etc. it can be hoped to generate “reliability” design rules from the materials point of view. During PBGA manufacture delamination under the die and passivations in the bonding areas / interfaces mentioned above have been detected after completion of the packaging process. This paper describes the selection and reliability evaluation of materials used in BGAs (internal and external interfaces) by measuring physical, mechanical and thermomechanical properties.

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