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Electroless plating is an important process in printed circuit board and electronics manufacturing but typically requires temperatures of 70‐95°C to give a suitable deposition rate. This is becoming problematic in industry due to the rising price of energy and is a major contribution to production costs. Previous studies have noted beneficial effects of ultrasonic irradiation upon electroless plating processes and it has been reported that sonication can increase the plating rate and produce changes to the chemical and physical properties of the deposited coating. The purpose of this paper is to reduce the operating temperature of an electroless nickel bath by introducing ultrasound to the process.

The deposition rate of an electroless nickel solution was determined by two techniques. In the first method, test coupons were plated in an electroless nickel solution at temperatures ranging from 50‐90°C and the plating rate was calculated by weight gain. In the second approach the mixed potential (and hence the current density at the mixed potential) was determined by electrochemical analysis of the anodic and cathodic reactions. In both cases the plating rate was found with and without the application of an ultrasonic field (20 kHz). The electroless nickel deposits obtained in the plating tests were also analysed to determine the phosphorus content, microhardness and brightness.

The plating rates under ultrasonic agitation were always higher than under “silent” conditions. Most importantly, considering the objectives of this study, the deposition rate under sonication at 70°C was significantly higher than that found with mechanical agitation at 90°C. In addition, the results indicated that the deposits produced in an ultrasonic field had consistently lower phosphorus content, higher microhardness and were brighter than those prepared in an electroless nickel bath that was not sonicated.

Although previous work has been performed on the effect of ultrasound on electroless plating, all these studies have been carried out at the normal operating temperature of the electroless process. In this paper, ultrasound has been applied at temperatures well below those normally used in electroless nickel deposition to determine whether sonication can enable low temperature electroless plating.

Electroless plating treatment is one surface modification technique. An added effect due to electroless plating is expected, and the vibration damping capacity of the structures may be improved by this technique. In the present study, the vibration damping capacity of such electroless plated structures is measured experimentally. Damping capacity can be improved regardless of the plated film materials. Improvement efficiency with an electroless plating film with dispersed foreign particles such as SiC ceramics is higher than with a uniform electroless plating film.

Data from laboratory and production scale tests are given that show that the efficiency of catalyst adsorption controls the coverage by the electroless copper deposit in plated‐through‐holes in FR‐4 laminate, and that this, in turn, governs the outgassing performance of the finished board. The nature of electroless copper nucleation and growth is discussed and the reasons for the formation of voids in the deposit are identified.

This study aims to elucidate the reaction mechanism of electroless NiP deposits on conductive but non-catalytic Cu films on the basis of their nucleation and growth without Pd catalyst and to measure the deposition rate and activation energy of electroless NiP deposits on the non-catalytic Cu film at various deposition times (60, 120, 240 and 480 s) and temperatures (70, 80 and 90°C) at pH 4.6.

Specimens with and without Pd catalyst on Cu film were prepared as follows: the Pd catalyst was deposited on half of the Cu film using a deposition protector, and the specimen containing the Pd catalyst deposited on half of its area was immersed in electroless NiP solution. The growth of NiP on the Cu films with and without the Pd catalyst was observed.

The number of Pd nanoparticles increased with Pd activation time; the nucleation of Pd dominated over growth at 60 s. Lattice images show that the d-spacing of Ni nanoparticles doped with less than 10 at% P increased to 2.050 Å. Nucleation of NiP deposits occurred simultaneously in the specimens with and without the Pd catalyst, because electrons could be transferred via the conductive Cu. Therefore, the reaction mechanism of the electroless NiP deposited on Cu film appears to be electrochemical. The activation energies for NiP deposits (15 s Pd with catalytic Pd, 15 s Pd without catalytic Pd, 60 s Pd with catalytic Pd and 60 s Pd without catalytic Pd) on the Cu film are 65.8, 64.0, 64.3 and 58.1 kJ/mol, respectively. This demonstrates that, regardless of the volume and the presence of catalytic Pd, the activation energy of electroless NiP has a consistent value.

It is necessary to study the relationship between the volume of Pd nanoparticles and the nucleation rate of NiP at an initial stage, as there are limited data regarding the effect of Pd volume on the nucleation rate of NiP.

The reaction mechanism of the electroless NiP deposited on conductive but non-catalytic Cu film involves electrochemical reactions because the nucleation of NiP deposits occurs on conductive Cu film regardless of the presence of the Pd catalyst.

The adhesion between electroless copper and a substrate is one of the most important factors in the reliability of thermoplastic printed circuit boards. The purpose of this paper is to investigate the effects of mechanical grinding and acid etching of thermoplastic substrate materials on the adhesion of copper deposited by an electroless copper plating process. The base material of the test substrates was a new high temperature thermoplastic polyphenylene oxide (PPO) compound.

The effects of pre‐treatment on plastic surfaces are analyzed by the following methods: Fourier transform infrared (FTIR), SEM, the Dyne surface energy test and the surface roughness test. The adhesion between electroless copper and thermoplastic substrate is measured with a peel strength test.

The results showed that mechanical grinding of the substrates significantly increased adhesion but the highest adhesion is gained by using an acid etch treatment before electroless plating. These results indicated that adhesion between copper and the substrates was not directly proportional to the roughness and surface energy values.

The conventional sweller/desmear treatment used in a printed circuit board factory for pre‐treating epoxy based laminates prior to electroless plating is not suitable for these PPO compound boards. The copper adhesion is adequate when the substrates are etched with sulphuric acid/chromate solution. In that case the bonding between the metal layer and the plastic surface is stronger than the bondings between the polymer chains of the thermoplastic material. The adhesion mechanism of electroless copper in these mechanically abraded samples is mechanical interlocking of metal particles.

For zero‐defect wave soldering of double‐sided and multilayer printed circuit boards, the copper through‐hole‐plating must be impervious to gas generated in the laminate for the period of a few seconds while the solder is molten. The kinetics of the gas evolution during soldering have been related quantitatively to the integrity of the plated copper. This in turn is related to the quality of the electroless copper (evaluated using the backlighting test), the nature and distribution of the catalyst (evaluated using X‐ray photoelectron spectroscopy) and the methods and effectiveness of the hole‐wall preparation (evaluated using scanning electron microscopy). The relevance of laboratory measurements has been confirmed on a wide range of industrial production plating lines.

To study the influence of storage time and environment on the solderability of electroless nickel plated samples with Sn‐3.8Ag‐0.7Cu and Sn‐3.5Ag lead‐free solders and to provide criteria for the use of an electroless nickel (Ni‐P) under bump metallization (UBM) without immersion gold protection.

Electroless nickel coatings were deposited onto pure aluminium foil through a procedure developed for the UBM of wafers prior to flip chip bumping. Their solderability with lead‐free solders was studied using the wetting balance technique. Samples stored in different environments for different periods of time were tested to study the dependence of the solderability of Ni‐P coatings on the storage time and temperature. The degree of oxidation of the Ni‐P coatings was examined by means of X‐ray photoelectron spectroscopy and the surface microstructure and roughness of the coatings were analyzed by scanning electron microscopy and atomic force microscopy.

It was found that the Ni‐P coatings were unacceptable for direct soldering without the assistance of a flux, due to poor wettability, even when using a freshly prepared Ni‐P coating. Therefore, a suitable flux with nitrogen inerting had to be applied to assist the soldering process. The results also show that the solderability of Ni‐P coatings was affected by the phosphorus content, and the Ni‐P coating with high phosphorus content had a good solderability. The storage time and temperature did not influence the wettability significantly with the assistance of strong flux.

The stability of the plating solution and the consistence of the phosphorus content in the coating are not easily controlled. This has resulted in implications for surface analysis and wetting testing. Ni‐P coatings with different levels of phosphorus content are being investigated in detail.

The value of the paper lies in its study on the solderability of lead‐free solders to Ni‐P coating after storage in different environments and for different periods, which can provide some criteria for the use of Ni‐P UBM without immersion gold protection.

The purpose of this paper is to investigate if copper nanoparticles could be utilized for two types of through hole plating in printed circuit boards, namely: as a catalytic material to initiate the electroless copper deposition process; and as a “conductive” layer which is coherent and conductive enough to allow “direct” electroplating of the through hole. The employment of nanoparticles means that an effective method of dispersion is required and this paper studies the use of mechanical agitation and ultrasound for this purpose.

The paper utilized drilled, copper clad FR4 laminate. The through holes were functionalized using a commercially available “conditioner” before being immersed in a solution of copper nanoparticles which were dispersed using either a magnetic stirrer or ultrasound (40 kHz). When the copper nanoparticles were utilized as a catalytic material for electroless copper plating, the efficacy of the technique was assessed using a standard “backlight” test which allowed the plating coverage of the through holes to be determined. As a control, a standard palladium catalysed electroless copper process was employed. The morphology of the electroless copper deposits was also analysed using scanning electron microscopy. In the “direct plate” approach, after immersion in the copper nanoparticle dispersion, the through holes were electroplated at 3 Adm⁻² for 15 min, sectioned and examined using an optical microscope. The distance that the copper electroplate had penetrated down the through hole was then determined.

The paper has shown that copper nanoparticles can be used as a catalytic material for electroless copper plating. The coverage of the electroless copper in the through hole improves as the copper nanoparticle concentration increases and, at the highest copper nanoparticle concentrations employed, good, but not complete, electroless copper coverage is obtained. Dispersion of the copper nanoparticles using ultrasound is critical to the process. Ultrasonically dispersed copper nanoparticles achieve some limited success as a conductive layer for “direct” electroplating with some penetration of the electroplated deposit into the through hole. However, if mechanical agitation is employed to mix the nanoparticles, no through hole plating obtaines.

The paper has demonstrated the “proof of concept” that copper nanoparticles can be utilized to catalyse the electroless copper process, as well as their potential to replace costly palladium‐based activators. The paper also illustrates the potential for copper nanoparticles to be used as a “direct plate process” and the necessity for using ultrasound for their dispersion in either process.

The literature on electroless gold deposition processes is reviewed both with respect to bath formulation and the kinetics of the electrochemical reduction and oxidation reactions involved. Some modified formulations are discussed in more detail and components processed by the use of electroless gold deposition are illustrated.

This, the third in a series of papers, reports an investigation of the semi‐additive process strategies of PWB manufacture. It will show that, although the adoption of a ‘naked’ palladium catalyst is the optimal production strategy in terms of cost, caution must be exercised in its implementation, particularly with regard to material process flow and solder mask type employed. It will be demonstrated that the use of a thin, sacrificial ‘flash’ of electroless copper prior to circuitisation and full build deposition will be required for the liquid photoimageable soldermask (LPISM) approach.