Reliability of Cu Wire Bonds. Randy Schueller, Ph.D. DfR Solutions Minneapolis, MN

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1 Reliability of Cu Wire Bonds Randy Schueller, Ph.D. DfR Solutions Minneapolis, MN

2 Outline/Agenda Cu Wire Bonding Intro and Trends Quality Concerns Reliability Concerns Failure Analysis Conclusions

3 Wire Bonding Types Wedge Au or Al (but mostly Al) Not very versatile. Ball Stitch Au Versatile and fast (but Au is $$$) So now Cu has been implemented Do we know enough about the failure mechanisms to be implementing in high volume on high value products?

4 Heraeus Cu WB Roadmap

5 Cu Volume STATS-ChipPAC claims to have shipped 100M Cu wire bond packages by the end of Growth rate of 75%/year Started with low leadcount power devices Originally used for low leadcount power devices, copper wire use has now expanded into mid- and high-end Input/Output (I/O) packaging, both leadframe and laminate substrate based, and has been proven on advanced wafer fabrication nodes and fine pitch devices The Edge Singapore, STATS chippac Ships More than 100m Units of Cu Wire Bond Packages, Nov. 30, 2010.

6 Growth Rate K&S, a leading supplier of wire bond machines, has stated that the number of fine pitch machines capable of bonding Cu wire had increased from 5% of the installed base in early 2009 to almost 25% by the end of Fine Pitch Copper Wire Bonding; Clauberg, Qin, Reid and Chylak [Kulicke and Soffa Industries, Inc.]; Chip Scale Review, Volume 14, Number 6, Nov/Dec 2010

7 Wire Cost $ Cost Incentive Larger savings with thicker wire (why power packages were first to adopt Cu) Wire Cost - per KM $1700/oz Cu Wire Diameter (micrometers)

8 Package Cost Breakdown Cu WB equipment costs slightly more and throughput is slower, but wire cost makes up for both. J. Ramos. Copper Wire Manufacturing Challenges. Presentation at Semicon Singapore 2008

9 Current Status Cu is cheaper but less proven Typically used on low cost products (not those where the cost of the IC is much greater than the package but migration is happening) Cu bonding is slower (5 wires/sec) so that adds process cost if high I/O (but it s getting faster)

10 Cu & Au Mechanical Properties Property Au Cu Elastic Modulus 8.8 MPa 13.6 MPa Tensile Strength N/m 2 > Oxidation No Yes Work hardening rate Low High Electrical Resistance 2.3 uohm-cm 1.7 uohm-cm Thermal Conductivity 293 W/mK 394 W/mK Cu oxidizes and is harder than gold (more difficult to deform) But, it has better electrical and thermal conductivity

11 How to deal with oxidation? Formation of ball takes place in an inert environment (forming gas or N 2 ) Pd coated wire aids stitch bond formation

12 Quality and Reliability Quality: the ability to consistently produce a strong WB in high volume production Reliability: the ability of that WB to maintain its strength over sufficient time in the user environment.

13 Common Quality Problems The process window has been made considerably smaller with Cu wire. Machine technology advancements have enabled improved control. Most common quality problems include: Ball Lift Die damage Second bond lift

14 Hardness Al Pad Splash Cu is harder and causes Al from pad to be forced outward potentially reducing bond strength Cu Au Al Splash

15 Al Splash Risk is that remaining Al is too thin.

16 Cratering Cu bonding typically requires higher force and vibration. Damage to underlying Si is the risk.

17 Die Stacking Challenges Combination of pyramid and cantilever stacked die. Example of stacked die memory package from Sharp (three flash and two SRAM die; 8mm x 11mm x 1.13mm) Cantilevered thin die will deflect during bonding - Cu wire would not be a good fit for this application.

18 Cratering Risks Bonding over active circuits is becoming more common (cratering damage carries higher risk). Use of Low K dielectrics (fragile and easily cracked) GaAs die are more fragile Die stacking Cratering can cause immediate or failure or become a latent failure (by initiating a crack that eventually grows to failure or allows metal migration worst case)

19 Poor Bond Strength - Ball The process window for an acceptable ball bond is considerably smaller with Cu compared with Au. Optimization and control of key variables is paramount. In an effort to avoid cratering, die cracking or excess Al splash, the tendency can be to overcorrect (too low force/temp/ vibration) and create weak bonds.

20 Poor Bond Strength Ball (cont) Bond strength can also decline due to variables such as: Capillary wear-out Excessive oxidation of the ball (forming gas control) Contamination of the bond pad (oxide thickness or foreign contaminant) Thickness of Al on pad Machine parameter variability Inadequate support under bond pad

21 Forming Gas vs. N2 Better surface condition with forming gas Lower flow rate required. Ref: Heraeus 19b3bafd0a79563c1e72de.html

22 Parameters to Assist Cu Ball Bonding Softer Cu (usually means higher purity) Clean bond pads Fresh wire (and good storage conditions) Use forming gas (instead of N2) Excellent machine parameter control

23 Weak Second Bond Challenges The stitch bond is created when the wire is crushed by the capillary against the bond surface. Oxidation of the wire is a greater concern Not newly formed like the ball Wire does not plastically deform as much as the ball to expose fresh metal

24 Stitch Bond Improvement Softer copper wire (will deform more) Properly stored wire (less oxide growth) Pd coated wire Plasma cleaned pads Larger amplitude motion of capillary. Roughened capillary (and stronger to resist wear-out damage) Use of machines with stitch bond enhancement (SBE) features table displacement capability.

25 Stitch Bond Enhancement H. Clauberg, et. al, Fine Pitch Cu Wire Bonding, Chip Scale Review, Nov/Dec, 2010.

26 Bonding Surface is Important

27 Reliability Concerns Most of the concern with Cu WB is creating a high quality bond. If this is achieved, the next concern is LONG TERM RELIABILITY. This potential risk of wear-out is of most interest to the non-consumer electronic environments such as: Military Medical Enterprise/Telecom Industrial and Automotive

28 Reliability: When do Wire Bonds Fail? Exposure to elevated temperature Intermetallic formation Exposure to elevated temperature/humidity Corrosion Exposure to temperature cycling Low cycle fatigue from expansion mismatch

29 Elevated Temperature Copper-aluminum forms intermetallics at a much slower rate than Au-Al Most common activation energy of ev Micron reported 0.63 ev L. England, ECTC, 2007 Molding compound has little effect HJ Kim, IEEE CPT, 2003 L Levine, Update on High Volume Copper Ball Bonding C. Breach, The Great Debate: Copper vs. Gold Ball Bonding

30 Pull Strength at Elevated Temp Au-Al Cu-Al Cu-Al shows improved pull strength over Au-Al at 200C Not to the extent expected based on intermetallic growth Different failure mode (gradual vs. sudden)

31 Shear Strength at Elev Temp Gold Wire a. b. Copper Wire Cu Cu Shear strength of Au and Cu ball bonds on Al pads. At lower temperatures (<150C) they are similar in strength loss. J. Onuki, M. Koizumi, I. Araki. IEEE Trans. On Comp. Hybrids & Manfg. Tech. 12 (1987) 550

32 Cu-Al and Elevated Temp Concerns Different intermetallics form at different temperatures Can a 150C/200C test be extrapolated to 85C? Fracture mode with pure Cu changed from within Cu to interfacial failure Pd addition reduced this. Shear Strength after S. Na, T. Hwang, J. Kim, H. Yoo, and C. Lee, Characterization of IMC growth in Cu wire ball bonding on Al pad metallization, ECTC, IEEE, 2011

33 Wire Bonds and Temperature/Humidity Even after large scale adoption of Cu wire bonding we are learning about new and unexpected failure mechanisms in T/H Conditions. For Example: Autoclave conditions have been found by the Fraunhofer Institute to cause catastrophic failure of the Cu wire bonds in some overmolded packages. Analysis showed Cl as a corrosion activator. Galvanic corrosion between the IMC and the Al caused delamination. Cl-free molding compounds can prevent this failure mode.

34 Temp/Humidity Testing Ball Shear Failure after autoclave testing (336 hours at 130C/85%RH/5V bias) Cu 9 Al 4 IMC induces galvanic corrosion Failure Rates of Wire Bonds Al Surface Underside of Cu Ball T. Boattcher, etal., On the Intermetallic Corrosion of Cu-AL Wire Bonds, EPTC, H. Clauberg, Chip Scale Review, Dec 2010

35 T/H Failure Mitigation Carefully select the appropriate mold compound when using Cu wire bonds. High ph and the presence of halides, especially chloride, can accelerate corrosive behavior Getters in the molding compound tend to reduce this risk Perform Autoclave testing Decapsulate the die and perform shear testing (examine failure mechanism)

36 Wire Bonds and Temperature Cycling Historically, temperature cycling has not been a major concern with gold wire bonds Significant amount of loop height creates low stress in ball and stitch bonds Cu Differences Not much Expect Cu to perform better due to higher ductility

37 Failure Analysis How do we decapsulate a Cu wire bonded die without dissolving the Cu? The following is one recipe (a place to start) Use 20% fuming sulfuric acid Use about a 3:1 or 5:2 ratio of nitric to sulfuric acid Use a low temperature (from 17 C to 25 C) Be patient (this could take awhile) Consider pre-decapsulation material removal, such as laser ablation or mechanical milling, to speed up the process

38 Summary Cu wire has been proven to provide real production cost savings Advances in Cu wire and machine technology is helpful in hitting the smaller process window Maintaining quality through strict control of variables is the largest challenge Reliability of Cu is generally better than Au, but do we know everything we don t know? Must continue to investigate Cu WB reliability in more challenging environments.

39 Thank You! Any Questions? Sr. Member of Technical Staff at DfR Solutions based in Minneapolis, MN

40 Competing Failure Mechanisms Shear is the first failure mechanism

41 Copper vs. Gold Temperature Cycling Copper clearly superior N. Tanabe, Journal de Physique IV, 1995 G. Pasquale, J. Microelectromech Sys.,, 2011

42 Copper Wire Bond and Temperature/Humidity (cont.) T. Boettcher believe early failures are due to galvanic corrosion of Cu-rich intermetallics (EPTC 2010) Induces the formation of copper oxides between the intermetallic and the copper bond wire Initial failures during JEDEC HTRB and Autoclave testing were reversed by increasing the amount of intermetallic through annealing Small anode (intermetallic) relative to cathode greatly increases corrosion rate

43 Copper Wire and Temperature Cycling Power module industry believes copper wire is more robust than aluminum Changes being implemented for electric drivetrain N. Tanabe, Journal de Physique IV, 1995 Part of improvement is believed to be due to reduced temperature variation from improved thermal conductivity Part of improvement could be due to recrystallization Can result in self-healing Part of improvement could be more robust fatigue behavior D. Siepe, CIPS 2010