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2012 Introduction wire bonding

A basic introduction into wire bonding technology based upon Harman's book and Tumala's Fundamentals of Microsystem Packaging

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2012 Introduction wire bonding

  1. 1. Workshop Wire Bonding for MEMS Technology Cluster (September 2012) Jan Eite Bullema
  2. 2. Contents Work Shop Wire Bonding Jan Eite Bullema Introduction Wire Bonding 2 Introduction Equipment Metallurgy Shear Testing Process Parameters Demonstration in the Lab
  3. 3. Wire Bonding in Micro Electronics Jan Eite Bullema Introduction Wire Bonding 3 Wire Bonding in Micro Electronics Georg G. Harman Third Edition 2010 ISBN: 978-0-07-164265-1
  4. 4. Fundamentals of Micro System Packaging Fundamentals of Micro System Packaging Rao R. Tummala 2001 ISBN-10: 0071371699 Jan Eite Bullema Introduction Wire Bonding 4
  5. 5. Wire Bonding is here to stay Jan Eite Bullema Introduction Wire Bonding 5 From 2 to 3 bonds in 1947 to 1013 bonds in 2008 (10.000.000.000.000) Wire bond trends • Increased yields ( < 25 ppm) • Decreased pitch 20 micron • Lowest possible and decreasing cost • New bond pad materials (e.g. Pd) • Higher Frequency US energy • Reliability issues in new compounds • Increasing wire sweep in new compounds
  6. 6. Process of fabricating DIP packages Jan Eite Bullema Introduction Wire Bonding 6
  7. 7. Ball Bonding Jan Eite Bullema Introduction Wire Bonding 7
  8. 8. Wedge Bonding Jan Eite Bullema Introduction Wire Bonding 8
  9. 9. Typical MEMS Wirebonding Jan Eite Bullema Introduction Wire Bonding 9
  10. 10. Recommended Maximum Current in Au & Al Jan Eite Bullema Introduction Wire Bonding 10 Au wire bonding (25 micron) is most commonly used due to process speed. Currently Au is replaced by copper due to cost Al wire bonding (200 micron) is common for ‘power’ applications
  11. 11. FEM Modeling of Wire Bonding Jan Eite Bullema Introduction Wire Bonding 11
  12. 12. Stud Bumps Jan Eite Bullema Introduction Wire Bonding 12
  13. 13. Stacked Chips: State of the Art 2010 (n>10) Jan Eite Bullema Introduction Wire Bonding 13
  14. 14. Fine Pitch Bonding (20 - 35 micron pitch possible) Jan Eite Bullema Introduction Wire Bonding 14
  15. 15. Ribbon Bonding Jan Eite Bullema Introduction Wire Bonding 15
  16. 16. Contents Work Shop Wire Bonding Jan Eite Bullema Introduction Wire Bonding 16 Introduction Equipment Metallurgy Shear Testing Process Parameters Demonstration in the Lab
  17. 17. Typical Wire Bond Equipment Jan Eite Bullema Introduction Wire Bonding 17
  18. 18. Typical Wire Bond Equipment Jan Eite Bullema Introduction Wire Bonding 18
  19. 19. An example of a typical ultrasonic transducer Jan Eite Bullema Introduction Wire Bonding 19 (A) is the transducer element; (B) is the mounting clamp, which is located on a vibration node and is clamped to the bonding machine; (C) is referred to as the horn (tapered to amplify the ultrasonic wave); (D) represents the ultrasonic wave amplitude; and (E) is the tool, or capillary, which is clamped perpendicular to the axis of the horn. This 60 kHz unit is about 12 cm (~5 in) long.
  20. 20. Contents Work Shop Wire Bonding Jan Eite Bullema Introduction Wire Bonding 21 Introduction Equipment Metallurgy Shear Testing Process Parameters Demonstration in the Lab
  21. 21. Three important metallurgical interfaces Jan Eite Bullema Introduction Wire Bonding 24 1. Metallurgical Bond: Pad on Package 2. Electrical Interconnect 3. Metallurgical Bond: Pad on IC
  22. 22. Common Metallurgies Wire Plating Au (Gold) Au (Gold) Au (Gold) Al (Aluminium) Au (Gold) Cu (Copper) Au (Gold) Ag (Silver) Al (Aluminium) Al (Aluminium) Al (Aluminium) Ag (Silver) Al (Aluminium) Ni (Nickel) Cu (Copper) Al (Aluminium) Jan Eite Bullema Introduction Wire Bonding 25
  23. 23. Intermetallic Growth Jan Eite Bullema Introduction Wire Bonding 26 No intermetallic growth Intermetallic growth
  24. 24. Effect of Plating Jan Eite Bullema Introduction Wire Bonding 27 Au wire on Al pad Intermetallic Compound Formed at interface Au wire on Al pad with ENIG Plating, no Intermetallic Compound formed at interface
  25. 25. Au – Au System Gold wire bonded to a gold bond pad is extremely reliable because the bond is not subject to interface corrosion, intermetallic formation, or other bond-degrading conditions. Even a poorly welded gold-gold bond will increase in strength with time and temperature. Gold wire welds best with heat although cold ultrasonic Au-Au wire bonds can be made. Either thermo compression or thermosonic bonds are easily and reliably made. Thermo compression bondability, however, is strongly affected by surface contamination. Jan Eite Bullema Introduction Wire Bonding 28
  26. 26. Kirkendahl Voiding A phenomenon called Kirkendahl Voiding (voids created at the boundary of two metals having different interdiffusion coefficients) may cause voiding along the boundary of the two metals (ribbon/contact) leading to intermetallic degradation and embrittlement of the bond itself making the lead/bond susceptible to failure during thermal cycling. Alternately, one or more barrier metals may be plated atop the chip contacts prior to the bond ribbon plating step to thereby ensure the compatibility of materials. Jan Eite Bullema Introduction Wire Bonding 30
  27. 27. Kirkendahl Voiding Jan Eite Bullema Introduction Wire Bonding 31
  28. 28. Au – Al System Au-Al welding system is the most commonly used in wirebonding process. However, this bonding system can easily lead to formation of Au-Al intermetallic compounds and associated Kirkendall voids. The formation can be accelerated with the temperature and time of the operational life. There are five intermetallic compounds that are all colored: Au5Al2 (tan), Au4Al (tan), Au2Al (metallic gray), AuAl (white), and AuAl2 (deep purple). Jan Eite Bullema Introduction Wire Bonding 32
  29. 29. Purple Plague AuAl2 can initially form in the interface between gold and aluminium during bonding process even at room temperature and could transform to other Au-Al compounds depending on the temperature, time and bonding configurations. Therefore, this system often presents a problem in reliability of the bonds Jan Eite Bullema Introduction Wire Bonding 33
  30. 30. Cu – Al System Copper wire can be bonded to both gold and aluminum substrate. Au- Cu system has been discussed before. For Cu-Al system, there exist five intermetallic compounds favoring the copper-rich side. Thus, there is the possibility of various intermetallic failures similar to those of Au-Al system. However, intermetallic growth in Cu-Al bonds is slower than in Au-Al bonds. The intermetallic growth in Cu-Al bonds does not result in Kirkendall voiding bur lowers the shear strength at 150-200oC due to the growth of a brittle CuAl2 phase. In the temperature range 300- 500oC, bond strength significantly decreases with the increase of the total intermetallic thickness. The rate of Cu-Al intermetallic formation relies on the ambient atmosphere composition. Jan Eite Bullema Introduction Wire Bonding 38
  31. 31. Crack propagation in the CuAl2 jumps through the intermetallics towards the Cu-rich layers Jan Eite Bullema Introduction Wire Bonding 39 M.M.H.Kouters, Characterization of intermetallic compounds in Cu-Al ball bonds, ESTC 2012
  32. 32. Contents Work Shop Wire Bonding Jan Eite Bullema Introduction Wire Bonding 41 Introduction Equipment Metallurgy (Shear) Testing Process Parameters Demonstration in the Lab
  33. 33. Shear force testing Jan Eite Bullema Introduction Wire Bonding 42
  34. 34. Shear Test Failure Modes Jan Eite Bullema Introduction Wire Bonding 43 Ball Shear Gold stays on PadBall Lift
  35. 35. Shear Test Failure Modes Jan Eite Bullema Introduction Wire Bonding 44 Cratering Wire Shear
  36. 36. Contents Work Shop Wire Bonding Jan Eite Bullema Introduction Wire Bonding 46 Introduction Equipment Metallurgy Shear Testing Process Parameters Demonstration in the Lab
  37. 37. Independent process variables Four principal process variables Ultrasonic power Temperature Bond force Bonding time Categorical variables Capillary Wire Jan Eite Bullema Introduction Wire Bonding 47 Most important variables Ultrasonic power C/V (impact force) Bond force Temperature Other Substrate cleanliness Alignment Touch down
  38. 38. Design of Experiments often used to find Process Window Jan Eite Bullema Introduction Wire Bonding 48 Example: Taguchi L8 (27) Orthogonal Design
  39. 39. The effect of ultrasonic power on ball-shear force Jan Eite Bullema Introduction Wire Bonding 49 The effect of ultrasonic power on ball-shear force. Bonds made with 25 μm (1 mil) diameter gold wire. The stage temperature for these measurements was 125°C, and the bonding force was 30 gf. This may be appropriate for one set of conditions/bonder and is given as an example.
  40. 40. Effect Bond Force on Cratering Jan Eite Bullema Introduction Wire Bonding 50 The incidence of cratering versus bond force for the ultrasonic bonding of 25-μm diameter Al, 1% Si wire of 15 to 16 gf breaking load. The data were obtained from bonding to various Si devices with 60 kHz power.
  41. 41. US Bonding on Soft Substrates Jan Eite Bullema Introduction Wire Bonding 53 Deformed bond pad and polymer structure after wire bonding. This illustrates the bond-pad cupping phenomena (metal and polymer yielding)
  42. 42. US Bonding on Soft Substrates Jan Eite Bullema Introduction Wire Bonding 54 The depth of bond-pad indentation (cupping) on polyimide flex substrates as a function of ultrasonic energy and bond force. (a) with only 2 μm thick Au over 18 μm thick Cu; (b) with a 3-μm thick Ni layer between the Cu and Au layers
  43. 43. Contents Work Shop Wire Bonding Jan Eite Bullema Introduction Wire Bonding 56 Introduction Equipment Metallurgy Shear Testing Process Parameters Demonstration in the Lab

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  • YiKuenLee

    Apr. 17, 2020

A basic introduction into wire bonding technology based upon Harman's book and Tumala's Fundamentals of Microsystem Packaging

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