Post-bond Compression Process for Bonding Thin-Die on BT Substrate using Thermoplastic Adhesive Tape Arthur C.M. Chong and Y. M. Cheung ASM Assembly Automation Ltd. 4/F., Watson Centre, 16 Kung Yip St., Kwai Chung, Hong Kong cmchong@asmpt.com and ken.cheung@asmpt.com Abstract Void formation along the bonding interface of bottom die and the BT substrate by using thermalplastic tape adhesives is studied in this article. Effect of surface (i) unevenness of the substrate, (ii) size and thickness of die as well as (iii) the die bonding parameters such as time and force on the interfacial void formation have been investigated experimentally. A post-bond compression process is introduced to reduce the size and number of voids. Finite element model is built to understand the characteristics of void under compression. Both experimental results and Finite Element Method (FEM) analysis show that higher compressive force is needed to reduce the void formation for a thicker die than for a thinner die for given tape application process – tape is applied to substrate first and then die bonding. It is due to a lower rigidity of the die can conform better to an uneven substrate. 1. Introduction Dice of thickness less than 75 μm become indispensable in high density stacked-die packages. Conventional die attachment by epoxy adhesives may not be the best choice for stacking large and thin dice since there may be high yield loss due to epoxy related failures such as voiding, epoxy coverage, die tilt, epoxy overflow, spread-out and etc. They are alternative adhesive media for thin-die attachment process such as thermoplastic adhesives tape. Thermoplastic adhesive tape is available in wafer lamination form or in tape-in-reel form. The handling process for the tape in tape-in-reel form is described as follows: the tape is un-reel from the reek and then cut by a special cutter. The singulated tape is picked and placed onto the substrate which is heated on an anvil block at an elevated temperature. This special tape handler is named as ‘cut and paste’ module for a die bonder. These adhesive tapes have advantages of producing a thin and uniform adhesive layer for the die attachment. However, one major failure mode for the bonding process by using adhesive tape is the void formation along the bonding interface [1]. It is a critical failure especially when bottom die is bonded onto an uneven bond pad of a BT substrate. In this article, we focus on bottom die bonding by using this so called ‘cut and paste’ tape application process. The adhesive tape is singulated, picked and bonded onto a BT substrate first. This adhesive tape is used as the bonding agent for subsequent die attachment process. Series of experiments are setup to investigate the mechanisms of void formation along the bonding interface. A post-bond compression process is introduced afterward, which applies an additional compressive pressure to the bonded specimen so as to investigate its effect on voids reduction along the bonding interface. This compression process is provided by a mechanization named ‘after press’ module. It is found that the additional compression process can reduce the resulting voids along the interface. However, its effectiveness depends on some other process variables. In order to exaggerate the effects of other process variables, an adhesive tape of 75 m sandwiched with thermoplastic layer is used as die attachment adhesive in our experiments. An axis-symmetry finite element model is built to understand the compression process. In this model, a void filled with air between the tape and the die is compressed by a tool – the bonding collet. Air is modeled as compressible fluid. The bending behavior of the die, compressible behavior of the void and the applied J-integral between the interface of the adhesive tape and die are investigated in this study. 2. Experimental setup and results Experiments are set up to investigate the following three aspects: (i) surface characteristics of the BT substrates, (ii) size and thickness effect of the die and (iii) die bonding parameters, bond time, and force. Four types of BT substrates with different surface qualities are used (a) uneven pad (b) less-uneven pad (c) flat-pad with small circular central copper pads and (d) large flat copper pad. The unevenness of the substrates comes from the copper traces of the circuit embedded in the BT laminates. The surface unevenness of the pads have been characterized using optical surface scanner and the depth of the unevenness is ranged from several to ten microns as shown in Fig.1. (a) uneven pad (b) less-uneven pad Fig. 1 Surface profile of the uneven pad in BT substrate. The dimension of test dice are: (a) 7.9x7.9x0.075 mm, (b) 7.9x7.9x0.2 mm and (c) 5.2x5.2x0.075 mm. The bond temperature is fixed at 180 degree C for both taping and die bonding process. A special collet is used to achieve