Wire Bonding

 

Wire bonding 

Gold wire ball-bonded to a gold contact pad 

 

 Aluminium wires wedge-bonded wedge-bonded to a KSY34 transistor  die

Germanium diode AAZ15 diode AAZ15 bonded with gold wire

The interconnection in a power package are made using thick (250 to 400 µm), wedge-bonded, aluminium wires

 

Wire bonding is the method of making

interconnection interconn ectionss (AT (ATJ) between an integrated circuit (IC) or other semiconductor device device and its packaging during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one printed circuit board (PCB) to another. Wire bonding is generally considered the most cost-effective and flexible interconnect technology and is used to assemble the vast majority of semiconducto semiconductorr packages. If properly designed, wire bonding can be used at frequencies above 100 GHz.[1]

 

Materials Bondwires usually consist of one of the following materials: Aluminum Copper Silver Gold Wire diameters start at 15 µm and can be up to several hundred micrometres for high-powered applications. The wire bonding industry is transitioning from gold to copper.[2][3] This change has been instigated by the rising cost of gold

 

and the comparatively stable, and much lower, cost of copper. While possessing higher thermal and electrical conductivity than gold, copper had previously been seen as less reliable due to its hardness and susceptibility to corrosion. By 2015, it is expected that more than a third of all wire bonding machines in use will be set up for copper.[4] Copper wire has become one of the

preferred materials for wire bonding interconnects in many semiconductor and microelectronic microelectr onic applications. Copper is used for fine wire ball bonding in sizes up to 0.003 inch (75 micrometres). micrometres). Copper

 

wire has the ability of being used at smaller diameters providing the same performance as gold without the high material cost.[5] Copper wire up to 0.020 inch (500 micrometres)[6] can be successfully wedge bonded with the proper set-up parameters. parameters. Large diameter copper wire can and does replace aluminum wire where high current carrying capacity is needed or where there are problems with complex geometry. Annealing and process steps used by manufacturers enhance the ability to use large diameter copper wire to wedge bond

 

to silicon without damage occurring to the die.[5] Copper wire does pose some challenges in that it is harder than both gold and aluminum, so bonding parameters must be kept under tight control. The formation of oxides is inherent with this material, so storage stor age and shelf life life are issues that must be considered. Special packaging is required in order to protect copper wire and achieve a longer shelf life.[5] Palladium coated copper wire is a common alternative which has shown significant resistance to corrosion, corrosion, albeit at a higher hardness than pure copper and a greater

 

price, though still less than gold. During the fabrication of wire bonds, copper wire, as well as its plated plated varieties, must be worked in the presence of forming gas [95% nitrogen and 5% hydrogen] or a similar anoxic gas in order to prevent corrosion. corrosio n. A method for coping coping with copper's relative hardness is the use of high purity [5N+] varieties.[4]

Red, Green, Blue surface mount  LED  LED package with gold  wire bonding details.

 

Pure gold wire doped with controlled

amounts of beryllium and other elements is normally used for ball bonding. This process brings together the two materials that are to be bonded using heat, pressur pressure e and ultrasonic energy referred to as thermosonic bonding. The most common approach in thermosonic bonding is to ball-bond to the chip, then stitch-bond to the substrate. Very tight controls during processing enhance looping characteristics and eliminate sagging. Junction size, bond strength and conductivity requirements typically

 

determine the most suitable wire size for a specific wire bonding application. application. Typical Typical manufacturers make gold wire in diameters from 0.0005 inch (12.5 micrometres) and larger. Production tolerance toler ance on gold wire diameter is +/-3%.[7] Alloyed aluminum wires are generally

preferred to pure aluminum wire except in high-current devices because of greater drawing ease to fine sizes and higher pulltest strengths in finished devices. Pure aluminum and 0.5% magnesium-aluminum are most commonly used in sizes larger than 0.004 inch (101 micrometer).

 

All-aluminum systems in semiconducto semiconductorr fabrication eliminate the "purple plague" (brittle gold-aluminum intermetallic compound) sometimes associated with pure gold bonding wire. Aluminum is particularly suitable for thermosonic bonding. In order to assure that high quality bonds can be obtained at at high production speeds, special controls are used in the manufacture of 1% silicon-aluminum wire. One of the most important impor tant characteristics of high grade bonding wire of this type is homogeneity of the alloy system. Homogeneity is given special attention

 

during the manufacturing process process.. Microscopic checks of the alloy structure of finished lots of 1% silicon-aluminum wire are performed routinely. Processing also is carried out under conditions which yield the ultimate in surface cleanliness and smooth finish and permits entirely snag-free de-reeling.[8]

Attachment techniques

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  aluminium wire between gold electrodes on a printed  circuit board and gold electrodes on a sapphire substrate, reverse bonding order.

The main classes of wire bonding: Ball bonding Wedge bonding Compliant bonding Ball bonding usually is restricted to gold and copper wire and usually require requiress heat. For wedge bonding, only gold wire requires heat. Wedge bonding can use large diameter wires or wire ribbons for power electronics application. Ball bonding is

 

limited to small diameter wires, suitable for interconnect application. In either type of wire bonding, the wire is attached at both ends using a combination of downward pressure, ultrasonic energy, and in some cases heat, to make a weld. Heat is used to make the metal softer. The correct combination of temper temperature ature and ultrasonic energy is used in order to maximize the reliability and strength of a wire bond. If heat and ultrasonic energy is used, the process is called thermosonic bonding.

 

In wedge bonding, the wire must be drawn in a straight line according to the first bond. This slows down the process due to time needed for tool alignment. Ball bonding, however, creates its first bond in a ball shape with the wire sticking out at the top, having no directional preference. Thus, the wire can be drawn in any direction, making it a faster process. Compliant bonding[9] transmits heat and pressure through a compliant or indentable aluminum tape and therefor therefore e is applicable in bonding gold wires and the beam leads that have been electroformed

 

to the silicon integrated circuit (known as the beam leaded integrated circuit).

Manufacturing and Reliability Challenges There are multiple challenges when it comes to wire bond manufacturing and reliability. These challenges tend of be a function of sever several al parameters such as the material systems, bonding parameter parameters, s, and use environment. Different wire bondbond pad metal systems such as Aluminum-Aluminum (Al-Al), GoldAluminum (Au-Al), and Copper-Aluminum (Cu-Al) require different manufacturing

 

parameters and behave differently under the same use environm environments. ents.

Wire bond Manufacturing Much work has been done to characterize various metal systems, review critical manufacturing parameters, and identify typical reliability issues that occur in wire bonding.[10][11] When it comes to material selection, the application and use environment will dictate the metal system. environment Often the electrical properties, mechanical properties, and even cost can play play a role. For example, a high current device for a space application might requir require e a large

 

diameter aluminum wire bond in a hermetically sealed ceramic package. If cost is big constraint then avoiding gold wire bonds may be a necessity. Some recent work has been done to look at copper wire bonds in automotiv automotive e applications.[12] This is only a small sampling, as there is a vast body of work reviewing and testing what material systems work best in differ different ent applications. From Fr om a manufacturing perspective, the bonding parameters play a critical role in bond formation and bond quality. Parameters such bond force, ultrasonic

 

energy,, temperature, temperature, and and loop geometr geometryy, to energy name a few, can have a significant effect on bond quality. There are various wire bonding techniques (Thermosonic bonding, Ultrasonic bonding, Thermocompression Thermocompr ession bonding) and types of wire bonds (ball bonding, wedge bonding) that affect susceptibility to manufacturing defects and reliability issues. Certain materials and wire diameters are more practical practical for fine pitch or complex layouts. The bond pad also plays an important role as the metallization and barrier layer(s) stackup will impact the bond formation.

 

Typical failure modes that result from poor bond quality and manufacturing defects include: fracture at the ball bond neck, heel cracking (wedge bonds), pad liftoff, pad peel, overcompression, and improper intermetallic formation. A combination of wire bond pull/shear testing, nondestructive testing, and destructive physical analysis (DPA) can be used to screen manufacturing and quality issues.

Wire bond Reliability While wirebond manufacturing tends to focus on bond quality, it often does not account for wearou wearoutt mechanisms related

 

to wire bond reliability. In this case, an understanding of the application and use environment environm ent can help prevent reliability issues. Common examples of environments environm ents that lead to wire bond failures include elevated temperatures, temperatures, elevated temperature and humidity, and temperature temperatur e cycling.[13] Under elevated temperatures, excessive intermetallics (IMC) growth can create brittle points of fractur fracture. e. Lots of work that has been done to characterize the intermetallic formation and aging for various metal systems. This not a problem in metal systems where the wire bond and

 

bond pad are the same material such as Al-Al. This does become a concern in dissimilar metal systems. One of the most well known examples is the brittle intermetallics formed in gold-aluminum IMCs such as purple plague. Additionally, diffusion related issues, such as Kirkendall Kirkendall voiding and Horsting voiding, can also lead to wire bond failures. Under elevated temperature temperature and humidity environments, corrosion can be a concern. environments, This is most common in Au-Al metal systems and is driven by galvanic corrosion. corrosio n. The presence of halides such as chlorine can accelerate this behavior.

 

This Au-Al corrosion is often characterized with Peck’s Law for temperature and humidity. This is not as common is other metal systems. Under temperature cycling, thermomechanical stress is generated in the wire bond as a result of Coefficient of Thermal Expansion (CTE) mismatch between the epoxy molding compound (EMC), the leadframe, the die, the die adhesive, and the wire bond. This leads to low cycle fatigue due to shear or tensile stresses in the wire bond. Various fatigue models have been used to predict the

 

fatigue life of wire bonds under such conditions. Proper understanding understanding of the use environment environm ent and metal systems are often the most important factors for increasing wire bond reliability.

Testing While there are some wire bond pull and shear testing techniques,[14][15][16][17] these tend to be applicable for manufacturing quality rather than reliability. They are often monotonic overs overstress tress techniques, where peak force and fracture location are the critical outputs. In this case the

 

damage is plasticity dominated, and does not reflect some wearout mechanisms that might be seen under environmental environmental conditions. Wire pull testing applies an upward force under the wire, effectively pulling it away from the substrate or die.[18] The purpose of the test is as MIL-STD-883 2011.9 describes it: "To measure bond strengths, evaluate bond strength distributions, or determine compliance with specified bond strength requirements". requirements". A wire can be pulled to destruction, but there are also non-destructive variants whereby one tests whether the wire can withstand a

 

certain force. Non-destructive test methods are typically used for 100% testing of safety critical, high quality and high cost prod products, ucts, avoiding damage to the acceptable wired bonds tested. The term wire pull usually refe refers rs to the act of pulling a wire with a hook mounted on a pull sensor on a bond tester. However, to promote certain failure modes, wires can be cut and then pulled by tweezers, also mounted on a pull sensor on a bond tester. Usually wires up to 75 µm diameter (3 mil) are classified as thin wire. Beyond that size, we speak about thick wire testing.

 

See also Purple plague (intermetallic) Kirkendall effect Ball Bonding Wedge bonding Thermosonic Bonding

References 1. V. Valenta et al., "Design and  experimental evaluation of compensated  bondwire interconnects above 100 GHz", International Journal of Microwav Microwavee and  Wirele Wir eless ss Techn echnolo ologie gies, s, 201 2015 5.

 

2. Mokhoff, Nicolas (March 26, 2012). "Red  Micro Wire encapsulates wire bonding in glass" gla ss" . EE Times Times.. San Fr Franc ancisc isco: o: UBM UBM plc. plc. ISSN IS SN 01 0192 92-1 -15 541 . OC OCLC 56 56085 8504 045 5.  Archived from the original on March March 20, 2014. Retrieved March 20, 2014.

3. "Product Change Notification - CYER27BVXY 27B VXY633 633"" . microch microchip. ip.com com.. August August 29, 29, 2013. 201 3. Archi Archived ved fr from om the the origina originall on Marc March h 20, 2014. Retrieved March 20, 2014.

4. Chauhan, Preeti; Choubey, Anupam;  Zhong, ZhaoWei; Pecht, Pecht, Michael (2014). Copp Co pper er Wi Wire re Bo Bond ndin ing g (PDF). New York: Springer. ISBN 978-1-4614-5760-2. OCL OC LC 864 864498 9866 662 2.

 

5. "Copper Bonding Wire: Electrical  Interc Int erconn onnect ect Materi Materials als"" . coining coininginc inc.co .com. m. March Mar ch 20, 20, 2014. 2014. Archi Archived ved fr from om the the origina original l  on March 20, 2014. Retrieved March 20, 2014.

6. Brökelmann, M.; Siepe, D.; Hunstig, M.; McKeown, M.; Oftebro, K. (October 26, 2015), Copper wire bonding ready for  indu in dust stri rial al mas masss prod produc ucti tion on (PDF) , , retrieved  January 30, 2016

7. "Gold Bonding Wire and Ribbon: Wire for   Automatic Bonders" Bonders" . coininginc.com. March Mar ch 20, 20, 2014. 2014. Archi Archived ved fr from om the the origina original l  on March 20, 2014. Retrieved March 20, 2014.

 

8. "Aluminum Bonding Wire and Ribbon: Silicon Silico n Alumi Aluminum num Wir Wire, e, Alumin Aluminum um Ribbo Ribbon n" . coininginc.com. March 20, 2014. Archived  from the original on March 20, 2014. Retrieved March 20, 2014.

9. A.Coucoulas, “Compliant Bonding”  Bonding”  Proceedings 1970 IEEE 20th Electronic Components Conference, pp. 380-89, 1970. http://commons.wikimedia.org/wiki/File:Co mpliantBondingPublic_1-10.pdf  https://www.researchgate.net/publication/2 25284187_Compliant_Bonding_Alexander_ Coucoulas_1970_Proceeding_Electronic_Co mponents_Conference_Awarded_Best_Pap er 

 

10. G. G. Harman, Wire Bonding in Microelectronics: Microelectron ics: Materials, Materials, Processes, Reliability and Yield. New York: McGraw Hill, 2010.

11. S.K. Prasad, Advanced Wirebond  Interconnection Technology. New York: Springer, 2004. Springer 12. http://www.dfrsolutions.com/hubfs/Resour  ces/services/Suitability-of-Cu-wire-bondedICs-for-auto-applications.pdf?  t=1506700543823

 

13. Hillman, C., “Predicting and avoiding die attach, wire bond, and solder solder joint joint failure failuress .”  International Symposium on 3D Power  Electronics Integration and Manufacturing (3D-PEIM), 2016.

14. MIL-STD-883: Test Test Method Standard for  Microcircuits, Method 2011.7 Bond Strength Microcircuits, (Destructive Bond Pull Test)

15. MIL-STD-883: Test Test Method Standard for  Microcircuits,, Method 2023.5  Microcircuits Nondestructive Bond Pull  16. ASTM F459-13: Standard Standard Test Test Methods for Measuring Pull Strength of  Microelectronic Microelectron ic Wire Bonds

 

17. JESD22-B116: Wire Bond Shear Test  Method 

18. How to test bonds: How to Wire Pull?   April 2016. 2016. Copper (Cu) Wire Bonding Challenging the limits of bonding wire

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