Gold Plating Bath Research Articles

Fabrication of a gold microelectrode for amperometric detection on a polycarbonate electrophoresis chip by photodirected electroless plating

A novel method of photoresist-free micropatterning coupled with electroless gold plating is described for the fabrication of an integrated gold electrode for electrochemical detection (ED) on a polycarbonate (PC) electrophoresis microchip. The microelectrode layout was photochemically patterned onto the surface of a PC plate by selective exposure of the surface coated without photoresist to 254 nm UV light through a chromium/quartz photomask. Thus, the PC plate was selectively sensitized by formation of reactive chemical moieties in the exposed areas. After a series of wet chemistry reactions, the UV-exposed area was activated with a layer of gold nanoparticles that served as a seed to catalyze the electroless plating. The gold microelectrode was then selectively plated onto the activated area by using an electroless gold plating bath. Nonselective gold deposition on the unwanted areas was eliminated by sonication of the activated PC plate in a KSCN solution before electroless plating, and the adhesion of the plated electrodes to the PC surface was strengthened with thermal annealing. Compared with the previously reported electroless plating technique for fabrication of microelectrodes on a microchip, the present method avoided the use of a membrane stencil with an electrode pattern to restrict the area to be wet-chemically sensitized. The CE with integrated ED (CE-ED) microchip was assembled by thermal bonding an electrode-plated PC cover plate to a microchannel-embossed PC substrate. The novel method allows one to fabricate low-cost, electrode-integrated, complete PC CE-ED chips with no need of a clean room. The fabricated CE-ED microchip was demonstrated for separation and detection of model analytes, including dopamine (DA) and catechol (CA). Detection limits of 0.65 and 1.03 microM were achieved for DA and CA, respectively, and theoretical plate number of 1.4 x 10(4) was obtained for DA. The plated gold electrode can be used for about 4 h, bearing usually more than 100 runs before complete failure.

The root cause of black pad failure of solder joints with electroless Ni/Immersion gold plating

This paper reports on a study of the reaction of solder with the electrolessnickel with immersion gold (ENIG) plating system, and the resulting interfacial structures. Aforcused-ion beam (FIB) was used to polish the cross sections to reveal details of the microstructure of the ENIG-plated pad with and without soldering. High-speed pull testing of solder joints was performed to expose the pad surface. Results of scanning-electron microscopy/energy-dispersive x-ray analysis of the cross sections and fractured pad surfaces support the suggestion that black pad is the result of galvanic hyper-corosion of the plated electroless nickel by the gold plating bath. Criteria are proposed for diagnosing black pad of ENIG plating.

Speciation Analysis of Au(I) Electroplating Baths Containing Sulfite and Thiosulfate

An analysis of the speciation of gold plating solutions containing sulfite and thiosulfate as complexing agents has been performed. In the case of gold sulfite baths, it was found that the observed stability and electrochemical behavior were inconsistent with the commonly assumed stability constants of for the species. The data are, however, consistent with a much larger stability constant of . In gold plating baths containing sulfite and thiosulfate ligands, the speciation model predicts that the dominant species at neutral or basic are mixed complexes of the form and . The thiosulfate complex is only dominant under acidic conditions. It is shown that the electrodeposition characteristics reported in many studies can be interpreted in terms of the existence of such mixed ligand complexes. Finally, the effect of the stability of various gold plating solutions on colloidal gold formation has been explored. For free-standing solutions, decomposition is more likely to occur via the disproportionation of Au(I) species such as AuOH, , and rather than free aurous ions. The stability of gold baths during electrodeposition is mainly influenced by the formation of dithionite ions which reduce Au(I) complexes to form colloidal gold.

Direct immersion gold (DIG) as a final finish

PurposeTo report on research on the alternative surface finish “direct gold on copper”, including reaction mechanism, methods of deposition and end uses.Design/methodology/approachExamines the deposition reaction of the electroless flash gold plating bath, and the effects of the copper surface roughness and deposition time on the deposit and solderability characteristics.FindingsDirect immersion gold is only partially immersion and mostly electroless in deposition mode. The surface is applicable to soldering for both leaded solder and lead‐free solders. The surface is also wire bondable.Originality/valueThe paper offers details of a new alternative surface finish for use in printed circuit board fabrication as well as in packaging applications. The paper shows the electroless deposition mode of the process. The finish is ideally suited where Rf losses must be minimized. It is suitable for soldering as well as for wire bonding.

数値解析による金めっき膜の特性予測

Methods for prediction of gold plating characteristics by numerical analysis were investigated. The gold plating bath for the evaluation contains 5,5-dimethylhydantoin (DMH) as the complexing agent. HAuCl4 was used as the gold source, and phosphate salt was contained as a pH buffer. Thallium ion was added to refine the deposited grains. The concentration of Au([Au]), the pH, and the temperature were manipulated to determine the composition and conditions of the plating bath. The appearance of gold films was evaluated by color measurement using the L*a*b* color system. This method of evaluation is advantageous in terms of quantification of the appearance. Values of the color measurement tended to change depending on the various compositions and conditions of the plating bath. The brightness of the color L* obtained as a result of the color measurement was analyzed numerically with a statistical technique. The following experimental formulas were established by the numerical analysis.L*(−) = K0{1−exp(−K1C)}−K2CK0(−) = 73.9−2180exp(−0.125T)K1(M−1) = 178K2(M−1) = 338−323exp[−{(pH−7.18)/1.17}2][C : [Au] (M = mol/dm3), T : temperature(ºC)]

ヒドラジン化合物を還元剤とする無電解金めっきの析出挙動

Polarization characteristics for electroless gold plating baths consisting of hydrazinium sulfate and sodium gold sulfite were investigated. Both anodic oxidation of a reducing agent and cathode reaction of gold ion were promoted by the bath temperature rise. Mixedpotential theory was materialized in this bath. And rise in bath temperature increased not only reduction rate but also substitution rate.

Evaluation of Substrate (Ni)‐Catalyzed Electroless Gold Plating Process

The substrate (Ni)‐catalyzed electroless gold plating process invented by Iacovangelo and Zarnoch has been investigated for plating gold on electroless nickel substrates for application to bonding of electronic devices. The process was found to yield uniform and adherent gold films on Ni‐B substrates, whereas on Ni‐P substrates acceptable gold films were obtained only when the P content was low and a pretreatment procedure was introduced to prevent the formation of surface oxide or to remove it completely. Unlike the conventional galvanic displacement process, the substrate‐catalyzed process does not attack the nickel substrates and yields gold films with a very low porosity. Compared with autocatalytic electroless gold plating baths, the substrate‐catalyzed bath is inherently more stable, although the maximum gold thickness obtainable is naturally limited. Results of these evaluations are presented in detail. © 2000 The Electrochemical Society. All rights reserved.

亜硫酸金塩を用いた自触媒型無電解金めっき浴

We studied cyanide-free autocatalytic electroless gold plating baths using sodium gold sulfite and found that gold was autocatalytically deposited when sodium thiocyanate or sodium iodide was added to the sodium gold sulfite solution.We began by studying the effect of pH, additional sodium thiocyanate, gold concentration, and bath temperature on the deposition rate in the bath to which sodium thiocyanate was added. The pH and gold concentration had more pronounced effect than the other parameters studied. Lowering pH increased the deposition rate and increasing the gold concentration proportionally increased the deposition rate. This bath yielded a smooth lemon-yellow gold deposit of 1.28μm at a deposition rate of 0.43μm/h.Gold was provided also autocatalytically deposited by adding sodium iodide. Using this bath, we studied the effect of gold concentration, pH, and the addition of sodium sulfite or sodium iodide on the deposition rate. The gold concentration and the pH had more pronounced effect than the other parameters studied, same as in the case of the bath containing sodium thiocyanate. But the deposit from sodium iodide bath was reddish and rough, then some additional agents were examined. Addition of nicotinic acid and polyethyleneimine gave smoother deposit. In sodium iodide bath, about 1μm/h of deposition rate was given, but according as increase of the plating thickness, growth of dendrite crystal was observed. As for this bath, continuous use more than 1M. T. O. was possible.

チオ尿素‐ヒドロキノン２元還元剤系を用いた極めて安定な非シアン無電解金めっき浴

The paper reports a new autocatalytic electroless gold plating bath that achieves almost neutral, low temperature bath conditions and excellent process stability without cyanide. The bath contains a thiosulfatesulfite mixed complex of Au (I) as the gold source and two reducing agents, thiourea and hydroquinone. The thiourea acts as the primary reductant for the gold complex, while the hydroquinone recycles thiourea by reducing the intermediate thiyl radical derived from thiourea oxidation. Therefore hydroquinone functions as a thiourea regenerator. This unique cooperative reductant system offers superb bath stability. The bath has been applied not only to the most advanced mainframe MLC boards but also to a variety of very fine pitch organic PWBs manufacturing process.

Thick film positive photoresist: Development and resolution enhancement technique

The development of a positive, thick film photoresist consisting of a diazonaphthoquinone sensitizer and a novolac resin is described which has the capability of meeting a wide range of thick film requirements necessary for magnetic recording head fabrication. This photoresist system can coat 4–15-μm-thick films over various types of topography on a variety of different metal and dielectric surfaces (copper, Permalloy, and alumina). Moreover, this resist is compatible with copper, permalloy, and gold plating baths as well as different metal etchants. The photoresist can be used to produce images with wall profiles greater than 80° with a resolution capability approaching 2 μm lines and spaces in a 7.5-μm-thick film. Process latitude is demonstrated over a wide range of exposure doses and focus positions. Enhancement of thick film photoresist resolution has been achieved using a method which involves the treatment of photoresist films with dilute surfactant solutions. A number of factors have been found to effect the outcome of this method including the surfactant structure and concentration, the method and period of application, and the order of processing steps. Under proper conditions significant enhancement of photoresist contrast is observed which is accompanied by improvement of photoresist image profile and resolution. The method is particularly useful in electroplating applications.

The Autocatalytic Deposition of Gold in Nonalkaline, Gold Thiosulfate Electroless Bath

The autocatalytic deposition of gold has been investigated in a novel, nonalkaline, noncyanide electroless gold plating bath. The bath consists of sodium L‐ascorbic acid as the reducing agent and sodium gold (I) thiosulfate as the gold complex. Ascorbic acid autocatalytically reduces gold thiosulfate at deposition rates of approximately 1 μm/h. The reaction sequence includes various side reactions responsible for consuming reactants and lowering the deposition rate. The rate of the electroless gold thiosulfate reduction is sensitive to the reactant concentration as well as the concentration of thiosulfate. The accumulation of thiosulfate and the depletion of the reactants explain the decreasing deposition rate with time. Periodic addition of hydrogen peroxide consumes the excess thiosulfate and increases the deposition rate and longevity of the bath.

Cracking Behaviour during Au‐Au TAB Inner Lead Bonding

Tape automated bonding (TAB) is an interconnection technique for integrated circuits (ICs) with a small lead pitch and a thin assembly thickness. During inner lead bonding the flying (Au plated Cu) leads of the TAB foil are connected to the Au bumps on the bondpads of an IC. The Au bumps are deposited in the openings of a thick Novolac based resist layer by electroplating. The resist is coated on a sputtered TiW‐Au metallisation; TiW is the barrier layer between Au bump and Al bondpad. Bonding of the leads to the Au bumps requires substantial plastic deformation of the bump and lead. As a result of this deformation, the TiW barrier layer underneath the bump may crack easily. A theoretical model has been used to describe the occurrence of these cracks. This theoretical model is compared with experimental results of deformation and cracking behaviour by visual inspection of the TiW barrier and the etched cross‐sections. Separate (single point) and simultaneous (gang) bonding techniques, different gold plating baths and TAB tapes are used to study the cracking behaviour.

‘The Purple of Cassius’ : A Recurrent Gold Finger, Tab‐plating Problem

Over the past 20 years there has been a recurrent problem of a purple‐bluish stain appearing on the laminate materials of finished printed circuit boards between the gold‐plated fingers. The origin of this staining has been traced back to the PWB fabrication plant and it is generally accepted to be related to either the gold plating bath and/or the solder stripper chemistry. In this paper the authors report their investigation of this phenomenon and show that, far from being a benign, cosmetic defect, this purple stain poses a potentially serious metallic contamination to the laminate surface of the PWB. The purple colour arises from generation of a colloidal gold chromophore known as the ‘Purple of Cassius,’ which has been known since ancient times and has been in commercial use in the glass and ceramics industry for at least 300 years.

Orientation and Property of Gold Plating in the Presence of Organic Additives

The addition of sodium laurylsulfate, decylamine, and dodecylamine to the cyanide gold plating bath was found to suppress the growth of the (111) facet. Similar trends were observed for deposits obtained at higher current densities and lower temperatures. The decylamine and dodecylamine enhance the growth of (220) and (311) facets and result in lowering of the microhardness of the deposits. A shell‐like morphology was obtained when the growth of (220) and (311) facets was enhanced. An increase in the concentration of sodium laurylsulfate tends to produce flake platings.

アスコルビン酸を還元剤とする無電解金めっき浴の電気化学的挙動

A novel electroless (autocatalytic) gold plating bath containing sodium L-ascorbate as the reducing agent has been developed. A suitable bath composition was formulated based on the results of an electrochemical study in which were determined at a gold electrode the anodic polarization curves of various reducing agents and the cathodic polarization curves of sodium tetrachloroaurate (III) dissolved in solutions of sodium sulfite and/or sodium thiosulfate.A typical bath containing 0.0125mol/dm3 sodium tetrachloroaurate (III), 0.1mol/dm3 sodium sulfite, 0.1 mol/dm3 sodium thiosulfate, and 0.25mol/dm3 sodium L-ascorbate, was operated under moderate conditions, typically at a pH of 6.0 and 333K. The rate of gold deposition obtained with this bath was in the same range or greater than that obtained with the classical cyanide-borohydride bath. The deposition rate of the new bath depended on both the ascorbate and gold concentrations, and the gravimetrically determined deposition rate was found to correspond well with the rate estimated from electrochemical polarization measurements.

Electroless Gold Plating Solution

SummaryThe behaviour of autocatalytic electroless gold plating baths contained DMAB as reducing agent was investigated by the cyclic voltammetry analysis method. It was found that autocatalytic electroless gold plating favourably proceeded at pH 13 or above, but cyanide ions greatly inhibited oxidation of DMAB. In the optimum bath condition, the deposition rate was about 2.0 μm/hr, and the electroless gold plating had fine crystal form of 1–5 μm in particle size and high adhesion strength at 9 g in gold wire bonding.

ChemInform Abstract: Electroless Deposition of Au:In Alloys for Ohmic Contacts Onto Pd‐Activated n‐GaAs Substrates.

AbstractThe new electroless plating bath for the formation of In:Au alloy deposits contains, like Okinaka's gold plating bath, KBH4 as reducing agent, KAu(CN)2, KOH, and KCN, and additionally In2(SO4)3 (> 10‐4 M) and EDTA to stabilize In3+.

Electroless Deposition of Au:In Alloys for Ohmic Contacts onto Pd‐Activated n ‐ GaAs Substrates

A new electroless plating bath for the formation of In:Au alloy deposits is described. Like Okinaka's gold plating bath, it contains potassium borohydride as the reducing agent but EDTA must be added to cyanide ions in order to stabilize In3+. This bath can operate at room temperature, as proved by deposits obtained on Pd‐activated substrates, at the price of a low deposition rate (0.1 μm h−1). As expected, better ohmic contacts were obtained by annealing heterostructures prepared by using In: Au (4:96) electroless deposits instead of the pure electroless gold.

厚付け無電解金めっきについて

A new type of electroless gold plating bath has been developed using tetracyanoaurate (III). The trivalent gold bath showd excellent stability even in the absence of KCN because the tetracyanoaurate (III) was reduced to monovalence by BH4- liberating the cyanide ions. The deposition rate of gold was increased by a factor of 8 to 9 with the addition of trace amounts of PbCl2 or TlCl. This enhancement of electroless gold deposition can be explained by a model for the catalytic action of adsorbed heavy metal atoms proposed by McIntyre. The adsorbed foreign atoms seemed to influence the crystallographic orientation of the gold deposits. In the initial stage of the deposition of gold on the base metal, displacement and autocatalytic reactions may occur in parallel, but the displacement reaction diminished and the deposition rate decreased significantly when the substrate was completely covered with gold film. The autocatalytic deposition of gold is enhanced by the forced convection of the plating solution, so that the rate of deposition of gold was increased markably by stirring the solution. The stirring also enhanced the incorporation of oxygen into the solution, and this improved the stability of the electroless gold plating bath. In contrast, it was confirmed that excessive agitation of air inhibited the deposition of gold. The stability of the electroless gold plating bath was markedly reduced by introducing a trace amount of nickel ion. It is, therefore, essential in the heavy deposition of electroless gold for nickel substrates to be pretreated by displacement gold plating.

Understanding gold plating

For effective operation, gold plating baths must require the minimum of attention in respect of replenishment and replacement, and must produce as little sub-standard or reject work as possible. To achieve this, an understanding of the processes involved and the factors affecting them is advantageous. This article attempts to provide such an understanding in a simple manner. Special attention is given to the plating of gold from acid baths containing monovalent gold complexes.