An Innovative High Throughput Thermal Compression Bonding Process Li Ming 2 September 2015
Outline Introduction Throughput improved TCB Process Liquid Phase Contact (LPC) bonding Flux-LPC-TCB under inert environment Fluxless-LPC-TCB under inert environment Summary Add Author s Name Here 2
Mass Reflow vs. Thermal Compression Bonding TCB MR TCB enables interconnects for: Fine bump pitch, small bump diameter, & small solder volume Thin & large die: Die warpage Thin & coreless substrate: Substrate warpage Stress sensitive ELK : white bumps due to CTE mismatch TCB major concern: Low throughput 40/20 50/25 60 /30 80 75 Bump Pitch / Diameter (um) Die Thickness (um)
How to Improve Throughput for TCB Process? Mechanical approach Process approach Flux-Liquid Phase Contact (LPC) TCB Flux-LPC-TCB under Inert environment Fluxless-LPC-TCB under Inert environment TCB Post-applied underfill (TCB-flux) (localized reflow) Pre-applied underfill (NCP/NCF) Traditional TCB-Flux (Flux-SPC) Liquid Phase Contact (LPC) Flux-LPC-Inert (CuOSP) Fluxless-LPC-Inert (Ni/Au or SOP) TM
TCB-flux (SPC & LPC) Process Flow Solid Phase Contact (SPC) 1. Applying flux BH 2. Alignment BH Solid Phase Solder 3. Contact, pulse heating,bonding and cooling Force, heat, Height, time BH 4. Underfilling & curing Flux dipping Liquid Phase Contact (LPC) Heat BH Liquid Phase Solder Force, heat, Height, time BH Flux printing/spraying Heat Pressurized Underfill cure To remove OSP To remove oxides Alignment Thermal compensation Pulse heating & cooling cover a large portion of bonding cycle time Underfill methods: CUF or MUF
High UPH of Flux-LPC-TCB Approach Flux-LPC-TCB Flux-SPC-TCB Temp Temp Force Force 1.0s 0.9s 2.5s 1.0s 1.0s LPC w/o cooling LPC with cooling Flux SPC TCB with cooling Process time 1.0s 1.9s 4.6s UPH (w/ 2s MHT) 1200 800 600
Advantages of Flux-LPC-TCB Improved throughput Compensation for solder height variation/ better wetting 3~5μm Low substrate temperature & short bonding time Low substrate warpage Bond head 280 C Bond stage: <100 C Better placement accuracy: align and bond at the same temperature Good stand-off height control
Limitation of Flux-LPC-TCB Process Solder cap exposed at high temperature Sn oxide formation Sn oxide affects wetting & causes Open Solution to overcome the issue: Inert environment Flux-LPC-TCB under Inert gas
Test Vehicles Die (5x5 mm) Substrate (15.4x15.4mm) CoS Bump diameter: Ø60μm Height: 25μmCu/27μm solder Bump pitch: 160μm Bump diameter: Ø40μm Height: 25μmCu/17μm solder Bump pitch: 160μm 2M layers BT laminate Metallization: CuOSP, Ni/Au Lead width:18μm Pad diameter: Ø80μm Cu lead/pad thickness: 15μm CoC Bump diameter: Ø40μm Height: 25μm Cu/17μm solder Bump pitch: 160μm Si substrate Metalization: Cu Cu pad diameter: Ø60μm Cu pad thickness: 10 μm Lead Pad Bump - Die Cu OSP substrate Add Author s Name Here 9
Bonding Parameter Set-up Temperature, force and position profiles Wetting test Si Si Cu Bonding parameters were not optimized, or temperature was not uniform Bonding parameters were optimized, and uniform wetting was achieved Add Author s Name Here 10
Flux-LPC-TCB : Good Solder joints BOL BOP C2C Cu Solder Cu trace Cu Solder Cu pad Cu Solder Cu pad BOL BOP C2C 45-46 µm 45-46 µm 37-38 µm WH T. ( 0 C) BH T. ( 0 C) BT (sec) 80 80 120 260 260 350 1 1 1 Add Author s Name Here 11
Thermal Aging and Multiple Reflow Comparison among LPC-TCB, SPC-TCB and MR As-bonded 150ºC, 4hrs 3 X reflow LPC-TCB Solder IMC Cu SPC-TCB MR Fracture surface showing solder residual Add Author s Name Here 12
LPC-TCB Process : Precise stand off height Initial reference level: Cu pillar touching Cu pad/trace Final level: The required stand-off height (position control) Temperature Force Position Bonding Profile
Solder Height Control To achieve good solder joint reliability To facilitate underfill process Controlling factors Coplanarity adjustment Z-direction position control Thermal compensation Effect of cooling step Various solder heights Add Author s Name Here 14
Effect of Cooling on Solder Height z 4μm Si Cu BT 7~8μm Z Experiment w/o cooling: Bond head releases the die when solder is still in molten state. w/ cooling: Bond head releases the die after solder solidification. 3.7~4.5μm Simulation Restoring force (10-5N) 5 4 Attractive force 3 2 1 Equilibrium point 0-1 0 5 10 15 20 25 30 35 40 45-2 -3-4 -5 Repulsive force -6 Solder thickness (um) Restoring force vs. solder thickness 10μm 20μm 7~8μm 7~8μm w/o cooling 10.7~12.0μm 21.0~22.0μm w/ cooling Cooling effect on solder height at different bonding levels Without a cooling step, an equilibrium height will be achieved With a cooling step, solder height could be controlled by a pre-determined level Add Author s Name Here 15
Effect of Cooling Temperature on Solder Height 220ºC 180ºC Solder solidification Signal indication for cooling temperatures Zone I Zone II Zone III Cu Liquid Solid Cu Bonding head releases at Zone I: Solder in a solid state (a position controlled height) Zone II: Solder in a solid/liquid mix state Zone III: Solder in a molten state (an equilibrium height) Add Author s Name Here 16
Flux-LPC on Embedded Trace Substrate with Hot Pick & No Cooling Process (with 10um pull back) 300 C 350ms 300gm 10μm BH Temp (DegC) BS Temp (DegC) Bond Time (ms) Bond Force (g) Cooling Temp. (DegC) 300 80 350 300g Without Cooling UPH 1800 Process Control Hot pick, Force mode change to position mode control, pull back 10um, without cooling September 7, 2015 ASM Pacific Technology Ltd. 2015 page 17
Flux-LPC on ETS : Hot Pick & No Cooling Process (with 10um pull back) Die Size (mm) 10 x 8 Die thickness (um) Bump Size (um) Bump Height (um) 760um 50um ~45um Cu / ~17um SnAg Bump Pitch (um) 110/55 No of Bump 1637 Left Middle Right September 7, 2015 ASM Pacific Technology Ltd. 2015 page 18
A New TCB Approach: Fluxless-LPC-TCB under Inert Environment Fluxless bonding No flux is needed (no dipping, no cleaning) Receiving pads: Ni/Au Advantages for Fluxless process Improving UPH significantly No flux residual Lower cost of maintanence Substrate with Ni/Au surface finish Si die with Cu/Ni/Au Metalization APT Add ASM Author s PT Ltd. Name Here 2015 19
Effect of N 2 Protection (C2S) With N2 Excellent Wetting Cu Solder Cu Ni Solder thickness: 5-6um Without N2 Non Wetting observed in some bumps solder thickness: 5-6um Inert environment improves the solder wetting significantly for fluxless-lpc-tcb process APT Add ASM Author s PT Ltd. Name Here 2015 20
Fluxless-LPC-TCB under Inert Gas for C2C Sample 1 Solder height 3.0 um 3.5 um 4.2 um Sample 2 Solder height 8.6 um 7.3 um 6.4 um Sample Bond Head Temp ( 0 C) Bond time (sec) Work Holder Temp ( 0 C) Bond Force (g) 1 350 0.5 120 500 No 2 300 0.5 160 200 No Cooling Good joints formed under different bonding conditions Add Author s Name Here 21
Fluxless-LPC-TCB under Inert Environment for Die Stacking Without using flux, UPH could be improved significantly Add Author s Name Here 22
Process Flow for Stacking Die Stack Die Bonding Molded Underfill No Flux Cleaning Add Author s Name Here 23
Conclusions Flux-LPC-TCB under Inert environment provides an improved throughput and reliable Interconnection method to address fine pitch/small solder volume, thin die and/or coreless substrate packages Fluxless-LPC-TCB under Inert environment is a suitable process for 3D TSV stacking die packages with high throughput and no flux residual TM bonding technology Inert Thermocompression Bonding Technology Add Author s Name Here 24