Numerical study on the bonding tool position, tip profile and planarity angle influences on TAB/ILB interconnection reliability

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Abstract

Tape automated bonding (TAB) is a widely used interconnection technology for high-pincount and fine-pitch IC packaging. In this study, a three-dimensional computational model was developed for analyzing TAB inner lead bonding (ILB) process. This experimental study on the thermomechanical properties of copper leads was achieved using high precision micro-force tensile tests. A stress–stain relation between the copper lead and different temperature ranges was successfully implemented into the finite element model to study large plastic deformation in ILB formation. The resulting ILB lead profile and bump sinking values obtained from the simulations agreed well with the experimental observations from actual manufacturing data with the same bonding parameters. The tool position and lead length effects are analyzed to study the residual stress distribution after ILB. A 10-lead model was developed to study how the tool tip profile and planarity ‘angle affect the co-planarity between the bonding tool and the stage. The numerical results show that the permissible tool profile variance should not exceed 1.25 μm and the acceptable planarity angle is 0.005° to achieve the minimum bump deformation requirement.

Introduction

Tape automated bonding (TAB) technology is a momentous interconnection method in first level packaging for portable and handheld electrical production. TAB offers many advantages such as finer pitch and high I/O, lower profile, more precise geometry and gang bonding. This assembly technology corresponds well with the current trend in electronics [1]. The first assembly step in the TAB process involves bonding silicon chips to patterned metal-on-polymer tape, called inner lead bonding (ILB). The objective of ILB is to form a strong metallurgical bond between the gold bump on the die pad and the tin plated beam lead on the carrier tape (see Fig. 1). This process does not compromise the quality and electrical reliability of the final component. Thermocompression (T/C) gang bonding is the most commonly used TAB/ILB technology. This method can simultaneously bond all TAB beam leads to gold bumps on a chip using a bonding tool (thermode, see Fig. 2) with proper temperature, pressure and dwell time. This bonding method does increase throughput, but on chips with high-density I/O, planarity problems can lead to missing bonds, cracked chips or inconsistent pressure. Improper tool temperature, bonding force and tool position can cause fractures in the beam lead span near the bond pad (see Fig. 3). Currently, the packaging industry relies heavily on the use of tests to adjust the bonding parameters to meet the design requirements. This trial-and-error process is time consuming and cost ineffective. Therefore, it is important to fully understand the dynamic characteristics of the TAB/ILB to develop better design tools that can quickly obtain optimum manufacturing parameters for various types of TAB packaging.

In the past, much research has been performed to examine the possible TAB packaging failure modes due to thermal mismatch or cyclic thermal loading [2], [3], [4], [5], [6], [7]. Few research studies attempted to investigate how the bonding parameters affect bonding strength and reliability. In experimental work, Atsumi et al. [8] studied the acceptable bonding conditions and the tool tip configuration for both the 182-lead and 504-lead test dies. Optimal bonding ranges for tool temperature, bonding pressure, and bonding time were found by examining the pull test bond strength. Lai et al. [9] investigated how bonding temperature and bonding pressure affected the microstructure of ILB joints. Oshige and Nakanishi [10] applied a simple two-dimensional numerical model to simulate the contact between a lead and bump pair. Only part of the lead close to the bump was modeled. Numerical results such as the deformation shape and strain distribution for the lead and bump were presented. Jung et al. [11] used the finite volume method to calculate the temperature distribution for each component during ILB, and concluded that the temperature of the heat cartridge in the bonding tool should not be above 520 °C to avoid thermal damage (delamination) in the adhesive layer. Takahashi et al. [12] employed two-dimensional contact finite element (FE) analysis to study the size ratio effect between the pad thickness and lead height in the ILB process. They found that if the pad thickness was decreased, the stress exhibited a peak under the pad that might cause damage to the die.

Up to now, no research has presented a numerical model to examine how the geometric parameters such as the tool compression location affect the stress distribution on the beam lead span. Most studies used only a two-dimensional simple FE model to perform the analysis. This approach cannot be used to discuss the tool planarity and flatness effects. Therefore, in this research the TAB/ILB formation phenomena during the T/C bonding process was simulated using a single-lead three-dimensional computational model and explicit FE analysis. Sn-plated Cu lead thin-strip samples were used as test specimens to perform micro-force tensile tests. The elasto-plastic stress–strain relations for the beam lead at various temperature ranges were combined into nonlinear dynamic FE code LS-DYNA3D to perform TAB/ILB formation simulations. The numerical results were used to study lead deformation, residual stress at the lead span after bonding and gold bump-downward length. A 10-lead FE model was also developed based on a single-lead model to study the effect of the tool tip profile and the tool and die coplanar surface. Both FE models were verified using actual TAB/ILB tests. The numerical simulation model presented here can provide better understanding of the fundamental formation phenomena during the ILB process and serve as a useful tool for better adjusting the bonding tool parameters to achieve the reliability requirements.

Section snippets

Copper lead constitutive model

To determine the appropriate elasto-plastic constitutive model to quantify the plastic flow characteristic effects on the Cu lead in the ILB process, thermomechanical micro-force tensile testing was developed. The test sample material investigated in this study was electrodeposited copper film used as the raw material for Sn-plated Cu beam lead. Experiments were performed on an MTS-Tytron micro-force tester equipped with a 125 N load cell. An environmental chamber was attached to the testing

Finite element modeling and validation

The analysis model configuration shown in Fig. 7 is based on the actual TAB/ILB process. Because of the symmetrical nature of the problem in hand, only half of the ILB model was used for the simulation. The dimensions of the gold bump were 100 μm × 40 μm × 17 μm. The thickness of the copper lead was 18 μm. The length of the copper lead beam was 274 μm. The FE model for analyzing the single lead TAB/ILB process is shown in Fig. 8. It consists of five major parts: the gold bump, the polyimide, the

Failure mode of beam lead and bonding force effects

From the simulation the TAB/ILB process might be separated into four steps, as shown in Fig. 12. Step one, the tool makes initial contact with the end of the lead beam. Step two, the lead slides along the tool tip and makes contact with the bump-top surface. The sliding distance is about 50 μm. Step three, the tool compresses the lead top surface and ties the lead bottom surface with the bump. Step four, the tool rises up after completing the ILB process. Fig. 13 shows the effective (Von-Mises)

Ten-lead finite element analysis and results

The co-planarity between the bonding tool and the stage is a critical element for reliable thermocompression gang bonding, especially as chip sizes become larger and the bump and lead geometry become smaller. In addition to the bonding tool planarity setup, the profile of the bonding tip surface at various elevated temperature is also critical to a successful gang ILB process. As the temperature increases up to 500 °C, the profile of the tool tip surface exhibits a concave shape with

Conclusions

This paper presents a three-dimensional FE computational model for simulating the TAB/ILB process using the explicit FE method. A new constitutive equation was developed to describe the stress–stain relationship of the copper beam lead at temperature ranges from 25 up to 250 °C. Both single-lead and 10-lead models were developed and validated by comparison with the bump deformation obtained from actual TAB/ILB test samples. FE analysis was performed to examine the influences of tool position,

Acknowledgements

This research was supported by the R.O.C. NSF Foundation Grant NSC-91-2212-E-194-012 to the National Chung Cheng University.

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