Low Temperature Bonding Based on Solder for Flexible IoT Platforms Kwang-Seong Choi, Haksun Lee, Hyun-Cheol Bae, and Yong-Sung Eom IT Materials and Components Lab. Kwang-Seong Choi, Slide 1
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
IoT (Internet of Things) Kwang-Seong Choi, Slide 3 Source: Microsoft
Supply Chain of IoT Kwang-Seong Choi, Slide 4 Source: Microsoft
Device + Service + Software Kwang-Seong Choi, Slide 5 Source: Microsoft
Device + Service + Software Kwang-Seong Choi, Slide 6 Source: Microsoft
Devices for IoT Kwang-Seong Choi, Slide 7 Source: Internet
Packaging Technologies of Devices for IoT Kwang-Seong Choi, Slide 8 Source: Internet Human
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
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
Soft Assemblies of Sensors for the Skin Kwang-Seong Choi, Slide 11 Source: Sheng Xu, et al. Science 344, 70 (2014)
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 ℃
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
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?
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.
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 Cross-sectional SEM of Bonded Joints
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
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
How to Reduce the Bonding Temperature? Kwang-Seong Choi, Slide 19 Applying Solder Alloy with Lower Melting Point
Solder with Lower Melting Point Kwang-Seong Choi, Slide 20 Tm : 84.35C
Solder Powder Kwang-Seong Choi, Slide 21 (> 38um) (20 ~ 38um) (< 20um) (< 20um) 2 nd Hopper 100um
Design of Test Vehicle Kwang-Seong Choi, Slide 22 FPCB SubstrateFPCB + Cu OSP Pad Length2650 µm Pad Pitch243 µm Pad Width86 µm
Maskless Printing Process using SBM Kwang-Seong Choi, Slide 23
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
SEM Images of Solder Bump Array Kwang-Seong Choi, Slide 25
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 sec.0gf Lower130ºC FU130-LI20
Cross-Sectional SEM Image Kwang-Seong Choi, Slide 27
Resistance Comparison Kwang-Seong Choi, Slide 28 SolderACF Contact Resistance 0.21 mΩ18.82 mΩ ACF Solder
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.
Kwang-Seong Choi, Slide 30 Thank you!