0-7803-8906-9/05/$20.00 ©2005 IEEE 2005 Electronic Components and Technology Conference Comprehensive Hygro-thermo-mechanical Modeling and Testing of Stacked Die BGA Module with Molded Underfill Xueren Zhang a,* , Tong Yan Tee a , Hun Shen Ng a , Jerome Teysseyre a , Shane Loo b , Subodh Mhaisalkar b , Fong Kuan Ng c , Chwee Teck Lim c , Xinyu Du d , Eric Bool d , Wenhui Zhu d , Spencer Chew d a STMicroelectronics, 629 Lorong 4/6 Toa Payoh, Singapore 319521. * Phone: (65) 63897056 Fax: (65) 62598662 Email: xueren.zhang@st.com b Nanyang Technological University, School of Material Engineering, Singapore. c National University of Singapore, Department of Mechanical Engineering, Singapore. d Cookson Electronics, 12 Joo Koon Road, Singapore. Abstract Package reliability is a great concern in developing new advanced packages. This paper presents some of the modeling and testing activities for the design of mixed flip-chip (FC)- wire bond (WB) stacked die BGA module with molded underfill (MUF). The success of the MUF application depends on its performance in thermal shock (TS) test and pressure cooker test (PCT). Mechanical properties (modulus and adhesion strength) of MUF after post mold cure (PMC), reflow and PCT are measured. Shear strength between die and MUF under various temperature and moisture conditions are also characterized. The results show that reflow process and PCT degrade the material properties and adhesion strength. Hygro-mechanical properties, i.e. coefficient of moisture expansion (CME) and saturated moisture concentration (C sat ), are also measured. Based on the measured mechanical and moisture properties, a combined hygro-mechanical and thermo-mechanical stress modeling is performed on the FC-WB stacked die BGA package to compare three types of MUF materials at various temperatures (-40°C, 25°C, 121°C and 150°C) and PCT condition. It is observed that MUF-D3 material induces the lowest stresses on the die active surface. Die stresses induced by MUF with that of conventional mold compound and underfill materials are also compared. The analysis helps in material selection of MUF to enhance the die and package reliability of BGA module. 1. Introduction Nowadays, stacked die or 3D packaging has become popular to reduce the footprint and total cost of package, and enable system-in-package design through functions integration. There are several types of stacked die BGA available in the industry. An example for memory application is stacking of SRAM and Flash dice, which can reduce the total footprint by 28% [1]. This is especially useful for mobile phones or PDAs, which emphasize on smaller and lighter product. The common types of stacked die interconnection are [2], WB-WB, WB-FC, and FC-WB. Mixed FC and WB stacked die BGA is a type of BGA module with both active and passive components (see Figure 1). It consists of two layers of stacked dice, which are connected by flip-chip and wire bonding methods respectively. The advantages of this kind of package are footprint reduction and cost saving. It is used in RF applications such as mobile phones [2]. Generally in FCBGA package, underfill is used to fill the gap between die and substrate to improve the reliability of solder bump, and mold compound is applied to encapsulate the whole die to protect it from the environment. While the flip-chip assembly process is fairly well optimized, the underfilling step is still a bottleneck that results in lower productivity and additional cost. MUF appears to be an ideal solution to recover the productivity and maintain the package reliability [3-5]. MUF material incorporates the functions of both underfill and mold compound, and only one process is needed to encapsulate the die, and thus saving time and cost (see Figure 1). Package reliability needs to be considered for the FC-WB BGA with MUF. The success of the MUF application depends on its performance in TS test (-40 to 150°C) and PCT. PCT, also called autoclave test or pressure pot test (PPT), is a reliability test performed to assess the ability of a product to withstand severe temperature and humidity conditions. It is used primarily to accelerate corrosion in the metal parts of the product, including the metallization areas on the surface of the die. It also subjects the samples to the high vapor pressure generated inside the autoclave chamber. PCT consists of soaking the samples for 168hr at 121°C, 100%RH, and 2 atm. Intermediate readpoints at 48hr and 96hr may also be taken. Solder bump BT Substrate Die Attach MC DIE 2 UF Die 1 (a) With underfill + mold compound Solder bump BT Substrate Die Attach Die 1 MUF DIE 2 (b) With MUF Figure 1. FC-WB stacked die BGA module 196