Parametric Analysis of TEBGA Reliability Using a Finite-Volume-Weighted Averaging Technique Hsien-Chie Cheng Computational Solid Mechanics Laboratory National Center for High-Performance Computing Kuo-Ning Chiang Power Mechanical Engineering National Tsing Hua University Chao-Kuang Chen Computational Solid Mechanics Laboratory National Center for High-Performance Computing

Outline Introduction Modeling The Finite-Volume-Weighted Averaging Technique Parametric Design of The TEBGA Reliability Conclusions

Introduction BGA: PBGA, CBGA, TBGA, CCGA PBGA over CBGA: Lower cost of substrate Smaller global CTE mismatch from PCB Lower profile Conventional Over Molded PBGA Packages Experience: Excessive moisture sensitivity of substrate Poor thermal performance of over molded compound TEBGA Provides: The enhanced thermal performance over PBGA Relatively cost-effective over CBGA With Solder Joint Reliability(SJR) in Concern: Similar to PBGA, potential SJR problems still occur in TEBGA.

Introduction (Cont.) Many Studies on C-up PBGA Reliability: Paydar et. al., 1994; Pan, 1994; Hong, 1997; Jung et. al,1997; Winter and Wallach, 1997; Lau and Pao, 1997, etc. Few Investigations on C-down TEBGA: Pao et. al, 1998; Lee and Lau, 1998, etc. Most focus on the prediction of SJR. SJR is function of many parameters. Appropriate combinations of these parameters can signigicantly enhance SJR. Two Techniques for this Purpose: Design of Optimization (DO) or Design of Experiment (Box, 1978) Require tremendous, time-comsuming trials Parametric FEA (Yeh et. al, 1996) More cost-effective Reflect the effect of each parameter on the problem in concern

Introduction (Cont.) Characterization of Strain/Stress Concentration Field is Critical. Fatigue life Stress/Strain Information Stress/Strain Information Mesh Desnsity ( Due to material and geometry singuarities) Techniques: Explicit geometry representation (Fillet) High mesh density Material-nonlinear modeling Work only for Stress Concentration Problem Averaging Technique (Clark and Mcgregor, 1993; Akay et.al., 1997) Considerably conservative result is obtained due to the whole domain area is averaged

Purposes An Improved Averaging Technique: Finite-Volume-Weighted Averaging Technique Instead of averaging the response within the whole domain, the structural response is averaged in a finite zone Determine the dimension of the finite zone using an engineering emprirical approach Design Parametric Finite Element Analysis Design parameters: Geomertry/Thickness : Die/PCB/Die Attach/Heat Spreader/BT Substrate Geometry/Size : Die Material/Young’s Modulus : FR-4/BT/Die Attach Material/CTE : BT/FR-4

PBGA and Thermal Enhanced BGA Heat spreader Die attach Die BT Solder Mask Solder Ball Encapsulant Copper Ring.

256-Pin TEBGA and Modeling Heat Spreader PCB Table: TEBGA Geometry Data Table: TEBGA Material Property

Solder Joint Material Property Temperature-dependent (Plasticity) Time-dependent (Creep) Garfolo Hyperbolic Sine Law

Fatigue Life Prediction An Empirical Coffin-Manson Relationship

Improved Volume-Weighted Averaging Technique Finite-Volume-Weighted Averaging Technique Rules to Determine the Dimension of the Finite Zone In an empirical engineering approximation manner: Small enough to capture the maximal strain field Large enough to obtain a converging solution as the mesh density increases

Determination of the Finite Zone # of Elements within the fan shape: 18, 66, 192, 504 Four different radii are defined: 0.01, 0.02, 0.04, 0.06 mm. A net termperature swing = ; plastic behaviors considered. 20 zone provide considerable agreement to the proposed criterion. Fan-shape circular sector 0.12 mm A CBD A B C D

Finite Element Modeling 1/2 package is modeled due to the symmmetric condition 2-D plane strain finite element model Elements: Nodes: The inelastic strain within the characterized finite zone will be averaged using a Volume-Weighted Technique.

Parametric Design of TEBGA Reliability Design Parameters: Geomertry/Thickness : Die/PCB/Die Attach/Heat Spreader/BT Geometry/Size : Die Material/Young’s Modulus : FR-4/BT/Die Attach Material/CTE : BT/FR-4 Thermal Cycling 5 min linear temperature loading/unloading 20 min low/high termperature dwell periods Test temperature range: -40 ~ 125 Stress free temperature: 25 Assume the is stablized within two cycles

Geometry: Die Size Nonlinear relationship is detected Between: Solder Joint Fatigue Life -vs.- Die Size The maximum fatigue life appears at die size=7 mm Cavity-down TEBGA / / Cavity-up PBGA Cavity-up PBGA: The larger the die size; the shorter the fatigue life The location experiencing with the largest CTE mismatch is right beneath the chip Cavity-down TEBGA: A nonlinear relationship is obtained A more complicated failure mechanism) Die Size

Geometry: Die Size (Cont.) Two Major Failure Mechanisms: Global Mismatch: CTE of PCB >> CTE of the Package The farther distance from the center of the package, the larger deformation Local Mismatch: CTE mismatch between the heat sprader and the die Ball Numbering

Geometry:Thickness Die ThicknessPCB Thickness Die Attach Thickness Heat Spreader Thickness BT Thickness

Materials: Young‘s Modulus/CTE PCB Y’s Modulus BT Y’s Modulus Die Attach Y’s Modulus BT CTE PCB CTE

Conclusions Introduction of a Finite-Volume-Weighted Averaging Technique: Dimension of the zone determined using an empirical approximation mehtod. The strain/stress concentrtion responses can be characterized, and would no more be mesh- sensitive/dependent. Further experimental verification to really predict fatigue life of Solder Joints. A parametric study of the reliability of the TEBGA assembly with respect to geometry and material parameters: The design guideline for obtaining optimal reliability: Educing a more accurate conclusion requires a sophisticated design optimization technique

Conclusions (Cont.) A Nonlinear Relationship Between SJR and Die Size/PCB CTE. Extensive Investigation on the Major Failure Mechanisms for the case of SJR and Die Size. A different failure mechcanism from that of C-up PBGA. Further investigation: Effect of other failure mechanisms, such as die cracking on the package reliability. The nonlinear relationship between SJR and PCB CTE.