Au Ball Bonds Research Articles

Bondability and Strength Evaluation of Gold Ball Bond Using Nanoindentation Approach

The evaluation of the strength and bondability of gold, Au ball bond requires a new approach to provide a more detail data. Nanoindentation test was used as a new approach to evaluate the strength distribution and bondability of Au ball. Au ball bonds that experienced different value of wire bonding parameter namely bonding force, bonding time, bonding power, and stage temperature were used as samples for the present analysis. The distribution of strength based on hardness and reduced modulus values located at the bonding area of Au ball bonds were found to be related with the values of the wire bonding parameter. Nanoindentation test was found to be a suitable approach to analyze and evaluate the bondability of Au ball bond in a localized and detailed manner. In addition, the responsible mechanism for the thermosonic Au wire bonding can be identified and analyzed by using the results obtained from the nanoindentation test.

Effect of Bond Geometry on Shear Strength and HTS Reliability for Au Ball Bond on Al Pad

The effect of bond geometry on bond strength and reliability is studied for 25-μm gold wire bonds on standard CMOS aluminum pads. An accelerated method for parameter optimization is used to produce ball bonds with optimized shear strength (SS) and bond geometry. Ball bonds have a diameter of 56.5 μm with a bond height of 12, 16, and 20 μm representing the low, medium, and high bonds, respectively. Low height bonds achieve 126.1 MPa strength and endure up to 41.9% ultrasound (US) before deformation begins, whereas high bonds achieve 109.8 MPa strength and withstand a maximum of 39.8% US. Low bonds achieve higher strengths possibly due to the higher level of US that can be used. Bond reliability is assessed in high-temperature storage tests using a shear test (a destructive method) and piezoresistive stress sensors (a nondestructive method). The bonds are aged at 200°C for ~500 h. Quantifying the reliability performance using shear test results found to be difficult due to poor time resolution. However, from features in the stress sensor signals, a characteristic time (tc) is derived during aging. The t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> value indicates the time it takes for the bonds to severely degrade due to intermetallics and crack formation. The tc values for low, medium, and high bonds were found to be 199.9 ± 8.7, 225.9 ± 9.5, and 231.9 ± 6.8 h, respectively. The result that suggests ball bonds with higher heights are more reliable than the bonds with lower heights for a constant ball bond diameter. This result is corroborated by the SS before and after aging being the same for the high bonds, whereas SS dropped by 3% for the low bonds. The nondestructive method was demonstrated to have an estimated time resolution better than 10 h, which allows for precise comparisons of the reliability of various processes.

Development of a fast method for optimization of Au ball bond process

An accelerated optimization method is developed to minimize required time and resources, and demonstrated for a 25μm diameter Au ball bonding process. After a preparation phase to pre-set many parameters based on literature values, the values for more significant process parameters, impact force (IF) and EFO time (tEFO) for a given target bond geometry are optimized in a second phase, utilizing a 32 full factorial experiment and the response surface method (RSM). The target bond strength of 120±2MPa is achieved in a third phase by optimizing the ultrasonic energy (US) parameter using an iterative method. For an example process with a target geometry of 58μm for the bonded ball diameter measured at the capillary imprint (BDC) and 16μm for the height of the bonded ball (BH), the optimized process parameters (phases 2 & 3) can be found in less than 4h. The values for IF and tEFO are found to be 424mN and 0.474ms, respectively. The bond is strengthened with incrementing US until additional ball deformation occurs. The bond strength achieved is >120MPa with 48.6% US. Other bonding parameters include EFO current (IEFO)=50mA, temperature (T)=158°C, bond time (Bt)=20ms, and bond force (BF)=185mN.

Influence of shear strength on long term biased humidity reliability of Cu ball bonds

This paper compares and discusses the influence of shear strength (in terms of shear-per mils-square) of Au and Cu ball bonds on the biased humidity reliability performance in SOIC 8LD leaded package. Biased (HAST) highly accelerated temperature and humidity stress test, 130 °C, 85 % RH) has been carried out to estimate the long term reliability of Au and Cu ball bonds. Lognormal reliability plots have been plotted for the three legs (control, leg 1 and leg 2) whereby Shear-per-mil-square of 6.68 is identified to have better mean-time-to-failure (t50) compared to other two legs. Open failure from biased HAST 96, 192 h are subjected for secondary electron microscopy cross-sectioning and found typical interfacial CuAl intermetallic compound corrosion microcracking. HAST failure rates have been analyzed and the Cu ball bond lifetime has been established by using Peck model. The obtained Cu ball bond lifetime, of SPMS of 6.68 is >25 years and belongs to wearout reliability data point. This proves significant influence of SPMS on biased HAST failure rate. The higher the ball bond shear strength the lower the failure rate of biased HAST test. Hence, we should implement control on the average SPMS of ≥7.50 g/mil2.

Evolution and investigation of copper and gold ball bonds in extended reliability stressing

This paper discusses the microstructure evolution of copper (Cu) and gold (Au) ball bonds after various extended reliability stresses such as biased highly accelerated temperature and humidity test (HAST), unbiased highly accelerated temperature and humidity test (UHAST), temperature cycling (TC), and high temperature storage life (HTSL) in BGA package. Objective of this study is to study the microstructure evolution and changes after long hours and long cycles of component reliability stressing and its predicted failure mechanisms and to determine the long-term reliability comparison with combination of bonding wires in HAST, UHAST, and TC. Secondary electron microscopy (SEM) and energy dispersive X-ray (EDX) have been carried out to understand the respective microstructure of failed samples in HAST, UHAST, TC, and HTSL long-term reliability failures. Respective failure mechanisms of copper and gold ball bonds carrion under HAST and UHAST, ball bond lifting in TC and HTSL have been analyzed and proposed. The evolution of surface morphology, including copper and gold ball bond micro cracking, gold ball bond Kirkendall microvoiding and intermetallic compound (IMC) formation, was studied in FBGA package with copper and gold ball bonds during various reliability stresses. Biased HAST, UHAST, TC, and HTSL mechanisms were proposed to explain the observed morphological changes and the resulting ball bond wear out modes after extended reliability stresses. Weibull reliability analyses have been established to compare the performance of copper and gold ball bonds under humid and dry environmental tests.

Effects of Bonding Wires and Epoxy Molding Compound on Gold and Copper Ball Bonds Intermetallic Growth Kinetics in Electronic Packaging

This paper discusses the influence of bonding wires and epoxy mold compounds (EMC) on intermetallic compound (IMC) diffusion kinetics and apparent activation energies (E aa) of CuAl and AuAl IMCs in a fineline ball grid array package. The objective of this study is to study the CuAl and AuAl IMC growth rates with different epoxy mold compounds and to determine the apparent activation energies of different combination of package bills of materials. IMC thickness measurement has been carried out to estimate the coefficient of diffusion (D o) and E aa various aging conditions of different EMCs and bonding wires. Apparent activation energies (E aa) of both wire types were investigated after high temperature storage life tests (HTSL) for both molding compounds. Au bonds were identified to have faster IMC formation, compared to slower IMC growth of Cu. The E aa obtained for CuAl IMC diffusion kinetics are 1.08 and 1.04 eV with EMC A and EMC B, respectively. For AuAl IMC diffusion kinetics, the E aa obtained are 1.04 and 0.98 eV, respectively, on EMC A and EMC B. These values are close to previous HTSL studies conducted on Au and Cu ball bonds and are in agreement to the theory of HTSL performance of Au and Cu bonding wires.Overall, EMC B shows slightly lower apparent activation energy (E aa) valueas in CuAl and AuAl IMCs. This proves that the different types of epoxy mold compounds have some influence on IMC growth rates.

Explaining Nondestructive Bond Stress Data From High-Temperature Testing of Au-Al Wire Bonds

The application of an alternative method of bond monitoring during high-temperature aging is reported using a custom made test chip with piezoresistive integrated CMOS microsensors located around test bond pads. The sensor detects radial stresses originating from the bond pad and can resolve changes because of intermetallic compound (IMC) formation, voiding, or crack formation at the bond interface. Optimized Au ball bonds are aged for over 2000 h at 175 °C. It is found that stress sensors next to the bonds are capable of showing the stages of IMC growth, consumption of pad Al layers, and monitoring the formation of low-density and Al-rich IMC (AuAl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) which shows an advanced stage of aging. In particular, a first stress signal increase corresponds to the conversion of all Al above the diffusion barrier into IMCs. The second increase in stress signal after a period of stability corresponds to conversion of all Al below the barrier into IMCs. The IMC formation in these periods causes shear strength increase. After complete bond Al consumption, the bond, however, reaches maximum strength. As bond degradation starts, e.g., by lateral IMC formation, voiding, and oxide formation, as well as because of lateral pad Al transformation to IMC, the signal exhibits a strong decrease. The findings are corroborated by results obtained from classical methods such as interruptive or destructive testing including visual inspection, shear testing, cross sectioning, and by bond resistance monitoring.

Superior performance and reliability of copper wire ball bonding in laminate substrate based ball grid array

PurposeThe purpose of this paper is to provide a systematic method to perform long‐term reliability assessment of gold (Au) and copper (Cu) ball bonds in fineline ball grid array package. Also with the aim to study the apparent activation energies (Eaa) and its associated wearout mechanisms of both Au and Cu wire in semiconductor device packaging. This paper discusses the influence of wire type on the long‐term reliability and mechanical performance after several component reliability stress tests.Design/methodology/approachA fineline ball grid array (FBGA) package with Cu and Au wire bonds was assembled with green molding compound and substrate. Samples are subjected for long‐term high temperature storage bake test at elevated temperatures of 150°C, 175°C and 200°C. Long‐term reliability plots (lognormal plots) are established and Eaa of both ball bonds are determined from Arrhenius plots. Detailed failure analysis has been conducted on failed sample and HTSL failure mechanisms have been proposed.FindingsReliability results show Au ball bond in FBGA package is observed with higher hour‐to‐failure compared to Cu ball bonds. The Eaa value of high temperature storage life (HTSL) reliability for Au ball bond is lower than Cu ball bond. Typical HTSL failure mechanism of Au ball bond is induced by micro‐voiding and AuAl intermetallic compound (IMC) micro‐cracks while CuAl IMC micro‐cracking (induced by Cl− corrosion attack and micro‐cracking) caused wearout opens in Cu ball bond. These test results affirm the test‐to‐failure data collected is a useful method for lifetime prediction and Eaa calculation.Practical implicationsThe paper reveals higher reliability performance of Cu ball bond in FBGA flash memory package which can be deployed in flash memory FBGA packaging with optimised package bill of materials.Originality/valueThe test‐to‐failure methodology is a useful technique for wearout reliability prediction and Eaa calculation.

Effect of Nanoindentation Loading Rate on Gold Ball Bond

Nanoindentation tests with loading rates of 0.05 mN/s, 0.1 mN/s, 0.5 mN/s, and 1.0 mN/s were conducted on the Au ball bond. The effect of different loading on the Au ball bond were analysed based on qualitative and quantitative results. The displacement burst was more pronounced with the increment of loading rates. The increase of hardness value and the decrease of the reduced modulus value when the loading rate was increased are due to the effect of creep. It was found that the loading rate of 0.5 mN/s is the appropriate and stable value for the nanoindentation test on the Au ball bond.

Strength distribution of Au ball bond using nanoindentation approach

The evaluation of gold, Au ball bond requires a new approach to obtain a more localized and detailed result. This is due to the limited data in terms of quality and quantity obtained from conventional tests. The nanoindentation test is proposed in order to evaluate the quality of ball bond in more detail. The nanoindentation test was carried out to analyze the strengthening of Au ball bond after being subjected to high temperature storage, HTS. It was observed that Au and intermetallic compounds, IMC, have a higher creep behavior compared to that of silicon, Si. The dislocation glide was the responsible indentation creep mechanism for Au and IMC of ball bond that have been subjected to 0 and 1000h of HTS. It was observed that the variation of creep behavior of Au ball bond was reduced after being subjected to 1000h of HTS. The variation of creep behavior of IMC decreased after being subjected to 1000h of HTS. It was also noted that the creep behavior of both Au and IMC decreased with the increase of HTS time interval. It was found that the hardness value for Au decreased with the increase of HTS time interval, whereas, the hardness value of IMC increased with the increment of HTS time interval. Based on the results obtained, it was indicated that the nanoindentation test has the ability to evaluate the strengthening of Au ball bond in a localized manner and in more detail compared to that of the conventional tests.

Reliability Assessment and Activation Energy Study of Au and Pd-Coated Cu Wires Post High Temperature Aging in Nanoscale Semiconductor Packaging

Wearout reliability and high temperature storage life (HTSL) activation energy of Au and Pd-coated Cu (PdCu) ball bonds are useful technical information for Cu wire deployment in nanoscale semiconductor device packaging. This paper discusses the influence of wire type on the wearout reliability performance of Au and PdCu wire used in fine pitch BGA package after HTSL stress at various aging temperatures. Failure analysis has been conducted to identify the failure mechanism after HTSL wearout conditions for Au and PdCu ball bonds. Apparent activation energies (Eaa) of both wire types are investigated after HTSL test at 150 °C, 175 °C and 200 °C aging temperatures. Arrhenius plot has been plotted for each ball bond types and the calculated Eaa of PdCu ball bond is 0.85 eV and 1.10 eV for Au ball bond in 110 nm semiconductor device. Obviously Au ball bond is identified with faster IMC formation rate with IMC Kirkendall voiding while PdCu wire exhibits equivalent wearout and or better wearout reliability margin compare to conventional Au wirebond. Lognormal plots have been established and its mean to failure (t50) have been discussed in this paper.

Extended reliability of gold and copper ball bonds in microelectronic packaging

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 (s) 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.

Reliability assessment and mechanical characterization of Cu and Au ball bonds in BGA package

Extended reliability and mechanical characterisation of Au and Pd-coated Cu (Cu) ball bonds are useful technical information for Au and Cu wire deployment in semiconductor device packaging. This paper discusses the influence of wire type on the package reliability and mechanical performance after several component reliability stress tests. Failure analysis has been conducted to identify the associated failure mechanisms of Au and Cu ball bonds after reliability tests. Wire pull strength and ball bond shear (with its break modes) of both wire types are investigated after unbiased highly accelerated temperature and humidity stress test (UHAST), temperature cycling (TC) and high temperature storage life test (HTSL) at various aging temperatures. Reliability Weibull plots have been plotted for each reliability stresses. Obviously Au ball bond is found with longer time-to-failure in Unbiased HAST stress compare to Cu ball bonds. Cu wire exhibits equivalent package and or better reliability margin compare to Au ball bonds in TC and HTSL tests. Failure mechanisms of UHAST and HTSL have been proposed and its mean-time-to failure (t50), characteristics life (t63.2, η) and shape parameter (s) have been discussed in this paper.

Effect of Applied Load in the Nanoindentation of Gold Ball Bonds

We have analyzed the effects of nanoindentation at applied loads of 10 mN and 20 mN on the micromechanical properties of gold (Au) ball bonds with and without cracks. The depth profile and the plastic zone size for each indentation were determined to identify the substrate effect and its relationship with the observed micromechanical properties. The substrate effect occurred for indentations at 20 mN applied load, but did not occur near cracks for either 10 mN or 20 mN loads. Because of the substrate effect and the presence of cracks, the average hardness or yield strength decreased for indentations on Au ball bonds. Therefore, to minimize the substrate effect, an applied load of 10 mN is best for characterizing Au ball bonds.

Wearout Reliability and Intermetallic Compound Diffusion Kinetics of Au and PdCu Wires Used in Nanoscale Device Packaging

Wearout reliability and diffusion kinetics of Au and Pd‐coated Cu (PdCu) ball bonds are useful technical information for Cu wire deployment in nanoscale semiconductor device packaging. This paper discusses the HAST (with bias) and UHAST (unbiased HAST) wearout reliability performance of Au and PdCu wires used in fine pitch BGA packages. In‐depth failure analysis has been carried out to identify the failure mechanism under various wearout conditions. Intermetallic compound (IMC) diffusion constants and apparent activation energies (Eaa) of both wire types were investigated after high temperature storage life test (HTSL). Au bonds were identified to have faster IMC formation, compared to slower IMC growth of PdCu. PdCu wire was found to exhibit equivalent or better wearout reliability margin compared to conventional Au wire bonds. Failure mechanisms of Au, Cu ball bonds post‐HAST and UHAST tests are been proposed, and both Au and PdCu IMC diffusion kinetics and their characteristics are discussed in this paper.

Study of Wirebonding on Thin Al Pads with Various Size Probe Marks

IC bond pads having thin pad aluminum (Al), less than 0.7μ thick, were probed multiple times by a variety of cantilever probe tips. Probe marks were created by an experimental high force probe card having various tip diameters. A finite element model of the probe tip was matched to the probe mark scrub length to gain a little more academic understanding of what to expect as probe parameters vary. Simulations using this model will be helpful in the future for situations where physical experiments are difficult or costly to run. Bond pads in the experiments include various ON Semiconductor circuit under pad (CUP) structures having Al metallization and silicon dioxide (SiO2) interconnect, previously demonstrated to be robust against cracking when compared against traditional IC bond pads. Smaller ball size and bond contact area are desirable as pads shrink on future products, but this will heighten any adverse effects of probe marks due to the relative % area increase beneath the bond. Experiment evaluations include bond pull strength (BPS), and bond shear (BS) for gold (Au) ball bonds of varying ball diameters over the wide range of probe marks, to begin checking the validity of a customary “probe mark area” maximum limit in wirebonding. Data indicates that large, deep probe marks can indeed bond ball lift fails, especially for non-optimized bonding recipes. It appears that probe mark depth, not area, is the most adverse factor in bond reliability. No issues relating to bonding over probe marks are expected in the more controlled and “gentle” manufacturing situations.

Packaging Approach for Integrating 40/45-nm ELK Devices Into Wire Bond and Flip-Chip Packages

There is a rapid transition in the semiconductor packaging industry of devices moving toward 40/45-nm extreme low k (ELK) from the development phase into mainstream semiconductor assembly manufacturing. The drive to achieve adoption without major changes to process and equipment infrastructure while meeting the superior yield necessary is high. The main goal of this paper is to share learning's and provide solutions for integration and qualifications of 40/45-nm ELK devices into flip-chip (FC) and wire-bond (WB) package technology. The scope of this paper covers the fine-pitch ball grid array for the WB packages and FC chip-scale package for the FC Devices. The main challenge and focus point for WB 45/40-nm interconnects is on the first bond process. This paper will share some detailed analysis on the following. 1) Characterization for 45 nm ELK ultrafine-pitch. 2) Key factors for good Au ball bond integrity. 3) Recommendations for achieving the required levels of reliability for 45-nm Cu ELK. The challenge of FC Technology today has been changing at a faster rate and more and more adoption has been made from the conventional eutectic solder bumps or high lead solder bumps toward the lead free solder bump mainly due to the Green initiatives. This paper will share analysis on the following areas. 1) Thermo-mechanical simulation on key input factors and its correlation to actual evaluations. 2) Integration of Pb Free ELK bumps with 40-nm ELK technology are various critical processes. 3) Moldable underfill technology for Pb Free + ELK devices. In addition, critical process control is also required at the wafer level process of wafer thinning and dicing, including the need of Laser Grooving will also be covered.

Effect of Pd Surface Roughness on the Bonding Process and High Temperature Reliability of Au Ball Bonds

A 0.3-μm-thick electrolytic Pd layer was plated on 1 μm of electroless Ni on 1 mm-thick polished and roughened Cu substrates with roughness values (R a) of 0.08 μm and 0.5 μm, respectively. The rough substrates were produced with sand-blasting. Au wire bonding on the Ni/Pd surface was optimized, and the electrical reliability was investigated under a high temperature storage test (HTST) during 800 h at 250°C by measuring the ball bond contact resistance, R c. The average value of R c of optimized ball bonds on the rough substrate was 1.96 mΩ which was about 40.0% higher than that on the smooth substrate. The initial bondability increased for the rougher surface, so that only half of the original ultrasonic level was required, but the reliability was not affected by surface roughness. For both substrate types, HTST caused bond healing, reducing the average R c by about 21% and 27%, respectively. Au diffusion into the Pd layer was observed in scanning transmission electron microscopy/ energy dispersive spectroscopy (STEM–EDS) line-scan analysis after HTST. It is considered that diffusion of Au or interdiffusion between Au and Pd can provide chemically strong bonding during HTST. This is supported by the R c decrease measured as the aging time increased. Cu migration was indicated in the STEM–EDS analysis, but its effect on reliability can be ignored. Au and Pd tend to form a complete solid solution at the interface and can provide reliable interconnection for high temperature (250°C) applications.

Power SO-8: An Interconnect Case Study

With the rising cost of Au base material, many power semiconductor device manufacturers are evaluating alternative interconnect technologies to reduce manufacturing costs. In addition, shrinking package sizes, increasing electrical performance requirements and environment-friendly packaging objectives are additional considerations when choosing an alternative interconnect technology. Presently, three primary technologies compete to become the dominant Au wire replacement in the small outline, discrete power semiconductor market. Each alternative, Cu Wire, Cu Clip, and Al Ribbon, attempts to strike the best balance of electrical performance, manufacturing cost, and production yield. The small outline, discrete power semiconductor market can be broadly described as the sub-category of discrete semiconductors presently utilizing multiple 2-mil (or larger) diameter Au ball bonds as the interconnect between the die and the package leads. This paper explores observed benefits of these competing technologies in two example Power SO-8 packages that are presently ball-bonded with 2-mil Au Wire. In this comparison, we will examine the electrical performance (Rds(on) reduction), reported productivity (UPH), production yields, manufacturing cost-per-part, product-line flexibility and lead-free compliancy for each interconnect method. From this case study, similar extrapolations can be made for applying these interconnect technologies to similar packages, such as Small Outline Non-leaded (SON or PDFN), Power QFNs (PQFN), Driver MOSFETs (DrMOS), and Bi-Polar Transistor packages.

Nanoindentation Approach for Evaluation of Process Parameters Effect on the Strength of Bonded Au Ball Bonds

The nanoindentation test and geometry measurement have been conducted to evaluate the hardness and geometry changes of bonded Au ball bonds towards the changes of the selected wire bonding parameters namely bonding power, bonding time and bonding force. Three indentations were made on the bonded ball bonds to evaluate the variation of hardness properties with the location of indentation. It was noted that the increase of bonding or ultrasonic power will increase the hardness value for the indentations 1 and 3 located at the periphery of bonded ball bonds. The increase of bonding power also increased the deformation of bonded ball bonds. It was also shown that the increment of bonding time will increase the hardness value across the bonded ball bonds in almost even distribution. The application of the bonding force in the wire bonding process has the least effect on the hardness and geometry changes on the bonded ball bonds.