Wire Bond Challenges of Stacked Dice Devices Charles J. Vath, III*, N. Srikanth, J. Premkumar, M. Sivakumar, M. Kumar ASM Technology Singapore Pte. Ltd. No. 2, Yishun Avenue 7, Singapore 768924 *Phone: +65 67503188, E-mail: vathcj(asmpt.com Abstract The integration of stacked dice into convention packag- ing formats provides a promising capability in reducing the cost, weight and size of the package while increasing the overall functionality of the system. It has been well accepted as enabling approach for smaller handheld devices such as cellular phones and digital camera. Among the various process steps in the packaging of these devices, wire bonding forms one of the critical processes. It applies a combination of mechanical force, ultrasonic power, and thermal energy to thin and, often, unsupported silicon die. This paper outlines the use of finite element in the design and development of a viable wire bonding approach for the interconnection of stacked dice in packages. We also show how improvements in the wire bonder are made to meet the needs of stacked die packaging. Introduction In the last three years, the concept of stacking silicon dice has moved from the laboratory to the manufacturing floor in the semiconductor packaging industry. This has provided a low cost solution for incorporating flash and SRAM chips used in cellular phones into smaller packages with less vol- ume, thus minimizing the size and weight of the overall de- vice [1-5]. Die stacking makes use of several different dice. They may be of the same or different sizes. The stacking sequence may also vary, depending on application. This gives rise to several different arrangements viz., pyramid, unidirectional overhang, criss-cross stacking, and same size stacking with spacers. Any overhang of the dice makes the bondable struc- ture mechanically weaker than conventional single die con- figuration, a fact that has been recognized to be an industry wide issue. Additionally to achieve minimal package thick- ness, thinner silicon dice are being used. This thinner mate- rial has lower stiffness, strength and poorer thermal charac- teristics. It behaves in a very different manner when com- pared to conventional thicker dies. Hence these package design characteristics pose significant challenges to down- stream package assembly processes and materials. This paper focuses on the wire bonding process step that involves the welding of gold wires on the wire bond pads of thin, stacked silicon dice. Using numerical methods, simula- tions have been performed which enable the package de- signer to relate the induced stress to the inherent bulk mate- rial's strength and the process parameters viz., compressive force, heat and ultrasonic energy. Furthermore, the paper discusses the thermal heat transfer characteristics, resonance effects; static deflection of the silicon dies in stack die con- figuration. The paper is intended to be a useful reference in explain- ing the overall methodology in the analysis and design of a stacked dice package. Numerical Model During thermosonic bonding, mechanical force, ultra- sonic power and thermal energy are applied simultaneously [6, 7]. Hence to understand the effect of each process pa- rameter separate analyses were performed to understand the process limits: . Application of a compressive force on the bonding pad of the overhanging die causes bending with excessive defor- mation and stress. A three dimensional model was built to understand the effect of die thickness and overhang dis- tance. * Ultrasonic energy is a high frequency vibration and in the present study a 138 KHz frequency is applied with mi- crometer amplitude. Generally mechanical systems have infinite resonances and the possibility of inducing such resonance is studied by performing a detailed modal analysis. . The presence of a stacked dice structure with multiple in- terfaces and die overhang causes poor temperature rise and a drop in temperature at the bond pad due to the thermal resistance of the various materials. Secondly, the applied thermal energy can cause out-of-plane deformations and shear stresses at the interfaces. A thermomechanical model that takes the thermal mismatch into account can be used to study this. Results Numerical Model Finite element based computer aided engineering (CAE) software such as Ansys and Flotherm have been used to perform such studies. Table 1 lists the necessary thermome- chanical properties of the various materials used for studying the stacked structure under various process parameters; bond force, ultrasonic energy and thermal energy. Bond force studies: Since an overhanging die is expected to bend and twist depending on the location of the bonding force, two locations, die side's mid-point, and edge, are con- sidered as shown in Fig. 1. The bond force was applied at the respective bond pads. Middle load Fig. 1. Typical corner and mid-span load applied to an overhang die during wire bonding. 81 Int'l Electronics Manufacturing Technology Symposium 1-4244-1336-2/07/$25.00 (02007 IEEE Authorized licensed use limited to: National University of Singapore. Downloaded on July 17,2010 at 03:08:20 UTC from IEEE Xplore. Restrictions apply.