Biography of Authors. September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 1

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1 Biography of Authors Jingshi Meng is a PhD candidate from CALCE (Center for Advanced Life Cycle Engineering), University of Maryland. His doctoral research, advised by Professor Abhijit Dasgupta, focuses on accelerated tesong and damage modeling of electronic packaging interconnects caused by impact and drop loading. September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 1

2 Drop Test Reliability of Flexible PWBs and Adhesive Interconnects in Portable Products J. Meng and A. Dasgupta CALCE Electronic Products and Systems Center Mechanical Engineering Department; University of Maryland, College Park, MD USA Phone: ; September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 2

3 Introduction Flex-on-Board (FOB) structure: Anisotropic conductive films (ACFs) and flexible printed circuits (FPCs) are widely used with PWBs in portable electronic products [1] Reliability studies: Many aspects of ACF reliability have been extensively investigated, e.g. thermo cycling tests, pressure cooker tests, and hydrothermal aging test [2-5] Drop/Impact loading: Electronics in portable devices are commonly exposed to shock impacts due to accidental drops [6]. Thus, a thorough investigation of the common failure sites in ACFs interconnections under drop loadings becomes necessary Objectives: Testing and modeling methodology for drop durability assessment of anisotropic conductive film (ACF) interconnections between flexible printed circuits (FPC) and rigid printed wiring boards (PWB). Method is based on materials characterization [7], assembly drop testing and PoF modeling September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 3

4 Approach and Overview Fixture FPC PWB (mass) Focus of this study September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 4

5 Test Vehicle for ACF Drop Testing [7] Fixture FPC PWB FPC-PWB is jointed by ACF Parameters of ACF ACF thickness before bonding (µm) 35 PWB joint area (mm 2 ) 5.5x5.5 Diameter of conductive particles (µm) 10 Conductive particles Au/Ni coated polymer Pitch width (µm) 700 Features of Flex designs Surface finish ENIG September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 5

6 ACF Drop Test Setup Commercially available Dual Mass Shock Amplifier (DMSA) accessory is added to the drop tower, in order to achieve a high acceleration capability (up to 100,000 G) Specimen design is a Flex-on-Board (FoB) assembly jointed by ACF In-situ resistance monitoring: 1Hz September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 6

7 ACF Drop Test Matrix With DMSA Test matrix Surface finish ENIG Drop height, mass 10 inch drop, 340 mg 18 inch drop, 340 mg 30 inch drop, 340 mg 18 inch drop, 170 mg 18 inch drop, 510 mg September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 7

8 Failure Mode I: FPC/Cu Trace Crack Before test Crack in FPC/Cu trace After test Cu trace Gold pads (on PWB) y x Two-wire measurement is used for in-situ monitoring of the joint connection during the drop test Sampling frequency=1hz Optical microscope pictures are taken to analyze the failure modes Complete FPC crack Partial FPC crack Cu trace crack z x z x Cu trace crack y z September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 8

9 Failure Mode II: FPC Tear ACF adhesion failure ACF adhesion secondary failure mode is also observed Before test Broken FPC ACF, not compressed Sometimes FPC separation occurs with secondary failure mode: ACF adhesion September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 9

10 Failure Mode III: ACF Adhesion ACF joint adhesion failure is the dominant failure mode: more than 70% in all the tests 340 mg mass potted under bending condition September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 10

11 Rigid fixture ACF Drop Test Simulation animation FPC & PWB Contour plots of the frame when the max. strain occurs September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 11

12 ACF Drop Test Durability Modeling I Two different types of functions (f) are compared for the durability model : power law (f 1 ) and exponential (f 2 ) equation Metric variable (x) for durability models: a. Peak true strain in x direction b. Peeling stress in z direction c. Acceleration in z direction c 1. peak inertia force c 2. impulse The form of function f(x) is selected by comparing how well each option agrees with the experimental durability data September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 12

13 ACF Drop Test Durability Modeling (Continued) N f =f(x) R Drop counts to failure x: metric value f: function of the durability model Highest R 2 f1. Power law f2. Exponential a. Strain b. Peel Stress c1. Inertia force c2. Impulse Metric Variable September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 13

14 Experimental Data and Distribution Unreliability 95% confidence bounds, 3P- Weibull Drop Counts to Failure (scaled value) Drops to failure 20 test data for this test condition Ranking of distributions Drop test severity (measured by peak peeling stress in PWB, Unit: MPa) September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 14

15 Summary and Conclusions From the perspective of model constants robustness wrt. changes in design and stress distribution, peak peeling stress in PWB is a more robust metric variable than inertial force Loading severity for five different test conditions are ranked by peak peeling stress in PWB; Drop durability decreases exponentially as drop severity increases 3-parameter Weibull distribution can describe the test data reasonably well September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 15

16 References 1. P. Opdahl, 65.1: Invited Paper: An Overview of Display Interconnect Technology Trends: Technologies & Markets, SID Symposium Digest of Technical Papers, vol. 38, no. 1, pp , A. Nagai, K. Takemura, K. Isaka, O. Watanabe, K. Kojima, K. Matsuda, and I. Watanabe, Anisotropic conductive adhesive films for flip-chip interconnection onto organic substrates, in IEMT/IMC Symposium, 2nd 1998, 1998, pp M. Teo, S. G. Mhaisalkar, E. H. Wong, P.-S. Teo, C. C. Wong, K. Ong, and C. F. Goh, Effects of anisotropic conductive adhesive (ACA) material properties on package reliability performance [flip-chip interconnects], in Electronics Packaging Technology, th Conference (EPTC 2003), 2003, pp Y. C. Lin, X. Chen, and Z. P. Wang, Effects of hygrothermal aging on anisotropic conductive adhesive joints: experiments and theoretical analysis, Journal of Adhesion Science and Technology, vol. 20, no. 12, pp , W.-S. Kwon, S.-Y. Yang, S.-B. Lee, and K.-W. Paik, The effect of Tg on thermo-mechanical deformation and reliability of adhesive flip chip assemblies during temperature cycling, in Electronic Components and Technology Conference, Proceedings. 54th, 2004, vol. 2, pp Vol S. Douglas, J. Meng, J. Akman, I. Yildiz, M. Al-Bassyiouni, and A. Dasgupta, The effect of secondary impacts on PWB-level drop tests at high impact accelerations, in Thermal, Mechanical and Multi- Physics Simulation and Experiments in Microelectronics and Microsystems (EuroSimE), th International Conference on, 2011, pp. 1/6 6/6. 7. J. Meng, P. Stark, A. Dasgupta, M. Sillanpaa, E. Hussa, J. P. Seppanen, J. A. Raunio, and I. J. Saarinen, Effect of strain rate on adhesion strength of Anisotropic Conductive Film (ACF) joints, in Electronic Components and Technology Conference (ECTC), 2013 IEEE 63rd, 2013, pp September 5, 2014 ASTR 2014, Sep 10-12, St. Paul, MN 16