Ball Shear Research Articles

A new omnibus SPRT chart for monitoring process mean and variability based on the average number of observations to signal

The recent development of the omnibus sequential probability ratio test (OSPRT) chart marks a significant contribution to the advancement of joint monitoring schemes. As the OSPRT chart is a variable-sample-size control chart, practitioners often wish to understand its inspection efficiency, i.e. the number of observations it samples before producing a signal. In this article, we propose two enhanced optimization designs for the OSPRT chart based on the average number of observations of signal (ANOS) and expected value of the ANOS (EANOS) metrics under deterministic and unknown shift sizes, respectively. The ANOS metric is central to our design as it perfectly combines both the average run length (ARL) and the average sample number. A comparative analysis reveals that the OSPRT chart outperforms four benchmarking control charts in terms of the ANOS and EANOS metrics. Finally, an implementation of the OSPRT chart is presented with a ball shear test dataset.

Microstructure and mechanical behavior of Sn15Bi-xAg\Cu solder joints during isothermal aging

Purpose This paper aims to focus on the reliability of Sn15Bi–xAg and Sn15Bi–xCu solder joints during isothermal aging. Design/methodology/approach The effects of Ag or Cu additions on the microstructure, interfacial metallic compound layer and shear strength of Sn–15Bi (Sn15Bi) based solder joints during were investigated. The effects of Ag or Cu additions on the microstructure and tensile properties of Sn15Bi-based bulk solders were also investigated to provide a comprehensive analysis. The interfacial morphology and microstructure were observed by scanning electron microscopy and the composition in the structure was examined by energy dispersive spectrometer. The shear tests were carried out on the as-soldered and as-aged joints using a ball shear tester. Findings The results revealed that by adding Ag or Cu, the microstructure of Sn15Bi solder can be refined. Ag addition increased the tensile strength of Sn15Bi solder but had little effect on elongation. However, Cu addition decreased the tensile strength and elongation of Sn15Bi solder. For solder joints, Ag addition increased the shear strength and toughness of Sn15Bi/Cu joints but Cu addition decreased the shear strength and toughness of Sn15Bi/Cu joints. Originality/value The authors can potentially provide a replacement for Sn40Pb traditional solder with Sn15Bi solder by alloying Ag or Cu due to its lower cost and similar melting point as Sn–Pb solder.

Silver Bonding Wire - an Alternative for Mechanical Sensitive Chip Configurations in Automotive Electronics Packaging

Silver (Ag) bond wires are mainly used in consumer products as a reliable low-cost alternative to gold (Au) bond wires. In previous investigations, we have shown that silver wires also have potential for use in the automotive sector (1, 2, 3, 4], where the materials are exposed to particularly harsh environmental conditions. In order to understand thc intluence of these conditions in the electronic package on the silver-aluminum material system, we carried out extensive studies on test packages built on laboratory level. For the future development of challenging automotive devices, it will be important to have a cooperation between semiconductor package developer and material supplier with an assist of characterization spccialists. In this project, we formed a threeparty project in which characterization specialists, package developer, and wire manufacturer collectivcly conduct a feasibility study of a new Ag wire material for automotive application. Two wire types (standard Ag wire and new type of Ag wire GX2s) were chosen for the investigation in direct comparison regarding long-term reliability. Samples were built under mass production conditions and then subjected to various reliability tests following AEC-Q006 requirements. Subsequently, mechanical characterizations by means of ball shear and pull tests as weil as high resolution SEM microstructure analyses on cross-section samples are applied. In the paper, results of the different tests will be summarized, but a more in-depth analysis will be given to the temperature cycle test (TCT), where the most significant differences between the sample variations occurred. Overall, the results show the importance of the knowledge for the material reliability upfront a device qualification process (robustness validation) and give further guidance for material selection.

A Comprehensive Study of Surface Finishes for High Frequency/High Speed Applications

The introduction of 5G/6G created growing demand for faster rates of data transfer and operation at higher frequencies, pushing signals to travel towards the outer edges of conductors. Therefore, the surface finish applied over the copper circuitry is now gaining more attention. It has been previously shown that the low conductivity and magnetic properties of the Electroless Nickel (EN) layer negatively affect electrical signals as they travel along the conductor outer surfaces, leading to insertion losses. Subsequent studies show that reducing the EN thickness can offset some of the insertion losses observed, leading to a reduction in insertion losses. In more recent times “Nickel Free’ finishes, such as EPIG (Electroless Palladium - Immersion Gold, no EN) have been promoted as solutions to improving signal losses at higher frequency. Exactly how much high frequency performance these new “nickel-free” finishes by the user has not yet been fully quantified, and what are the considerations or trade-off’s for solderjoint reliability with smaller diameter solder spheres when assembled using these newer surface finishes? Comprehensive studies of signal loss, solderjoint reliability testing (ball shear, drop shock, electromigration) and other critical to quality performance data were undertaken for standard ENEPIG, Thinner EN ENEPIG’s, EPIG (no EN) as well Immersion Silver, OSP and a new Ag/Au surface finish. All surface finishes were compared across the performance requirement matrix and a Quality Function Deployment was constructed to produce a data driven tool to align surface finish performance against design needs, allowing for optimal surface finish selection for each application.

Improving Reliability of Fan -Out Wafer Level Package Through Doping of Lead-Free Solder Balls

Embedded wafer level ball grid array (eWLB) or FO-WLP (Fan-out wafer-level packaging) is investigated as a package for MMICs (Monolithic Microwave Integrated Circuit) for automotive radar applications in the 77GHz range. Doping of Bi and/or Sb was applied to improve the resistance of lead-free SAC balls against thermal cycling stress. We have shown that by increasing the doping level the lifetime can be increased and determined the maximum doping level before extrinsic failure modes dominate. It is well-known from the literature that doping of Bi and/or Sb improves the resistance of lead-free SAC balls against thermal cycling stress. It was shown that lifetime of FC-BGA package can be multiplied using Bi and/or Sb doping. The lifetime gain is achieved by reduced creepage of the hardened balls caused by the doping. This reduced creepage increases the risk of causing other failures than ball cracks in the eWLB package or the PCB during thermal cycling stress. We have tested robustness of 7x8,5 mm^2 eWLB packages against thermal cycling stress T = -40 to 125 °C using SAC-balls with different Bi-doping content from 0% to 2,5% Bi concentration. As could be expected the solder ball fatigue mode is delayed more and more increasing the Bi content. We found other failure modes than solder ball fatigue emerging at doping around 1%. The fatigue modes are shifted towards cupper lines on PCB or into the package. Cu line cracks can be overcome by switching the PCB layout from NSMD (non solder-mask defined) to SMD (solder-mask defined) solder pads. In general there is a lifetime penalty when switching from NSMD to SMD, but by avoiding the Cu line cracks this lifetime loss is overcompensated using SMD pads for optimum performance. As input for simulations we have characterized the doped solder material using specimens produced in glass columns. We have also tested the components on PCBs with different top layer laminate with different CTE and elastic modulus to assess the influence of the PCB properties on the lifetime. It is a trend in automotive radar systems to use stiffer laminates which are cheaper but have negative influence on component lifetime. Solder ball shear tests were also carried out to characterize the influence of ball doping on other board-level reliability beyond thermal cycling performance. As expected ball shear force is increased with increased doping level. At increased shear-force the shearing is still done through the solder ball and does not lead to different shear-mode. Finally drop-tests were done to check influence of doping on board-level reliability performance other than thermal-cycling performance.

Effect of Sb Addition on Microstructure and Shear Strength of Sn-3.0Ag-0.5Cu Solder Ball Joints

The effect of a small amount of Sb addition to Sn-3.0Ag-0.5Cu (mass%) solder on microstructures and impact properties of the solder ball joint was investigated. Cross-sectional microstructural observation revealed that scalloped Cu6Sn5 is formed at the solder/Cu interface, and Cu3Sn is formed at the interface between Cu6Sn5 and Cu with aging. It was confirmed that the growth of the reaction layer is suppressed by the addition of Sb. Moreover, the result of the impact ball shear test showed that the decrease in impact properties with aging can be suppressed in Sn-3.0Ag-0.5Cu-1.5Sb (mass%). It was also suggested that the suppression of linear crack propagation at the Cu3Sn/Cu6Sn5 interface is effective to prevent the reduction of absorbed energy.

Bonding Property of Middle Temperature Sn-Ag-Cu Solder Ball Joint of Ball Grid Array Package

Alloying effects of Nickel and Bismuth in SAC solder to achieve mechanical and microstructural evolution were investigated by comparing three different chemical composition of solders. Sn-3.0Ag-0.5Cu (SAC305) is the most commonly used for Pb-free solder and solder balls are based on Sn-Ag-Cu alloy composition. Compositions of solder balls were Sn-3.0Ag-0.5Cu (SAC305), Sn-1.2Ag-0.5Cu-0.05Ni (SAC1205N) and Sn-Ag-Cu-Ni-Bi based solder (SACNB). Organic solderability preservative (OSP) surface finished substrate, ball grid array (BGA) package arranged by 300 ㎛ diameter solder ball and Type 4 solder paste were used. Solder ball shear test of the BGA package was conducted before the reflow soldering. Board level package bonding process was carried out by hot air reflow soldering. Void content of the BGA solder joint was investigated after soldering. The microstructure evolution before and after thermal cycle test (TCT) was observed using scanning eletron microscopy (SEM) and energy dispersive spectroscopy (EDS). After TCT, intermetallic compound (IMC) refinement and solid solution strengthening effects of Bi element addition were observed in the solder matrix, and also any segregation of Bi element was not founded. Based on these result, it was found that the solder joint reliability of the middle and low temperature solder joints was improved.

Effects of Process Parameters on Bond Properties of Ag-2.35Au-0.7Pd-0.2Pt-0.1Cu Alloy Wire.

Bond properties were performed on Ag-2.35Au-0.7Pd-0.2Pt-0.1Cu alloy wire with a diameter of 25 µm under different process parameters. The effects of electrical flaming off (EFO) current and EFO time on the deformability of the free air ball (FAB) were investigated using scanning electron microscopy (SEM), as well as the effects of ultrasonic power and bonding force on the bond characteristic. The experimental results show that FAB grows from a preheated tip to a small ball, a regular ball, and finally to a golf ball with increasing either the EFO current or the EFO time, and the FAB presents an optimal shape at 25 mA and 650 μs. Moreover, a nonlinear relationship between FAB diameter and EFO time is obtained at an EFO current of 25 mA, which could be expressed by a cubic equation. Further, at a constant bonding force, as the ultrasonic power increased, the mashed ball diameter grew larger and larger, the capillary hole imprint became more and more obvious, and the tail width also increased, and vice versa. The optimal ultrasonic power and bonding force are 70 mW and 45 gf for ball bonding and 90 mW and 75 gf for wedge bonding, respectively. Finally, for all the bonded wire samples prepared under optimal process parameters, no ball and wedge bond lifts happened after the destructive pull test, and full intermetallic compound coverage with perfect morphology occurred on the bond pad after the ball shear test, which meant that the bonded wire samples had high bond strength and hence improved the reliability of microelectronic products. It provided technical support for the reliability research of Pt-containing Ag-based bonding alloy wires.

DEVELOPMENT AND CHARACTERIZATION OF SELF-HEALING TRANSDERMAL PATCH OF LIDOCAINE FOR THE MANAGEMENT OF PAIN

Pain management is a significant challenge due to the side effects associated with commonly prescribed medications like NSAIDs and opioids. Local anesthetics, such as lidocaine, offer an alternative with fewer side effects when formulated in topical patches. Transdermal delivery systems, including heated patches, enhance drug absorption and provide targeted pain relief. This research paper aims to develop and characterize a self-heating transdermal patch of lidocaine for pain management. The drug sample of lidocaine was characterized using UV-visible spectrophotometric analysis, melting point determination, and differential scanning calorimetry. The results confirmed the purity of the drug sample. Pre-formulation studies were conducted to determine the solubility and partition coefficient of lidocaine, as well as drug-excipient interaction studies. The formulation of the lidocaine transdermal patch included the selection of excipients such as solvents, adhesives, solubilizers, and permeation enhancers. The patch was developed in three trial batches, with the final batch prepared using a drug-in-adhesive type approach. The developed patch was evaluated for physical characteristics, solvent residual content, rolling ball test, shear strength, drug crystallization, drug content, and in-vitro permeation. The results of the evaluation showed that the developed lidocaine transdermal patch had the desired physical characteristics, uniform thickness, good folding endurance, and appropriate solvent residual content. It exhibited satisfactory rolling ball test and shear strength. Microscopic examination confirmed the absence of drug crystallization. The drug content of the patch was determined to be within the desired range, and the in-vitro permeation study demonstrated successful drug release through the dialysis membrane. In conclusion, the development and characterization of a self-heating transdermal patch of lidocaine for pain management provide a promising approach for effective and targeted pain relief. The patch formulation demonstrated suitable physical properties, drug content, and permeation characteristics, suggesting its potential as an alternative pain management solution. KEYWORDS: Lidocaine, transdermal patch, pain management, self-heating, drug delivery

Mechanical Properties of Solderable Polymer Composites with Low- and High-Melting-Point Solder Mixed Filler

A solderable polymer composite (SPC) with a low-melting-point solder (LMPS)/high-melting-point solder (HMPS) mixed filler was formulated to solve the bonding property and reliability problems in an SPC system with an LMPS filler. In addition, the possibility of enhancing the mechanical bonding properties of SPC with LMPS/HMPS mixed fillers was investigated. Three types of SPCs (mixing ratios for LMPS and HMPS: 100:0, 50:50, and 0:100) were prepared, and two types of mechanical property investigations (i.e., ball shear and microhardness tests) were performed to measure the mechanical bonding properties of SPCs according to the LMPS/HMPS mixing ratios. The SPC with an LMPS/HMPS mixed filler exhibited enhanced mechanical bonding properties compared with the Sn-58Bi solder paste and SPC with LMPS or HMPS only due to the change in composition and subsequent microstructure transformation of the solder joint after adding HMPS.

Solderability and Reliability of Sintered Nano-Ag Bond Pads of Printed Re-Distribution Layer

Abstract Redistribution layer (RDL) is one key enabling technology for advanced packaging. RDL is usually fabricated at wafer level by a photolithography process. An alternative approach for implementing RDL by additive manufacturing (AM) method is proposed in this study. This allows RDL to be fabricated on a singulated chip. Nanosilver (nano-Ag) ink is printed on the silicon chip to form routing traces and bond pads. However, the Ag pad may be consumed by the solder quickly if the reflow process is not properly controlled. The mechanical integrity of the solder joint was evaluated by both solder ball shear and die shear tests. A high temperature storage test and humidity test were carried out to evaluate the solder joint reliability. Experiment results showed that the Ag pad fabricated by AM is compatible with surface mount technology (SMT). Reliable solder joints on the Ag pad were achieved. There was no significant change in the Ag3Sn intermetallic compound (IMC) layer thickness and mechanical strength after aging at high temperature and high humidity conditions. There was no early solder joint failure observed. The finding of the present study will serve as a very useful reference for the future practice of forming solder joints on sintered nano-Ag pads.

Au-coated Ag alloy bonding wires with enhanced oxidation resistance for electronic packaging applications

PurposeThe purpose of this paper is to develop high-performance Au-coated Ag alloy wires (ACAA wires) and demonstrate the effect of Au coating layers on the bonding performance and oxidation resistance for stable and reliable electronic packaging applications.Design/methodology/approachACAA wire with a diameter of approximately 25 µm and Au layer thickness of approximately 100 nm were prepared by the continuous casting, plating and wire drawing method. The bonding performance of the ACAA wires were studied through bonding on 3,535 chips. The oxidation resistance of ACAA wires and Ag alloy wires (AA wires) were comparatively studied by means of chemical oxidation tests, accelerated life tests and electrochemical tests systematically.FindingsACAA wires could form axi-symmetrical spherical free air balls with controllable diameter of 1.5∼2.5 times of the wire diameter after electric flame-off process. The ball shear strength of ACAA wire was higher than that of AA wires. Most importantly, because of the surface Au coating layer, the oxidation resistance of ACAA wires was much enhanced.Research limitations/implicationsACAA wires with different lengths of heat affected zone were not developed in this study, which limited their application with different loop height requirements.Practical implicationsWith higher bonding strength and oxidation resistance, ACAA wires would be a better choice than previous reported AA wire in chip packaging which require high stability and reliability.Originality/valueThis paper provides a kind of novel ACAA wire, which possess the merits of high bonding strength and reliability, and show great potential in electronic packaging applications.

Experimental and Statistical Study of the Fracture Mechanism of Sn96.5Ag3Cu0.5 Solder Joints via Ball Shear Test.

Ball shear testing is an efficient approach to investigate the mechanical reliability of solder joints at the structural level. In the present study, a series of low-speed ball shear tests were conducted to study the deformation and fracture characteristics of Sn96.5Ag3Cu0.5 solder joints at continuous speeds from 10 μm/s to 200 μm/s. In order to account for randomness, the quantity of tests was repeated for each shear rate. The relationship between mechanical properties and shear speeds was calculated in detail via effective statistical analysis. In addition, by utilizing SEM imaging and ingredient analysis the interfacial effect and fracture mechanism of solder balls were obtained and their fracture mode classified into two types, viz., bulk fracture and interface fracture. Furthermore, by means of statistical analysis and approximate calculation it was proven that bulk fracture balls have greater adhesive powers and reliability compared with interface fracture balls.

Effect of Ni(P) Layer Thickness on Interface Reaction and Reliability of Ultrathin ENEPIG Surface Finish.

Electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) is a common surface finish in electronic packaging, while the Ni(P) layer increases the impedance of solder joints and leads to signal quality degradation in high-frequency circuits. Reducing the thickness of the Ni(P) layer can balance the high impedance and weldability. In this paper, the interfacial reaction process between ultrathin ENEPIG substrates with different Ni layer thicknesses (0.112 and 0.185 μm) and Sn–3.0Ag–0.5Cu (SAC305) solder during reflow and aging was studied. The bonding ability and reliability of solder joints with different surface finishes were evaluated based on solder ball shear test, drop test and temperature cycle test (TCT), and the failure mechanism was analyzed from the perspective of intermetallic compound (IMC) interface growth. The results showed that the Ni–Sn–P layer generated by ultrathin ENEPIG can inhibit the growth of brittle IMC so that the solder joints maintain high shear strength. Ultrathin ENEPIG with a Ni layer thickness of 0.185 μm had no failure cracks under thermal cycling and drop impact, which can meet actual reliability standards. Therefore, ultrathin ENEPIG has broad prospects and important significance in the field of high-frequency chip substrate design and manufacturing.

Simulation Analysis of SnAgCu Brazed Joints Based on ANSYS-Workbench

With the continuous development of computer technology, simulation technology has been more widely used. Through the ANSYS-Workbench software to simulate the solder ball shear experiment, the strain stress and the overall deformation cloud map were obtained, and the distribution law of the solder ball and IMC stress and strain in the shear test was found. There is no significant difference compared with the actual experimental results. The reliability is high, which provides a certain reference for the development and experiment of SAC solder joints, and accelerates the development process.

Ultrasonic Bonding Interface Degradation Characteristics of Gold-Coated Silver Wire for Semiconductor Packaging

Gold-coated silver wire was developed to alleviate the high cost of Au wire used in semiconductor packaging. Ball-bonding and stitch-bonding techniques were used to fabricate the dummy packaging material, comprising 97.3 % Ag, 97.3% Au-Coated Ag, and 99.99 %Au wires. The wire ball shear test (BST), the wire ball pull test (BPT), and the microstructural attributes of the ultrasonic bonding interfaces were compared with the initial properties, both before and after the highly accelerated stress test (HAST), conducted at 130 ℃ and 85% relative humidity (RH). The initial bonding strength for all the wire variants was ?23~24 gf. Following the HAST, the bonding strength of the Ag wire, the Au-coated Ag wire, and the Au wire decreased by approximately 75 %, 47 %, and 17 %, respectively. The microstructure analysis revealed that cracks developed and propagated at the ends of the interface and that the Au-rich Au-Al intermetallic compound (IMC) inhibited the propagation of the crack at the ACA/Au wire interface. Additionally, it was discovered that the presence of the Ag-Au-Al IMC at the interface of the ACA wire reduced Kirkendall voids, which act as a barrier to Au-Al interdiffusion.

Enhanced Ball Shear Testing Configuration For Substrate LGA Sensor Devices

Semiconductor assembly mass production environment has means of testing and verifying bond consistency and reliability during wire bonding. Common bond integrity assessment is ball shear testing (BST). This test enables analysis of the strength between the bond pad and a ball bond. This paper presents significant procedure on how ball shear testing parameters should be treated during wirebond integrity check. Device complexity in terms of performing ball shear testing specifically on sensor dice has different output responses. Frequent shearing on die resulted as bond pads are elevated by 30 µm (microns). To address manufacturing in-process controls challenges, shearing tool position, dage settings, and optical scopes are taken into consideration. Also, a study was performed on the execution correctness in combination with proper dage parameters was explored to meet good ball shear test process capability and break modes.

IMC microstructure modification and mechanical reinforcement of Sn–Ag–Cu/Cu microelectronic joints through an advanced surface finish technique

Intermetallic compound(s) (IMC) that nucleates at the interface between solder and Cu trace during a soldering reaction, is one of the most crucial factors for microelectronic packaging reliability. This study was conducted to modify the IMC microstructure and to reinforce the mechanical strength of Sn–Ag–Cu/Cu microelectronic joints through various surface finish coatings, including organic solderability preservative (OSP), immersion Ag (ImAg), immersion Sn (ImSn), Au/Pd (electroless palladium/immersion gold, EPIG), and Au/Pd/Au (IGEPIG) layer(s). We confirmed that the type of surface finish dominated the IMC growth morphology and mechanical characteristics of the Sn–Ag–Cu/Cu solder joints, even though these surface finishes were eliminated in a few seconds of the soldering process. A dense Cu6Sn5 layer with a scallop-like appearance was obtained for the traditional OSP case, while a prismatic, loose Cu6Sn5 microstructure was produced for the alternative cases (metal films). This loose Cu6Sn5 microstructure offered numerous molten solder channels for the in-diffusion of Sn to Cu, retarding undesired Cu3Sn growth at the Cu6Sn5/Cu interface; consequently, a brittle-to-ductile transition in the joint fracture mode with a high shear strength and fracture energy was obtained in the high-speed ball shear (HSBS) test. A significant mechanical reinforcement of the Sn–Ag–Cu/Cu microelectronic joints can be achieved with the replacement of the traditional OSP coating by the newly developed IGEPIG trilayer surface finish.

Two-Tailed Fuzzy Hypothesis Testing for Unilateral Specification Process Quality Index

The quality characteristics with unilateral specifications include the smaller-the-better (STB) and larger-the-better (LTB) quality characteristics. Roundness, verticality, and concentricity are categorized into the STB quality characteristics, while the wire pull and the ball shear of gold wire bonding are categorized into the LTB quality characteristics. In terms of the tolerance, zero and infinity (∞) can be viewed as the target values in line with the STB and LTB quality characteristics, respectively. However, cost and timeliness considerations, or the restrictions of practical technical capabilities in the industry, mean that the process mean is generally far more than 1.5 standard deviations away from the target value. Researchers have accordingly proposed a process quality index conforming to the STB quality characteristics. In this study, we come up with a process quality index conforming to the LTB quality characteristics. We refer to these two types of indices as the unilateral specification process quality indices. These indices and the process yield have a one-to-one mathematical relationship. Besides, the process quality levels can be completely reflected as well. These indices possess unknown parameters. Therefore, sample data are required for calculation. Nevertheless, interval estimates can lower the misjudgment risk resulting from sampling errors more than point estimates can. In addition, considering cost and timeliness in the industry, samples are generally small, which lowers estimation accuracy. In an attempt to increase the accuracy of estimation as well as overcome the uncertainty of measured data, we first derive the confidence interval for unilateral specification process quality indices, and then propose a fuzzy membership function on the basis of the confidence interval to establish the two-tailed fuzzy testing rules for a single indicator. Lastly, we determine whether the process quality has improved.

Multi-response optimization of wire bonding process for evaluating alternative wire material

Finding an alternative material to gold wire is important to cost reduction for the wire bonding process. The wire materials for laser diodes are required to pass ball shear test, wire pull test and several reliability tests, so that it can be used in production. This research employs the Multi-response Quality Test procedure that investigates the feasibility and optimal conditions of using silver alloy wire in the wire bonding process. The weighted Principal Components Analysis-based Taguchi method is used to determine the optimal parameter combination of the wire bonding process, which can minimize quality loss in multiple tests. The result shows that under the optimal parameter combination of the wire bonding process, silver alloy wire can outperform gold wire by having higher ball bond strength, higher wire strength, and smaller operation current changes after the reliability tests. Substituting gold wire with silver alloy wire can significantly reduce cost by 30% while improving wire bond quality.