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Breach C.D.,ProMat Consultants | Wai Mun T.,ITE College Central | Lee T.K.,ITE College Central | Holliday R.,World Gold Council
Proceedings of the IEEE/CPMT International Electronics Manufacturing Technology (IEMT) Symposium | Year: 2010

High gold prices have led to renewed interest in replacing gold with copper in existing packages and new packages in order to save costs. Although reliability is often cited as a reason for using copper, the main driving force for its use is undoubtedly cost. Perceptions that copper wire is more reliable are based on the notion that the intermetallics grow more slowly and that thinner intermetallics are more reliable and yet old data tend to support the idea that copper is as reliable as gold. More recently however, copper ball bonds on aluminium metallization have been found to fail more than gold during temperature cycling (TMCL) and pressure cooker testing (PCT) [13]. The key feature of these tests is the presence of moisture than appears to accelerate corrosion. A proposed solution to this problem is Pd-coated Cu wire, which looks to be a promising but relatively untested solution at present [14]. While Pd-coated copper wire may improve reliability, cost-savings will be less compared with bare copper wire. It is important to note however that a recent SEMI survey shows that industry is generally not as confident in copper as wire equipment and materials suppliers [5]. Localized corrosion of aluminium bond pads is well known in microelectronics packaging and in Al-Cu and Al-Cu-Si bond pads, CuAl 2 acts as a cathode and aluminium corrodes in the presence of water (electrolyte) [6]. The situation is complicated by the presence of chlorine and other ionic contaminants in addition to moisture. In copper ball bonds, intermetallic coverage in as-bonded balls is very difficult to see but it appears that CuAl 2 and Cu 9Al 4 are the compounds that form initially [7]. Intermetallic growth of Cu-Al compounds is slow compared to Au-Al compounds, which means that in finished packages, a large amount of Al remains whereas in gold ball bonds, with thin 1m bond pads, aluminium can easily be consumed after encapsulation and moulding and surface mounting. It is plausible that during extended periods of exposure to moisture, stress and ionic contaminants, Cu ball bonds may be more susceptible to localized corrosion than Au ball bonds because slow intermetallic growth permits aluminium corrosion. This paper discusses ball bond corrosion and suggests that for high reliability applications or applications in moist environments, it may be necessary to accelerate the growth of Cu-Al intermetallics to mitigate potential corrosion and failure. © 2010 IEEE.


Breach C.D.,ProMat Consultants | Holliday R.,World Gold Council
Proceedings - 2010 11th International Conference on Electronic Packaging Technology and High Density Packaging, ICEPT-HDP 2010 | Year: 2010

Copper and gold ball bonds were bonded on 1.2μm thick Al-0.5%Cu-1%Si. One set of devices was aged at 175°C up to 1000 hours. A second set of devices was pre-conditioned for 168 hours at 85°C and 85% relative humidity followed by ageing at 175°C up to 1000 hours. No cratering was found with either wire in as-bonded devices but devices bonded with copper developed cratering after ageing. Neck and stitch pull strengths of copper decreased with ageing and moisture preconditioning slightly accelerated loss of strength. Shear strength of copper wires increased during ageing above the levels observed with gold. © 2010 IEEE.


Lee T.K.,ITE College Central | Breach C.D.,Promat Consultants | Chong W.L.,ITE College Central | Goh C.S.,ITE College Central
ICEPT-HDP 2012 Proceedings - 2012 13th International Conference on Electronic Packaging Technology and High Density Packaging | Year: 2012

As a result of increasing gold (Au) price, the use of copper (Cu) bonding wire has gained popularity for mass production. The different properties in hardness, oxidation and electrochemical potential between Au and Cu have a significant impact on the wire bonding process and reliability. This paper focuses on the oxidation and corrosion of Au/Al and Cu/Al ball bond for the wire bonding processes and their bond reliability in pressure cooker test (PCT) and halides environment. A design of experiment was conducted to optimize the wire bonding process prior to subjecting the bonded samples to oxidation and corrosion studies. It is noted that a higher ultrasonic energy is necessary for Cu wire bonding. This is likely due to higher energy necessary to break the oxides of copper and aluminum to result in formation of bonds between the wire and the bonding pads. In PCT analysis, the Au ball shear test results remained stable, while the Cu ball shear test results improved with soaking. CuO blisters are found on the surface of the Cu with corrosion apparent on the circumference of the Cu ball. The corrosion resulted in delamination between Al and Cu and generated undesired interface failure modes. A significant drop in stitch pull strength with large results deviation was observed for Cu wire bonding due to oxidation and stress resulting in cracks of the stitch. In halide environment, an increase in failure rate was observed for Cu/Al pad. This is a result of galvanic corrosion due to the very limited intermetallic formation beneath the Cu ball which allowed electrolyte to seep in to initiate galvanic corrosion. This paper provides an insight to the failure mechanisms due to oxidation and corrosion which can weaken the Cu/Al bond as compared to Au. © 2012 IEEE.


Lee T.K.,ITE College Central | Breach C.D.,Promat Consultants | Chong W.L.,ITE College Central
Proceedings of Technical Papers - International Microsystems, Packaging, Assembly, and Circuits Technology Conference, IMPACT | Year: 2011

The replacement of gold with copper bonding wire has taken a prominent position in the microelectronics packaging industry in recent years. This is largely driven by the increase of gold prices. The hardness, oxidation and electro-chemical potential of copper properties have a significant impact of its wire bonding process and reliability as compared to gold. This paper focuses on the relationship of Au and Cu material properties to wirebonding processes. A higher ultrasonic energy was needed for Cu than Au bonding and this resulted in pad cratering for Cu wire after thermal aging. A drop in stitch strength was observed due to oxidation under thermal aging condition. The resilient of corrosion for Au and Cu wire in moisture, DI water and NaCl conditions were also investigated. The IMC ball coverage appears to have an impact to initiate corrosion with the ingress of electrolyte. For Cu, the galvanic corrosion was suspected for the moisture and DI water soaking with additional pitting corrosion for NaCl condition. This paper provides a good initiation point for further investigation into the mechanisms of corrosion for Cu bonding. The paper summarises the effect of Au and Cu wires in wirebonding process and its reliability relating to its material properties. © 2011 IEEE.


Breach C.D.,ProMat Consultants | Lee T.K.,ITE College Central
Journal of Electronic Materials | Year: 2012

The shear test failure modes of "as-bonded" gold and copper ball bonds are fundamentally different. Soft gold balls are typically sheared by the tool, leaving a lower section of the ball bonded to the aluminum metallization. In contrast, copper balls do not undergo appreciable plastic deformation and are sheared completely away from the bond pad. Evidence is presented to show that the different failure modes of gold are copper are due to the relative strength of the balls. Copper balls are generally harder than gold balls, and the failure mode changes to the weakest interface, which is at the ball- aluminum bond pad. Gold ball bonds fail due to plastic deformation of the ball, while copper ball bonds fail by a process of plastic deformation in the aluminum bond pad. © 2012 TMS.


Breach C.D.,ProMat Consultants | Wulff F.W.,W. C. Heraeus Gmbh
Microelectronics Reliability | Year: 2010

Thermosonic ball bonding is a metallurgical process that until recently was rarely the subject of metallurgical analysis. However, in recent years greater focus has been given to the materials science of thermosonic ball-wedge bonding in an attempt to better control and advance its application as an interconnect technology in advanced packaging. As with most materials processes, establishing a scientific understanding of the process requires knowledge from various sub-disciplines of physical science. This article briefly reviews selected aspects of the materials science of ball bonding, focussing on 1st and 2nd bond formation and intermetallic growth. © 2009 Elsevier Ltd. All rights reserved.


Breach C.D.,ProMat Consultants
Gold Bulletin | Year: 2010

Thermosonic ball bonding is a major interconnect process in microelectronics packaging and is positioned to remain one of the key process technologies available to package designers in the near future. However, the main wire material used in fine pitch (FP) and ultra-fine pitch (UFP) ball bonding is gold and with significant increases in gold price, gold ball bonding has become a more costly process that has a considerable economic effect on the assembly of packages used in consumer electronics. An alternative wire material to gold is copper, which is much cheaper and has several technical benefits including better electrical conductivity and has been widely used in discrete and power devices with wire diameters typically larger than 30 μm in diameter for many years. However, copper wire behaves quite differently than gold due to its different physical properties, some of which are beneficial and others detrimental to bonding performance. In this article, we briefly review some of the advantages and difficulties with using copper wire advanced packaging and explain why copper cannot replace gold in many applications and why gold offers significant benefits.


Goh C.S.,Institute of Technical Education | Chong W.L.E.,Institute of Technical Education | Lee T.K.,Institute of Technical Education | Breach C.,Promat Consultants
Crystals | Year: 2013

A comparison study on the reliability of gold (Au) and copper (Cu) wire bonding is conducted to determine their corrosion and oxidation behavior in different environmental conditions. The corrosion and oxidation behaviors of Au and Cu wire bonding are determined through soaking in sodium chloride (NaCl) solution and high temperature storage (HTS) at 175 °C, 200 °C and 225 °C. Galvanic corrosion is more intense in Cu wire bonding as compared to Au wire bonding in NaCl solution due to the minimal formation of intermetallics in the former. At all three HTS annealing temperatures, the rate of Cu-Al intermetallic formation is found to be three to five times slower than Au-Al intermetallics. The faster intermetallic growth rate and lower activation energy found in this work for both Au/Al and Cu/Al as compared to literature could be due to the thicker Al pad metallization which removed the rate-determining step in previous studies due to deficit in Al material. © 2013 by the authors; licensee MDPI, Basel, Switzerland.

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