Intermetallic growth kinetics in gold ball bonds on Al-1%Si-0.5%Cu bond pads at 175∘C

Purpose The purpose of this study is to demonstrate that isothermal intermetallic growth data for gold ball bonds can be non-parabolic with explanations of why deviation from parabolic kinetics may occur. Design/methodology/approach Intermetallic thickness measurements were made at the centre of cross-sectioned ball bonds that were isothermally annealed at 175°C. Intermetallic growth kinetics were modelled with a power law expression(x(t) − x0)2 = α1tα2. The parameters of the power law model were obtained by transformation of the response and explanatory variables followed by data fitting using simple linear regression (SLR). Findings Ball bonds made with 4 N (99.99%Au) and 3 N (99.9%Au) gold wires exhibited two consecutive time regimes of intermetallic growth denoted Regime I and Regime II. Regime I was characterised by reactive diffusion between the gold wire and the aluminium alloy bond pad, during which Al was completely consumed in the formation of Au–Al intermetallics with non-parabolic kinetics. In Regime II, the absence of a free supply of Al to sustain intermetallic growth led to the conclusion that thickening of intermetallics was caused by phase transformation of Au8Al3 to Au4Al. Ball bonds made with 2 N (99%Au) wire also exhibited non-parabolic kinetics in Regime I and negligible intermetallic thickening in Regime II. Research limitations/implications The analysis of intermetallic growth is limited to total intermetallic growth at a single temperature (175°C). Originality/value The value of this study lies in showing that the assumption that only parabolic intermetallic growth is observed in isothermally aged gold ball bonds is incorrect. Furthermore there is no need to assume parabolic growth kinetics because with an appropriate data transformation, followed by fitting the data to a power law model using SLR and with the use of statistical diagnostics, both the suitability of the kinetic model and the nature of the growth kinetics (parabolic or non-parabolic) can be determined.

Influence of Initial Shear Strength on Time-to-Failure of Copper (Cu) Wire Bonds in Thermal Aging Condition

Copper (Cu) wire bond is used in a majority of microelectronic devices but has not been fully accepted by all industries due to potential reliability issues. Good quality bond is believed to provide high reliability. Shear strength and intermetallic (IMC) coverage are being used as indicators, however there is no developed model. High shear strength is a result of large IMC coverage laterally under the ball, but the reliability of the bond is also related to IMC longitudinal growth (thickness). This work involves studying effect of shear strength on reliability of a variety of experimental Cu ball bonds by performing temperature aging experiments on test devices, in QFN packages. The ball bonds are made by altering ultrasonic power and time to obtain different IMC coverage. All test packages are monitored for resistance change at specific intervals by performing four point resistance measurement. Resistance increase is analyzed with initial shear strength to determine whether “good quality” bonds always lead to high reliability.

High Temperature Storage Reliability of Bond Resistance of Palladium-Coated Copper Ball Bonds

Reliability of wire bonds made with palladium-coated copper (PCC) wire of 25 μm diameter is studied by measuring the wire bond resistance increase over time in high temperature storage at 225 °C. Ball bonds are made on two bond pad thicknesses and tested with and without mold compound encapsulation. Bond pads are aluminum copper (Al-0.5%Cu), 800 nm and 3000 nm thick. The wirebonding pattern is arranged to facilitate 4-wire resistance measurements of 12 bond pairs in each 28-pin ceramic test package. The ball bonding recipe is optimized to minimize splash on 3000 nm Al-0.5%Cu with shear strength at least 120 MPa. Ball bond diameter is 61 μm and height is 14 μm. Measurements include bond shear test data and in-situ resistance before and during high temperature storage. Bonds on 3000 nm pads are found to be significantly more reliable than bonds on 800 nm pads within 140 h of aging.

Reliability of Coated and Alloyed Copper/Silver Ball Bonds

The paper discusses on the reliability of coated and alloyed copper/silver ball bonds on both epoxy molded and unmolded conditions:Moisture resistance test using unmolded device at 130°C 85%RH (humidity chamber) revealed no ball lift failure until 96hours for the ball size of 1.65 times the diameter of wireCorrosion resistance test using unmolded device at room temperature by dripping (or soaking) dilute chlorine (Cl) solution revealed no ball lift failure for gold wires. Copper base wire bonds failed after 6min of storage while silver (Ag) base wire bonds showed a few bond lifts within 2min. This shows that Ag base wires are more sensitive to Cl environment than copper (Cu) base wiresA case study of epoxy molded device using green mold compound and four types of Cu and Ag base ball bonds passed on thermal ageing (HTS) at 175°C for 4000h and on +5V bHAST for 500h. The test response is by measuring electrical resistance in order not to reduce below 10% of contact resistance measured during time zero bonding○ Cross-section analysis of the samples showed intact bonding of Cu and Ag base ball bonds after 4000h of HTS○ The result shows when Cu and Ag base wire bonds molded with a good compatible green mold epoxy compound satisfy the automotive electronic council (AEC – Q006 & Q100 Rev-H, for Cu) requirements of 2X stress test with respect to electrical resistance measurementAnother case study of epoxy molded device revealed Cu and Ag base wire bonds pass 3000cycles of thermal cycling (−55°C to +150°C) without any neck/heel cracks and stitch lifts In addition, fine wires are baked at elevated temperature under vacuum or by purging nitrogen to find the quality of wire surface. As expected, Au, Cu and Ag base wires show clean surface. For palladium (Pd) coated Cu wire, a good Pd adhesion to Cu core surface without blisters is evident.