Copper Pads Research Articles

Investigation of Thermosonic Wire Bonding Resistance of Gold Wire Onto Copper Pad

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.

A Novel Technique and Soldering Method to Improve Performance of Transparent Polymer Antennas

A novel technique and a nonthermal soldering method to improve the performance of AgHT-8 transparent polymer antennas are proposed in this letter. The proposed technique involves the removal of the coating layer at areas on the coplanar waveguide (CPW) ground and feed line where the connectors of the coaxial feed or legs of the SMA connectors will be attached and applying a coat of silver paint on the exposed areas before cold-soldering the coaxial connections or SMA connector legs. The nonthermal or cold soldering using electrically conductive paste enables direct soldering of the coaxial feed points or connector legs, which cannot otherwise be done with hot or thermal soldering. This type of connection greatly enhances the performance of the AgHT-8 polymer antennas compared to coaxial feed point connections through hot-soldered copper pads glued to the surface of the polymer coating. The proposed technique also gives a stronger connection bond than directly cold-soldering the feed points or connectors to the smooth surface of the AgHT-8 material. Furthermore, the copper pad connection technique also introduces additional losses contributed by the adhesive properties of the glue used. This proposed novel technique and soldering method may be extended to enhance antenna performance made from other similar transparent conductive polymers like ITO.

Interaction Effect of Voids and Standoff Height on Thermomechanical Durability of BGA Solder Joints

This paper documents simulation studies on the interactive effect of standoff height and void volume on the thermomechanical durability of ball-grid-array solder joints using a 3-D viscoplastic finite element analysis. Surface Evolver software was used to find the optimized shape of the solder joints and standoff height by minimizing the surface energy as voids with different sizes were placed in solder balls. A global-local modeling approach was then utilized to model the thermomechanical durability of voided solder joints. Void-area-fraction ranges of 14%-40% were analyzed. A nonmonotonic behavior of durability versus void area fraction was observed. The results showed that, if the void is completely inside the solder ball and has no interface with the boundaries of the joint, it does not have a detrimental effect and even improves the durability as the void size increases. However, voids located at the interface of the solder joint and copper pads were found very detrimental to durability. Factors such as load bearing area, stress concentration factor, and overall compliance of the structure were found responsible for the nonmonotonic behavior of the joints. An analytical micromechanics approach was used to calculate the compliance of the structure, and a nonmonotonic trend in phase with the durability trend was observed. The stress concentration factor also showed the same nonmonotonic trend. The rise of these two factors for the void interfacing with copper pads in addition to the decreased-load-bearing-area effect resulted in a drastic decrease in durability.

Effect of multiple reflow cycles on ball impact responses of Sn‐Ag‐Cu solder joints

PurposeThe purpose of this paper is to report the effect of multiple reflow cycles on ball impact test (BIT) responses and fractographies obtained at an impact velocity of 500 mm/s on Sn‐4Ag‐0.5Cu solder joints.Design/methodology/approachSolder balls were mounted on copper substrate pads with immersion tin surface finish, supplied by two vendors. For these particular test vehicles and test conditions, fracture near the interface between the interfacial Cu6Sn5 intermetallic compound (IMC) and copper pad was identified as the only failure mode induced by BIT.FindingsMeasurement results indicate that BIT characteristics in general degrade as the number of reflow cycles increases. Furthermore, scanning electron microscopy observations show that the thickness and grain size of interfacial Cu6Sn5 increase with increasing number of reflow cycles. This correlation confirms the familiar notion that a thicker Cu6Sn5 degrades the interfacial strength.Originality/valueThere are few reports that can attribute failure directly to the IMC(s) at the interface. This paper, however, successfully correlates the weakening solder joints with the thickening and shape changes of IMC(s) in a direct way.

Cracking phenomena on flexible‐rigid interfaces in PCBs under thermo cycling loading

PurposeThe purpose of this paper is to develop simulation models for flexible‐printed circuit boards' flex‐rigid interfaces and to perform experimental tests in the laboratory in order to evaluate the cracking phenomena when these devices are submitted to thermal cycling.Design/methodology/approachA device was proposed in order to evaluate the reliability of flex‐rigid interconnections. Thermal cycling tests were performed in an environmental chamber and failure mechanisms investigated using optical and scanning electron microscope techniques. The failure modes were analyzed using computational modelling via the finite element method.FindingsThrough failure analysis, it was observed that device design and thermal expansion/contraction could contribute to the occurrence of high stresses in the copper pad connections. The simulation model proposed was able to identify the critical regions, as they had occurred in the tests. Thus, the qualitative approach may be considered successful enough to be applied in the early stages of design.Research limitations/implicationsThe materials proprieties were considered as being linear. Thus, in order to enhance the simulation models to provide a quantitative model for the evaluation of product life, non‐linear (visco‐elastic) behaviour in the material properties and a fracture model should be considered.Originality/valueTypical causes of cracking failure in electronic devices are related to mechanical stress, moisture, heat from transportation and field use. Thus, finite element analysis is an important tool in order to evaluate the reliability of electronic components regarding these types of loads in the early stages of design.

Lead-Free Bumping Using an Alternating Electromagnetic Field

A novel lead-free bumping technique using an alternating electromagnetic field (AEF) was investigated. Lead-free solder bumps reflowed onto copper pads through AEF have been achieved. A comparison was conducted between the microstructures of the lead-free solder joints formed by the conventional thermal reflow and AEF reflow. Keeping the substrate temperature lower than that of the solder bumps, AEF reflow successfully created metallurgical bonding between the lead-free solders and metallizations through an interfacial intermetallic compound (IMC). The AEF reflow could be finished in several seconds, much faster than the conventional hot-air reflow. Considering the morphology of the interfacial Cu6Sn5 IMC, a shorter heating time above the melting point would be a better choice for solder joint reliability. The results show that AEF reflow is a promising localized heating soldering technique in electronic packaging.

Self-Aligned Growth of Organometallic Layers for Nonvolatile Memories: Comparison of Liquid-Phase and Vapor-Phase Deposition

Deposition methods for the self-aligned growth of organometallic charge-transfer complexes for use as a conductivity-modulated, nonvolatile switching layer are presented. First, a deposition from liquid phase is investigated and a model for film growth is derived. Then, a deposition technique for charge-transfer complexes, namely selective organic vapor-phase deposition, is introduced which excels in selectivity and homogeneity. In particular, silicon dioxide layers exposed to the precursor molecules during processing remain completely uncovered, whereas copper pads in contact holes are rapidly filled with the desired layer of the charge-transfer complex. Then, both deposition methods are compared regarding reproducibility, selectivity, homogeneity, and the ability to selectively grow thin films in small structures with diameters below . Finally, electrical switching properties are investigated and the switching mechanism for the present charge-transfer complex is discussed.

A study aimed at characterizing the interfacial structure in a tin-silver solder on nickel-coated copper plate during aging

This paper highlights the interfacial structure of tin-silver (Sn-3·5Ag) solder on nickel-coated copper pads during aging performance studies at a temperature of 150°C for up to 96 h. Experimental results revealed the as-solidified solder bump made from using the lead-free solder (Sn-3·5Ag) exhibited or showed a thin layer of the tin-nickel-copper intermetallic compound (IMC) at the solder/substrate interface. This includes a sub-layer having a planar structure immediately adjacent to the Ni-coating and a blocky structure on the inside of the solder. Aging performance studies revealed the thickness of both the IMC layer and the sub-layer, having a planar structure, to increase with an increase in aging time. The observed increase was essentially non-linear. Fine microscopic cracks were observed to occur at the interfaces of the planar sub-layer and the block sub-layer.

Intermetallics Characterization of Lead-Free Solder Joints under Isothermal Aging

Solder interconnect reliability is influenced by environmentally imposed loads, solder material properties, and the intermetallics formed within the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminal plating, board pad plating, and solder materials. These metallurgies react together and form intermetallic compounds (IMCs) that affect the metallurgical bond strength and the reliability of solder joint connections. This study evaluates the composition and extent of intermetallic growth in solder joints of ball grid array components for several printed circuit board pad finishes and solder materials. Intermetallic growth during solid state aging at 100°C and 125°C up to 1000 h for two solder alloys, Sn-3.5Ag and Sn-3.0Ag-0.5Cu, was investigated. For Sn-3.5Ag solder, the electroless nickel immersion gold (ENIG) pad finish was found to result in the lowest IMC thickness compared to immersion tin (ImSn), immersion silver (ImAg), and organic solderability preservative (OSP). Due to the brittle nature of the IMC, a lower IMC thickness is generally preferred for optimal solder joint reliability. A lower IMC thickness may make ENIG a desirable finish for long-life applications. Activation energies of IMC growth in solid-state aging were found to be 0.54 ± 0.1 eV for ENIG, 0.91 ± 0.12 eV for ImSn, and 1.03 ± 0.1 eV for ImAg. Cu3Sn and Cu6Sn5 IMCs were found between the solder and the copper pad on boards with the ImSn and ImAg pad finishes. Ternary (Cu,Ni)6Sn5 intermetallics were found for the ENIG pad finish on the board side. On the component side, a ternary IMC layer composed of Ni-Cu-Sn was found. Along with intermetallics, microvoids were observed at the interface between the copper pad and solder, which presents some concern if devices are subject to shock and vibration loading.

Reliability study of surface mount printed circuit board assemblies with lead‐free solder joints

PurposeThe purpose of this paper is to evaluate the lead‐free solder joint reliability of a variety of surface mount components assembled onto printed circuit boards (PCBs) under a number of different tests.Design/methodology/approachLead‐free solder with a composition of Sn96.5‐Ag3.0‐Cu0.5 was used in a surface mount reflow process. Different types of surface mount dummy components with a daisy chain, such as CBGAs, BGAs, PLCCs, CSPs, and QFNs, were assembled onto PCBs. Both the mechanical and thermo‐mechanical reliability of the solder joints were evaluated by several tests. The experiments included package shear, package pull, three‐point bending and accelerated thermal cycling testing for 2,000 cycles. The packages were examined by X‐ray and C‐SAM before the reliability tests were carried out. The maximum load and the corresponding load‐displacement curve were recorded in the mechanical test.FindingsThe results from the mechanical tests show the major failure mode is on the copper pad. Weibull analysis shows that the characteristic lives of most packages are between 1,100 and 2,400 cycles. For the CBGA, the characteristic life of 96 cycles is relatively short, due to the serious CTE mismatch. Cross‐section inspection shows failures occur at the solder joint. Copper pad failure is also observed.Originality/valueThis paper provides both the mechanical and thermal‐mechanical reliability of lead‐free solder joints. The experimental data are very useful in the lead‐free SMT industries.

Microstructure and Intermetallic Formation in SnAgCu BGA Components Attached With SnPb Solder Under Isothermal Aging

The global transition to lead-free (Pb-free) electronics has led component and equipment manufacturers to transform their tin-lead (SnPb) processes to Pb-free. At the same time, Pb-free legislation has granted exemptions for some products whose applications require high long-term reliability. However, due to a reduction in the availability of SnPb components, compatibility concerns can arise if Pb-free components have to be utilized in a SnPb assembly. This compatibility situation of attaching a Pb-free component in a SnPb assembly is generally termed backward compatibility. This paper presents the results of microstructural analysis of mixed solder joints which are formed by attaching Pb-free solder balls (SnAgCu) of a ball-grid-array component using SnPb paste. The experiment evaluates the Pb phase coarsening in bulk solder microstructure and the study of intermetallic compounds formed at the interface between the solder and the copper pad.

Microstructural changes of lead‐free solder joints during long‐term ageing, thermal cycling and vibration fatigue

PurposeThe purpose of this paper is to investigate the microstructural development of SnAgCu solder joints under different loading conditions (isothermal storage, thermal cycling and vibration).Design/methodology/approachThe observed microstructural changes have been studied with respect to grain growth and grain refinement, crack formation and crack growth. The growth kinetics of the intermetallic phases encountered as particles in the bulk as well as a reaction layer on the copper pad, were studied in the temperature range of 125‐175°C.FindingsDynamic recrystallisation of the tin matrix leads to a change in the diffusion controlled growth mechanism, which causes an increase of the particle growth rate compared to isothermal storage. Thus, these grain boundaries are separated forcibly by crack growth during thermal cycling. This stress causes intergranular cracks while vibration stress induces transgranular cracks.Originality/valueThe paper adds insight into microstructural changes of lead‐free solder joints during long‐term ageing, thermal cycling and vibration fatigue.

Chemical flip-chip bonding method for fabricating 10-.MU.m-pad-pitch interconnect

A flip-chip bonding method has been developed for fabricating ultrafine-pitch pad-to-pad interconnects. The method utilizes the preferential growth of Ni-B bridge layers on resin walls in a microscale cavity structure fabricated in the underfill resin between copper pads facing each other under some conditions of electroless Ni-B plating. In this method, the interconnect can be fabricated without loading and/or heating. By controlling the growth of the bridge layer on the resin walls in the microscale cavity under optimized plating conditions, the feasibility of ultrafine-pitch flip-chip bonding with a 10-µm pad pitch is experimentally demonstrated at a temperature of 60°C.

Development of digital imaging device based on the gas electron multiplier

We fabricated a gas electron multiplier (GEM) for application to digital imaging devices such as X-ray or γ -ray detectors. Sixty-four square ( 3 × 3 mm ) copper pads were used to collect avalanched electron charge clusters from the GEM chamber. A 128-channel charge sensitive preamplifier, which also includes sample/hold amplifiers and a multiplexer, was used as the front end electronics of the readout circuits for data acquisition. With this system, we successfully took a two-dimensional digital image under the irradiation of the 55Fe radiation source.

Morphology and Pull Strength of Sn-Ag(-Co) Solder Joint with Copper Pad

In order to clarify the effect of the addition of Co to the Sn-Ag solder, the formation and growth of an intermetallic compound (IMC) at the interface between Sn-Ag(-Co) solders and a Cu pad were investigated, and the joint strength of the solder with a Cu pad was also evaluated by a bump pull test. Binary Sn-3.5mass%Ag solder was used as the basic solder, and Sn-3.5mass%Ag-xCo solders (x = 0.1 mass%, 0.3 mass%, and 0.5 mass%) were specially prepared as Co-added solders. For the reflow process, specimens were heated in a radiation furnace at 523 K for 60 s. For the aging process, some specimens were then heat-treated in an oil bath at 423 K for 168 h, 504 h, and 1008 h. The results show that the addition of Co to the Sn-Ag solder strongly affected the formation and growth of the IMC at the interface. The results of the pull test clearly show that all solders had similar pull strengths, regardless of the Co addition, although the IMC morphology at the interface of the Sn-Ag-Co solder was quite different from that of the binary Sn-3.5Ag solder.

Metal–Polymer Composite Interconnections for Ultra Fine-Pitch Wafer Level Packaging

The decrease in feature sizes of microelectronic devices has underlined the need for higher number of input-outputs (I/Os) in order to increase its functionality. This has spurred a great interest in developing electronic packages with fine and ultra fine pitches (20-100 mum). Most of the compliant interconnects that are currently being developed have inductance and resistance higher than desirable. This paper presents a novel low-temperature fabrication process that combines polymer structures with electroless copper plating to create low stress composite structures for extremely fine-pitch wafer level packages. Analytical models for these structures justify the stress reduction at the interfaces and superior reliability as integrated circuit (IC)-package interconnects. Low coefficient of thermal expansion (CTE) polyimide structures with ultra-low stress, high toughness, and strength were fabricated using plasma etching. The dry etching process was tuned to yield a wall angle above 80deg. The etching process also leads to roughened sidewalls for selective electroless copper plating on the sidewalls of polymer structures. This work also describes a selective electroless plating synthesis route to develop thin IC-package bonding interfaces with lead-free solder. Lead-free alloy films were deposited from aqueous plating solutions consisting of suitable metal salts and reducing agents at 45degC. The lead-free solder composition was controlled by altering the plating bath formulation. Solder film formed from the above approach was demonstrated to bond the metal-coated polymer interconnects with the copper pads on the substrate using a standard reflow process. Metal-coated polymer structures in conjunction with the thin solder bonding films can provide low-cost high-performance solutions for wafer-level packaging.

Electromigration in the Flip Chip Solder Joint of Sn-8Zn-3Bi on Copper Pads

Electromigration in Sn-8Zn-3Bi flip chip solder bumps on Cu pads has been studied at 120°C with an average current density of 4 × 103 A/cm2 and 4.5 × 104 A/cm2. Due to the polarity effect, the thickness of the intermetallic compound Cu-Zn (γ-phase) formed at the anode is much greater than that at the cathode. The solder joint fails after 117 h of stressing at 4.5 × 104 A/cm2, and void formation at the cathode can clearly be seen after polishing. However, it is the melting at the edge of the bump that causes the solder joint to fail. A simulation of the current density distribution indicates that the current density is not distributed uniformly, and current crowding occurs inside the bump. The results indicate that the increase of current density associated with Joule heating has affected melting and enhanced damage in the solder joint during electromigration.

Diamond Conditioner Wear Characterization for a Copper CMP Process

Conditioner wear, copper polish rates, pad temperature and coefficient of friction (COF) are measured for two novel Mitsubishi Materials Corporation designs during an extended wear and polishing test. Both designs are coated with a film to reduce substrate wear and chemical attack. Using optical interferometry, changes in the coating that result in gradual changes in diamond exposure are measured. Theories of the COF, conditioning, and polishing are applied to explain the observed performance differences between the designs.

Increasing bondability and bonding strength of gold stud bumps onto copper pads with a deposited titanium barrier layer

A flip-chip assembly is an attractive scheme for use in high performance and miniaturized microelectronics packaging. Wafer bumping is essential before chips can be flip-bonded to a substrate. Wafer bumping can be used for mechanical-single point stud bump bonding (SBB), and is based on conventional thermosonic wire bonding. This work proposes depositing a titanium barrier layer between the copper film and the silver bonding layer to achieve perfect bondability and sufficiently strong thermosonic bonding between a stud bump and the copper pad. A titanium layer was deposited on the copper pads to prevent copper atoms from out-diffusing during thermosonic stud bump bonding. A silver film was then deposited on the surface of the titanium film as a bonding layer to increase the bondability and bonding strength for stud bumps onto copper pads. The integration of the silver bonding layer with a diffusion barrier layer of titanium on the copper pads yielded 100% bondability between the stud bump and pads. The strength of bonding between the gold bumps on the copper pads significantly exceeds the minimum average values in JEDEC specifications. The diffusion barrier layer of titanium effectively prevents copper atoms from out-diffusing to the silver bonding layer surface during thermosonic bonding, which fact can be interpreted with reference to the experimental results of energy dispersive spectrometry (EDS) and analyses of Auger depth profiles. This diffusion barrier layer of titanium efficiently provides perfect bondability and sufficiently strong bonding between a stud bump and copper pads with a silver bonding layer.

Nanoindentation on SnAgCu lead-free solder joints and analysis

The lead-free SnAgCu (SAC) solder joint on copper pad with organic solderability preservative (Cu-OSP) and electroless nickel and immersion gold (ENIG) subjected to thermal testing leads to intermetallic growth. It causes corresponding reliability concerns at the interface. Nanoindentation characterization on SnAgCu solder alloy, intermetallic compounds (IMCs), and the substrates subjected to thermal aging is reported. The modulus and hardness of thin IMC layers were measured by nanoindentation continuous stiffness measurement (CSM) from planar IMC surface. When SAC/Ni(Au) solder joints were subject to thermal aging, the Young’s modulus of the NiCuSn IMC at the SAC/ENIG specimen changed from 207 GPa to 146 GPa with different aging times up to 500 h. The hardness decreased from 10.0 GPa to 7.3 GPa. For the SAC/Cu-OSP reaction couple, the Young’s modulus of Cu6Sn5 stayed constant at 97.0 GPa and hardness about 5.7 GPa. Electron-probe microanalysis (EPMA) was used to thermal aging. The creep effect on the measured result was analyzed when measuring SnAgCu solder; it was found that the indentation penetration, and thus the hardness, is loading rate dependent. With the proposed constant P/P experiment, a constant indentation strain rate h/h and hardness could be achieved. The log-log plot of indentation strain rate versus hardness for the data from the constant P/P experiments yields a slope of 7.52. With the optimized test method and CSM Technique, the Modulus of SAC387 solder alloy and all the layers in a solder joint were investigated.