Development of a thermosonic wire-bonding process for gold wire bonding to copper pads using argon shielding (original) (raw)
Related papers
Journal of Electronic Materials, 2006
A copper pad oxidizes easily at elevated temperatures during thermosonic wire bonding for chips with copper interconnects. The bondability and bonding strength of a gold wire onto a bare copper pad are seriously degraded by the formation of a copper oxide film. A new bonding approach is proposed to overcome this intrinsic drawback of the copper pad. A silver layer is deposited as a bonding layer on the surface of copper pads. Both the ball-shear force and the wire-pull force of a gold wire bonded onto copper pads with silver bonding layers far exceed the minimum values stated in the JEDEC standard and MIL specifications. The silver bonding layer improves bonding between the gold ball and copper pads. The reliability of gold ball bonds on a bond pad is verified in a high-temperature storage (HTS) test. The bonding strength increases with the storage time and far exceeds that required by the relevant industrial codes. The superior bondability and high strength after the HTS test were interpreted with reference to the results of electron probe x-ray microanalyzer (EPMA) analysis. This use of a silver bonding layer may make the fabrication of copper chips simpler than by other protective schemes.
Investigation of Thermosonic Wire Bonding Resistance of Gold Wire Onto Copper Pad
IEEE Transactions on Electronics Packaging Manufacturing, 2010
This paper discusses the electric performance for thermosonic wire bonding of gold wire onto copper pads. Various methods normally used to improve bondability were investigated including the bare copper pads with argon shielding gas and the copper pads with cupric oxide film, cuprous oxide film, and silver film. The micro-contact theory was used to determine the effective contact area. The circuit contact resistance was measured for each sample and was presented in terms of ultrasound power and effective contact area. The results show that the increase in the effective contact area leads to a lower circuit contact resistance before reaching a minimum value, and further increase in the effective contact area would not have noticeable effect on the resistance.
Thermosonic bonding of gold wire onto a copper pad with titanium thin-film deposition
Journal of Electronic Materials, 2004
A novel thermosonic (TS) bonding process for gold wire bonded onto chips with copper interconnects was successfully developed by depositing a thin, titanium passivation layer on a copper pad. The copper pad oxidizes easily at elevated temperature during TS wire bonding. The bondability and bonding strength of the Au ball onto copper pads are significantly deteriorated if a copper-oxide film exists. To overcome this intrinsic drawback of the copper pad, a titanium thin film was deposited onto the copper pad to improve the bondability and bonding strength. The thickness of the titanium passivation layer is crucial to bondability and bonding strength. An appropriate, titanium film thickness of 3.7 nm is proposed in this work. One hundred percent bondability and high bonding strength was achieved. A thicker titanium film results in poor bond-ability and lower bonding strength, because the thicker titanium film cannot be removed by an appropriate range of ultrasonic power during TS bonding. The protective mechanism of the titanium passivation layer was interpreted by the results of field-emission Auger electron spectroscopy (FEAES) and electron spectroscopy for chemical analysis (ESCA). Titanium dioxide (TiO2), formed during the die-saw and die-mount processes, plays an important role in preventing the copper pad from oxidizing. Reliability of the high-temperature storage (HTS) test for a gold ball bonded on the copper pad with a 3.7-nm titanium passivation layer was verified. The bonding strength did not degrade after prolonged storage at elevated temperature. This novel process could be applied to chips with copper interconnect packaging in the TS wire-bonding process.
Wire bonding is a process that is used to form solid-state bonds to interconnect metals such as gold wires to metalized pads deposited on silicon integrated circuits. Typically, there are 3 main wire bonding techniques; Thermo-compression, Ultrasonic and Thermosonic. This experiment utilizes thermosonic bonding which applies heat, ultrasonic energy and force on an Au-Al system. Sixteen groups of bonding conditions at various temperature settings were compared to establish the relationship between ball deformation and temperature. The results of this study will clearly indicate the effects of applied bonding temperature towards bond strength and deformation characteristics of gold ball bonding.
Effect of Conduction Pre-heating in Au-Al Thermosonic Wire Bonding
This paper presents the recent study by investigating the vital responses of wire bonding with the application of conduction pre-heating. It is observed through literature reviews that, the effect of pre-heating has not been completely explored to enable the successful application of pre-heating during wire bonding. The aim of wire bonding is to form quality and reliable solid-state bonds to interconnect metals such as gold wires to metalized pads deposited on silicon integrated circuits. Typically, there are 3 main wire bonding techniques applied in the industry; Thermo-compression, Ultrasonic and Thermosonic. This experiment utilizes the most common and widely used platform which is thermosonic bonding. This technique is explored with the application of conduction pre-heating along with heat on the bonding site, ultrasonic energy and force on an Au-Al system. Sixteen groups of bonding conditions which include eight hundred data points of shear strength at various temperature settings were compared to establish the relationship between bonding strength and the application of conduction pre-heating. The results of this study will clearly indicate the effects of applied conduction pre-heating towards bonding strength which may further produce a robust wire bonding system.
Extended reliability of gold and copper ball bonds in microelectronic packaging
Gold Bulletin, 2013
Wire bonding is the predominant mode of interconnection in microelectronic packaging. Gold wire bonding has been refined again and again to retain control of interconnect technology due to its ease of workability and years of reliability data. Copper (Cu) wire bonding is well known for its advantages such as cost-effectiveness and better electrical conductivity in microelectronic packaging. However, extended reliabilities of Cu wire bonding are still unknown as of now. Extended reliabilities of Au and Pd-coated Cu (Cu) ball bonds are useful technical information for Au and Cu wire deployment in microelectronic packaging. This paper discusses the influence of wire type and mold compound effect on the package reliability and after several component reliability stress tests. Failure analysis has been conducted to identify its associated failure mechanisms after the package conditions for Au and Cu ball bonds. Extended reliabilities of both wire types are investigated after unbiased HAST (UHAST), temperature cycling (TC), and high-temperature storage life test (HTSL) at 150, 175, and 200°C aging temperatures. Weibull plots have been plotted for each reliability stress. Obviously, Au ball bond is found with longer time to failure in unbiased HAST stress compared to Cu ball bonds for both mold compounds. Cu wire exhibits equivalent package and or better reliability margin compared to Au ball bonds in TC and HTSL tests. Failure mechanisms of UHAST and TC have been proposed, and its mean time to failure (t 50 ), characteristic life (t 63.2 , η), and shape parameter (ß) have been discussed in this paper. Feasibility of silver (Ag) wire bonding deployment in microelectronic packaging is discussed at the last section in this paper.
Study of Ag-alloy wire in thermosonic wire bonding
2012 IEEE 14th Electronics Packaging Technology Conference (EPTC), 2012
Lower cost materials, such as copper (Cu) and palladium coated copper (PdCu) are the commonly chosen alternatives of gold (Au) wire in the package industry. However, the high hardness of Cu and PdCu wires brings concerns over the bonding quality and the long-term reliability of the packages. Silver (Ag) has drawn more attention in the package industry since it has similar properties like hardness, elongation and breaking load as Au, while having a comparable price to PdCu. Bondability of Ag-alloy wire, including performance of free air balls (FAB) and bonding capability on aluminum (Al) die pads, was first investigated. Inspection of intermetallic compound (IMC) and unmolded baking of the bonded packages with the Ag-alloy wire were also carried out for further understanding the reliability performance of the wire. Investigations of bonding capability comparison between pure Ag, Ag-alloy, and PdCu wires in processes with stand-offstitch-bond (SSB) and peel sensitive dies were included in the study as well. Generally, Ag-alloy wire delivers good and stable bonding capability using N 2 as the cover gas. For applications with SSB and peel/lift sensitive bond pads which are normally difficult using PdCu wire, Ag-alloy wire also possesses good performance.
An analysis of intermetallics formation of gold and copper ball bonding on thermal aging
Materials Research Bulletin, 2003
In IC packages, thermosonic wire bonding is the preferred method for making electrical connections between the die pad and lead frame. These interconnect are made using fine metal wires. On thermal aging (under 175 8C for 5 h) gold aluminide easily forms in gold ball bonds while formation of intermetallic compound is absent in case of copper ball bonds. An analysis of the atomic property of the elements bonded explains that atomic radii and electronegativity factors favor gold aluminide formation. #
Microelectronics International, 2010
Purpose -Optimization of the process parameters remains a challenging task in thermosonic wire bonding due to relatively poor understanding of the bonding mechanism. The purpose of this paper is to understand initial bond formation in thermosonic gold wire bonding on aluminium metallization pads and the effect of bonding time on the initiation of bonding. Design/methodology/approach -A gold wire (20 mm diameter/99.99 per cent wt%) was bonded to aluminium metallization pads (1 mm thick) on a silicon chip using a commercial ball/wedge automatic bonder. Bonding parameters were selected specifically to produce underdeveloped ball bonds so that ball lift-off occurred during looping process. The lift-off footprints on the aluminium metallization pads and their evolution were carried out using optical microscopy and scanning electron microscopy. A model is proposed to elaborate the effect of bonding time on initiation of bonding. Findings -The obtained results showed that metallurgical bonding initiated at the peripheral areas of the contact area situated along the direction of ultrasonic vibration. Those areas extended inwards with bonding time, eventually covering the entire contact area. Originality/value -This paper describes how bond initiation and its evolution in thermosonic gold wire bonding on aluminium metallization is ascertained by observing lift-off footprints. The understanding of bonding mechanism benefits the optimization of process parameters and improvement of bondability in thermosonic wire bonding.