1. Low Temperature Bonding Based on Solder for Flexible IoT Platforms Kwang-Seong Choi, Haksun Lee, Hyun-Cheol Bae, and Yong-Sung Eom 2014. 10. 16. IT Materials and Components Lab. Kwang-Seong Choi, Slide 1

2. Outline Kwang-Seong Choi, Slide 2 I.IoT (Internet of Things) II.Motivations III.Bumping Process using Solder Bump Maker (SBM) IV.Bonding Process using Fluxing Underfill V.Resistance Comparison VI.Summary

3. IoT (Internet of Things) Kwang-Seong Choi, Slide 3 Source: Microsoft

4. Supply Chain of IoT Kwang-Seong Choi, Slide 4 Source: Microsoft

5. Device + Service + Software Kwang-Seong Choi, Slide 5 Source: Microsoft

6. Device + Service + Software Kwang-Seong Choi, Slide 6 Source: Microsoft

7. Devices for IoT Kwang-Seong Choi, Slide 7 Source: Internet

8. Packaging Technologies of Devices for IoT Kwang-Seong Choi, Slide 8 Source: Internet Human

9. Packaging Technologies of Devices for IoT Kwang-Seong Choi, Slide 9 Source: Internet Human Low cost Miniaturization High performance Human-compatible Low cost High performance

10. Sensors on Human Body Kwang-Seong Choi, Slide 10 Source: Rogue Valley Microdevices Blood monitoring Blood levels of sodium, glucose, kidney function, etc Smart contact lens Controlling intra- ocular pressure levels Mounted on a flexible substrate Treatment and prediction of cardiac disorders 3D elastic membrane made of a soft, flexible, silicon material

11. Soft Assemblies of Sensors for the Skin Kwang-Seong Choi, Slide 11 Source: Sheng Xu, et al. Science 344, 70 (2014)

12. Motivations Kwang-Seong Choi, Slide 12 Example of Flexible Display Source: Internet  One of the Technical Key words: Flexible Candidate Substrates – PET (Tg = 67 to 81 ℃ ) – PEN (Tg = 120 ℃ ) – PES (Tg = 185 ℃ ) Requirements for Interconnections – Lower Bonding Temperature e.g. Lower Than 150 ℃

13. Motivations Kwang-Seong Choi, Slide 13  ACF used as Interconnection Material for Display Process Flow of ACF Bonding Source: Internet Characteristics of ACF Bonding – Randomly distributed particles – Mechanical Contacts – High Electrical Contact Resistance – Reliability Issues Characteristics of ACF Bonding – Randomly distributed particles – Mechanical Contacts – High Electrical Contact Resistance – Reliability Issues

14. Motivations Kwang-Seong Choi, Slide 14  How to Reduce the Bonding Temperature?  How to Reduce the Electrical Contact Resistance? ACF Bonding Bonding using Solder and Fluxing Underfill Bumping Process? Bonding Process?

15. Bumping Process using Solder Bump Maker (SBM) Kwang-Seong Choi, Slide 15  Bumping Process SnAgCu305 Ref.: Haksun Lee, et al, “Novel interconnection technology for flex-on-glass (FOG) applications,” EMPC 2013.

16. Bonding Process using Fluxing Underfill Kwang-Seong Choi, Slide 16  Bonding Process Ref.: Haksun Lee, et al, “Novel interconnection technology for flex-on-glass (FOG) applications,” EMPC 2013. Cross-sectional SEM of Bonded Joints

17. Solder Bump Maker (SBM) Technology Kwang-Seong Choi, Slide 17 ComponentFunctions and Requirements Polymer matrix - Carrying solder powder - Proper viscosity with temperature - Proper surface tensions Deoxidizing agent and Additives - Elimination of the oxide layer on the solder powder - Minimal chemical reactions - No void formation - No out-gassing Solder powder - Sn3.0Ag0.5Cu (SAC305), SnBi, 52InSn - Solder bump formation - Size distribution

18. How to Reduce the Bonding Temperature? Kwang-Seong Choi, Slide 18  Applying Nano-sized Solder Powder Behavior of Melting Temp. of Nano powder Source: Prof. Lee, SNUST

19. How to Reduce the Bonding Temperature? Kwang-Seong Choi, Slide 19  Applying Solder Alloy with Lower Melting Point

20. Solder with Lower Melting Point Kwang-Seong Choi, Slide 20 Tm : 84.35C

21. Solder Powder Kwang-Seong Choi, Slide 21 (> 38um) (20 ~ 38um) (< 20um) (< 20um) 2 nd Hopper 100um

22. Design of Test Vehicle Kwang-Seong Choi, Slide 22 FPCB SubstrateFPCB + Cu OSP Pad Length2650 µm Pad Pitch243 µm Pad Width86 µm

23. Maskless Printing Process using SBM Kwang-Seong Choi, Slide 23

24. Solder Bump Array Kwang-Seong Choi, Slide 24 Number of bumps46 Minimum height23.1 µm Maximum height33.4 µm Average height27.44 µm Standard Deviation2.39

25. SEM Images of Solder Bump Array Kwang-Seong Choi, Slide 25

26. Bonding Conditions Kwang-Seong Choi, Slide 26 (a) Dispensing (b) Bonding(c) Post Cure Bonding TempRampHold TimeForce Upper130ºC 0.2ºC/sec.180 sec. 200gf (4gf / bump) Lower130ºC Post Cure (Hot Plate) TempRampHold TimeForce Upper- -600 sec.0gf Lower130ºC FU130-LI20

27. Cross-Sectional SEM Image Kwang-Seong Choi, Slide 27

28. Resistance Comparison Kwang-Seong Choi, Slide 28 SolderACF Contact Resistance 0.21 mΩ18.82 mΩ ACF Solder

29. Summary Kwang-Seong Choi, Slide 29  For the flexible IoT flatform, the low temperature bonding is required to obtain the reliable interconnections.  Solder Bump Maker and Fluxing Underfill technologies are introduced, and their features are low temperature process and low electrical contact resistance.

30. Kwang-Seong Choi, Slide 30 Thank you!