Trend of Solder Alloys Development

Transcription

1 Trend of Solder Alloys Development Dr. Ning-Cheng Lee Indium Corporation 1

2 Solder Is The Choice of Most Electronic Bonding For Years To Come (Indium) (Aprova) (Toleno) Property Soldering Wire Bonding Conductive Adhesive Reliability Good Good Medium Electrical resistivity (10-6 ohm-m) Thermal conductivity (W/m.K) Bonding temperature Medium Medium to high Low Pitch constraint None Bonding tool size Metal heterogeneity Footprint Area array Linear Area array Cost Low Low to high Medium Bonding speed Mass reflow, fast Slow Medium 2 Reworkability Good Poor Poor

3 Popular Solder Alloys Pb-free 300C 97Pb3Sn; 95Pb5Sn 97.5Pb2.5Ag 92.5Pb5In2.5Ag Pb 80Au20Sn 1 st Level Interconnect 92.5Pb5Sn2.5Ag 90Pb10Sn 10Sn88Pb2Ag High Pb 250C 95Sn5Sb 99.3Sn0.7Cu 96.5Sn3.5Ag 95.5Sn4Ag0.5Cu; 95.5Sn3Ag0.5Cu 91.8Sn3.4Ag4.8Bi 89Sn8Zn3Bi 2nd Level Interconnect 200C 10Sb85Pb5Sn 63Sn37Pb; 62Sn36Pb2Ag 3 58Bi42Sn 150C

4 Electronic Solder Joint Feature Consideration PCB level Package level Joint dimension 1000µ 100µ 10µ 1µ Corrosion Shock Solder bump Cu pillar (2012) Thermal fatigue Heterogeneity Electromigration Composition stability 4 Solder joint feature requirement Corrosion resistance (PCB level reliability) Shock resistance (PCB level reliability) Thermal fatigue resistance (PCB & package level reliability) Heterogeneity stability (PCB & package level stability) Electromigration resistance (> 10 3 amp/cm 2 application, PCB & package level reliability) Composition stability (TLPB, Au, Cu, package level reliability)

5 Business Drivers Smaller & smarter Smaller feature size (smaller, thinner, lighter) Higher I/O density Faster More reliable Non-fragile Fatigue resistant Stable & uniform microstructure Corrosion resistant Electromigration resistant Lower cost Lower processing cost (air, lower temperature, shorter cycle time, higher yield) Cheaper material (solder, components, boards) Green Less toxic (Pb-free) Low carbon Higher power density Higher temperature tolerance, higher current resistance 5

6 More Oxidation Resistant, Lower Surface Tension, Higher Fluidity Smaller & smarter (driver 1) Smaller feature size (smaller, thinner, lighter) Although solder size can shrink with feature size, solder oxide thickness does not shrink, unless solder is more oxidation resistant. Wetting Time (sec) Surface Tension (N/M) Anti-oxidation dopants such as P, Ge, desired for alloying. Ingredients prone to oxidize should be avoided. Since oxide thickness on parts also does not shrink, solder need to wet better, with lower surface tension or higher fluidity. Elements such as Bi, Ni, Co beneficial. 6 solder

7 Lower Melting Point, Lower Process Temperature For Miniaturized Applications Smaller & smarter (driver 2) Higher I/O density Lower processing cost (lower temperature) Reduced via-via spacing increase risk of CAF (conductive anodic filament) formation due to thermal damage. A lower melting temperature solder desired, possibly with addition of Bi, In, Zn, etc. e.g. 57Bi42Sn1Ag ( C) (Intel) 7 (Intel)

8 8 Lower Hardness For Portable Devices. Dopants Reduce And Stabilize IMC More reliable (driver 3) Non-fragile Drop Test Results of As-Reflowed Samples (Min, Max, 2X-StDev) Mn Ce Bi Y Ti Small joints more vulnerable to shock. Joints with low fragility desired Lower hardness (such as low Ag SAC) Dopant which reduce IMC thickness or fragility, such as Mn, Ti, Ni, Co, Pt Dopants which reduce Kirdendall void formation or spalling, such as Ni, In, and high Cu Sn1.1Ag0.45Cu0.1Ge Sn1.1Ag0.47Cu0.06Ni Sn1.07Ag0.47Cu0.085Mn Sn1.1Ag0.64Cu0.13Mn Sn1.13Ag0.6Cu0.16Mn Sn1.1Ag0.45Cu0.25Mn Sn1.07Ag0.58Cu0.037Ce Sn1.09Ag0.47Cu0.12Ce Sn1.05Ag0.56Cu0.3Bi Sn1.16Ag0.5Cu0.08Y Sn1.0Ag0.49Cu0.17Y Sn1.05Ag0.73Cu0.067Ti Sn1.0Ag0.46Cu0.3Bi0.1Mn Sn1.05Ag0.46Cu0.6Bi0.067Mn Sn1.19Ag0.49Cu0.4Bi0.06Y Sn1.15Ag0.46Cu0.8Bi0.08Y Sn1.05Ag0.64Cu0.2Mn0.02Ce SAC305 SAC387 SAC105 Sn63 No. of Drops to Failure (Indium) (Kao)

9 High Ag & Cu For Fatigue Resistance. Dopants Refine Grain And Stabilize Microstructure More reliable (driver 4) Fatigue resistant High Ag for slow creep. High Cu for stable IMC on Ni. Dopants Mn, Ce stabilize microstructure. Dopants which refine grain size may ease anisotropic Sn crystal issue. (Kao) 9

10 Nano-filler May Also Render Fatigue Resistance By Pinning Down Grain Boundary More reliable (driver 4) Fatigue resistant The fatigue resistance may also be introduced by adding nano-filler into the solder SnBi SAC SnBi doped BiSn solder doped with nano SAC305 can be reflowed at low temperature but exhibit high strength and high thermal fatigue resistance. Nano SAC particles reinforced BiSn matrix, and pinned down the grain boundary. 10

11 Low Ag, Dopants (Zn, etc.) Which Suppress Undercooling To Suppress Ag3Sn IMC More reliable (driver 5) Stable & uniform microstructure (Lee etc.) (SAC387) Small joint more vulnerable to IMC plate (such as Ag 3 Sn) formation, which can cause early failure. Alloys with low tendency of forming large IMC plate needed. Reduced Ag content or rapid cooling process through whole manufacturing process critical. Addition of dopants such as Zn will suppress formation of plate through reducing undercooling. 11 (IBM)

12 Avoid Composition With Galvanic Corrosion Potential 12 More reliable (driver 6) Corrosion resistant corroded Noncorroded (Song et al) Small joint more sensitive to damage induced by corrosion, such as cyclic bending load. Presence of Ag appear to aggravate galvanic corrosion. New alloys with composition with low propensity toward galvanic corrosion desired.

13 Ag & IMC Joint Resist EM More reliable (driver 7) Electromigration resistant Ag > 1% resist EM. The diffusivity of Ag in Sn is ~ 3000X slower than Cu at 150 C. IMC joint more resistant toward EM. The current density needed to cause EM damage for Cu6Sn5 IMC joint is at least one order of magnitude larger than Sn based solder. Cu/SnAg w EM Cu dissolved IMC helps Ag helps Cu/SnAg w aging + EM No Cu dissolved 13

14 Wide Pasty Range Alloy Critical For Slow Wetting Hence Low Reflow Defect Rate Tombstoning Rate (%). Lower cost (driver 8) Lower processing cost (higher yield) 7% 6% 5% 4% 3% 2% 1% 0% Sn2Ag0.5Cu Sn2.5Ag0.8Cu Sn3Ag0.5Cu Sn3.5Ag1Cu (Indium) Sn3.8Ag0.7Cu 63Sn37Pb Tombstoning Rate (%). 8% 6% 4% 2% Sn63 y = 0.082e x R 2 = % 0% 20% 40% 60% 80% Mass Fraction of Solid (%) (Indium) Reduced chip size cause greater vulnerability toward chip disturbance at reflow soldering (Tombstoning, swimming, billboarding, wicking) Need alloy with a slower wetting speed at melting temp, such as a pasty alloy with high mass fraction of solid at melting temp. 14

15 Low Ag, and Lower Lower cost (driver 9) Cheaper material (solder) Metal Price (USD/Kg) Ag 1,043 Reduced Ag content. Elements introduce hardening, slow in diffusion, and dopants stabilize microstructure and IMC desired. Au 53,376 Bi 29 Co 37.1 Cu 7.20 In 760 Ni 18.9 Pb 1.95 Pd 19,292 Ag Sb 15.6 Sn Ti 27.8 Zn 1.88

16 Sb Is Next? Environmental friendly (driver 10) Less toxic (Pb-free) Low carbon Pb & Cd free Sb and many of its compounds are toxic, and the effects of antimony poisoning are similar to As poisoning. Region Limits for Sb in tap water (µg/l) EU 5 Germany 5 US (EPA) 6 Japan 15 World Health Org 20 16

17 Ag Or Cu Desired For Future High Power Semiconductor Devices Higher power density (driver 11) Higher temperature tolerance, higher current resistant For die-attachment, the joint of future high power devices is expected to see a higher service temperature which may cause early solder fatigue failure. Ag or Cu bonding will be promising alternative in this regard, if processable at soldering temperature. (Baliga) (Ning et al.)

18 Nano-Ag May Serve As Ag-Solder For High Temp High Power Applications Nano-Ag may seal the gap between solder & high melting metal, and serve as a Ag solder. Cu may have similar potential. Material Thermal Conductivity (W/m*C) Melting Points SAC SnCu Nano-Ag paste Virginia Tech study showed the transient thermal impedance of the device attached by nano Ag paste (sintered at 275C/30min, < 5MPa) is 12.1% lower than those by the solder alloys. (IMAPS 4/2010). Fraunhofer Institute reported high bonding strength > 40 Mpa can be achieved with sintering at 275C for 60 sec and pressure at 2 Mpa (290 psi). (CIPS 2010). Volkswagen, Danfoss, Bosch, GM, TI, BAE pursue or explore this. Low or zero pressure sintering desired.

19 Summary Smaller & Smarter More oxidation resistant, lower surface tension, higher fluidity Lower melting temperature for miniaturized applications More Reliable Lower hardness for portable. Dopants for stabilize microstructure High Ag & Cu for fatigue resistance. Dopants refine grain and stabilize microstructure Nano-filler may also render fatigue resistance by pinning down grain boundary Low Ag, plus dopants (Zn, etc.) which suppress undercooling to stabilize Ag3Sn IMC. Avoid composition with galvanic corrosion potential Employ high Ag and IMC for EM resistance Lower Cost Wide pasty range alloy critical for slow wetting hence low reflow defect rate Low Ag for low cost Green More environmentally friendly. Sb next to ban? Higher Power Density 19 Nano-Ag/Cu bonding desired for future high power semiconductor devices

20 Trends of Solder Alloys Development Trends of getting better diversify according to applications, mainly into portable devices, high thermal reliability, and high temperature high power applications. Regulating Ag and Cu content plus doping or adding nano-filler will be the primary means for improvement. Quasi-solder joint formation such as forming IMC joint and sintering of nano-high-melting metal are new twist of achieving improvement. 20