As die have increased in complexity and density, there has been an associated growth in the number of die layers. To maximize field reliability interlayer die stress over use conditions should be minimized, which will minimize the occurrence of die layer delamination and associated die cracking failures. Interlayer die stress is affected by various packaging and assembly parameters, such as die thickness, die attach epoxy fillet geometry, molding compound, and saw cut process. Twenty-four lots of plastic ball-gid array (PBGA) packages were assembled in a 35 x 35" PBGA-352, as separate legs of a design of experiments (DOE). The die thickness was varied between 6 and 14 mils, in increments of 2 mils. The die were attached with three different fillet height geometries; standard fillet height (50% all around with no mismatch), hibow fillet height (90% on one side of the die and 25% fillet height on the side opposite), and hi/even fillet height (90% all around with no mismatch). Each lot was subjected to reliability testing to determine which combination of assembly parameters yielded the most robust PBGA package. A PBGA package assembled with a 12 mil thick die and standard fillet produced the most robust product. The data indicates that the molding compound type and saw cut process did not affect the robustness of the package over the range of molding compounds and saw cut processes studied. The data also indicates that the thickness of the die is the parameter that most directly affects die cracking. In addition, the geometry of the fillet height also contributes to mechanical stress on the die, though the magnitude of its contribution is not as great. Two-dimensional mechankal modeling supports the experimental results. Furthermore, mechanical modeling provides a qualitative analysis of the stress induced on the die from the fillet geometry as well as the relationship between die thickness and package induced die stress. BackgroundA PBGA package is comprised of dissimilar materials that expand at different rates upon heating [l]. The various materials within the package must accommodate the stress induced by differential expansion. A silicon die has a coefficient of thermal expansion (CTE) of approximately 3 parts per million (ppm) per degree Celsius, and therefore does not expand significantly when heated. In a PBGA package, the silicon die is attached to the organic substrate (CTE E 22

Introduction A joint meeting of the Engineering Group and the Institution of Highway Engineers was held at Burlington House on Tuesday, 10 January 1984. The topic of the meeting inspired practising engineers, academics and researchers alike, and this was reflected in an extremely well attended meeting. The audience listened to a number of speakers who have been actively involved with construction of particular sections of the M25 and who related their experiences in dealing with various ground engineering problems. There is currently no project so prestigious nor so intently observed in the UK as the M25, and this fact together with the fascination of ground engineering meant that the meeting was destined to be of great interest. The Chairman of the Engineering Group of the Geological Society, Dr A. B. Hawkins, introduced the meeting and related some facts about the M25. The 120 miles of 3–4 lane dual carriageway will constitute the largest orbital road in the world. The most heavily trafficked sections, for instance between the A30 and M4 in the southwest sector, are expected to accommodate between 70 000 and 80 000 vehicles during a 16-h day in 1987. The M25 will provide a much needed fast and problem-free route to cross London and its connections to 18 major radial link roads will greatly improve communications to and from central London. It is anticipated that it will generate traffic and that its excellent communications will encourage development of a new network of industry. Figure 1 shows the present

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