REPRIME: The application of advanced ultrasonics to replace explosive precursors and poisons used in industrial metal plating processes
Most surface modification chemical treatments by their very nature contain hazardous and oxidising chemicals; there is a major concern that these chemicals are used maliciously with the intent to manufacture explosives or formulate poisons to harm the public. Cyanide based plating solutions and high concentrations of hydrogen peroxide used in current techniques are a significant use of these materials in industry.
The use of cyanide and hydrogen peroxide is concentrated in a few industries: large polymer manufacturers, chemical manufacturers, and surface finishing companies. Surface finishing / PCB manufacturing is a particular risk as in general the sites where these processes are run are small distributed and significantly less secure than large chemical plants. Consequently, amounts of these dangerous substances are found across the UK with varying and sometimes limited control over their use, representing a significant risk.
Barriers to implementation of cyanide free plating solutions are related to quality of product and operational requirements. Introduction of the novel ultrasound technology can overcome these barriers, but must be validated on full scale processes and over a range of plating systems.
The introduction of ultrasound to current industrial protocols has been demonstrated in the first phase of this REPRIME project to allow the removal of cyanide from metal plating processes, and significantly reduce the amount of hydrogen peroxide use. This work has now been extended to larger scale to prove industrial relevance. A plating line designed to match small production scale (around 300litres of plating chemistry) was set up at C-Tech Innovation to perform the work.
Results from zinc plating trials on fixings, showed that cyanide free solutions could be improved in terms of the weight addition (up to 20%) and the coverage consistency with appropriate application of ultrasound. The technique particularly aids coverage when used on unusual shapes and through holes which are typically problematic with cyanide free solutions. Limited throwing power can be compensated by improved mass transfer created by ultrasound addition.
Plating of copper has also demonstrated improved finishes when ultrasound is applied to items plated with cyanide free chemistry. Results shown below use large flat plates on which are typically difficult to achieve a consistent coating finish because the current density naturally varies across the surface. The normal system has a dull colour and various imperfections in the coating quality over the item. When ultrasound is applied, the colour is much brighter and the imperfections are reduced. However, when continuous ultrasound is applied to items plated on jigs it is not always the optimum solution because standing waves can be formed, focusing the ultrasound in certain areas. By applying a bespoke power sweep function to the ultrasound, the power can be applied across the whole vat evenly. Thus, the best resulting finish on these difficult items was achieved using the sweep function.
Further tests were carried out to prove industrial application of the technology. It is important that this solution can be easily and cheaply retrofitted into existing equipment. The ultrasonic equipment has been designed to enable the transducers to be submersed in the solution. This was used at a surface finishing company Poeton Limited in one of their existing plating lines on a Zinc / Nickel alloy process. This non-cyanide based chemistry is being used as a replacement for cadmium cyanide. Improvements in thickness of coating of up to 25% was achieved.
Small scale work on etching of PCB materials has been performed. Ultrasound was shown to reduce the levels of hydrogen peroxide required in etchant solutions used in PCB manufacturing. However, the biggest improvements were seen when considering bath life. Peroxide based etchants deteriorate with time and with copper concentration. Baths treated with a particular configuration of ultrasound could perform for significantly longer timescales without performance degradation. Therefore, bath chemistry can be replenished at a less frequent rate, reducing chemical use and handling requirements. Deterioration in etch rates both with and without ultrasound are shown in the graph below.
Energy consumption is increase when using ultrasound, however it was also found that ultrasound can enable operation at lower temperature, thus offsetting the additional power requirements. The graph below shows that when temperature of the etching bath was reduced from 60oC to 40oC the etching performance drops from the nominal level of around 8% weight loss. However, when ultrasound is applied, the required weight loss can be achieved at lower temperature.
To go beyond the current process development in PCB etching and achieve impact in industry, the scope of work must be increased to test at an industrially relevant scale. This work will continue at C-Tech innovation using standard 24’’ x 18’’ circuit boards.
This work is possible due to funding received from The Home Office.
MACFEST: Manufacturing Advanced Coatings for Future Electronic Systems
Dr Andrew Ballantyne and Prof Karl Ryder
Materials Centre, Chemistry Department, University of Leicester
Now in its second year, MACFEST is a collaborative research project involving partners A-Gas Electronic Materials, C-Tech Innovation, MTG Research, The Institute of Circuit Technology, The University of Leicester and Merlin Circuit Technology. Part funded by Innovate UK (formerly the Technology Strategy Board), the aim of the project is the development of new “universal” PCB surface finishes which are suitable for both solder reflow and gold wire bonding. This will help PCB manufacturers meet the performance demands for high value electronic systems, ensuring long term reliability, even in harsh environmental conditions. In addition, the use of deep eutectic solvents (DESs), a patented technology of the University of Leicester, offers the ability to significantly reduce the environmental impact of a number of PCB plating technologies, reducing the requirements for use of cyanide and toxic/corrosive acids in plating baths.
DESs are a novel class of solvents similar to ionic liquids (ILs). Whereas ILs are composed exclusively of ions, DESs are liquids composed of a salt and complexing agent, commonly a tetraalkylammonium salt, such as choline chloride, and a hydrogen bond donor (HBD), such as 1,2-ethanediol or urea.1 When mixed together the HBD binds to the anion, resulting in a large depression of the melting point. At Leicester we have used DESs in the development of immersion silver,2, 3 electroless nickel-immersion gold (ENIG)4 and hot air solder levelled electroless nickel (HASLEN) processes, each of which offer its own benefits over existing processes5 from the reduced safety and environmental concerns mentioned above as well as, in some cases, the removal of existing failure mechanisms such as “black pad”.
The MACFEST project is building on this previous work in the development of a new, state of the art PCB surface finish for use in both reflow and wire bonding applications. Taking inspiration from the electroless nickel – electroless palladium – immersion gold (ENEPIG) surface finish, we have developed a novel nickel/palladium/gold coating where both the palladium and gold have been deposited though an immersion process from DESs. This coating is termed electroless nickel – immersion palladium – immersion gold (ENIPIG).
Because of their inherent thermodynamic properties, electroless processes are fundamentally unstable. This can lead to spontaneous bath breakdown or extraneous plating which requires either bath replacement or scrapping of the Pd plated PCB, both of which can be very costly. However, immersion plating baths are fundamentally stable and not prone to either of these issues. Nevertheless, it is still essential that a uniform adherent coating is produced to prevent the underlying nickel substrate from oxidation. Using the DES Ethaline 200, a uniform Pd coating of c.a. 100 nm thick can be achieved in 20 minutes using palladium chloride as the Pd source.
An example SEM image is shown in Figure 1 where the characteristic nodular structure of the electroless nickel can be observed. Few other features are apparent, despite the presence of c.a. 100 nm palladium on the surface, because of the uniformity of the coating.
Currently, immersion gold plating processes from aqueous chemistries for ENIG and ENEPIG coatings utilise potassium gold cyanide as the Au source. However, by using DESs, gold (I) chloride can be used as the gold source. In addition, acid is required in aqueous media to help prevent passivation of the nickel surface; this is linked to the “black pad” phenomenon where hyper-corrosion occurs near the grain boundaries resulting in a poorly solderable coating. Because of the unique behaviour of metal salts in DESs, no acid is required in the plating formulation helping to minimise the risk of “black pad”. A bright uniform gold coating, as shown in Figure 2, is possible in a short period of time (c.a. 5 minutes).
With nine months left, the project is proceeding according to the initial timeframe. Laboratory scale trials at the University of Leicester, while still ongoing, have resulted in a number of interesting plating formulations for both immersion palladium and gold plating processes, some examples of which have been described above. We are now engaged in further validation of the coatings. Working with Bob Willis, of The Smart Group, our coatings will form part of a trial to evaluate the properties of a number of surface finishes in a variety of assembly and soldering conditions.
This exciting opportunity will provide us with the chance to directly quantify the quality of our coatings against the current industry standards with testing being carried out by a well-known, impartial, industry expert.
In addition, C-Tech and Merlin will further develop scale-up protocols for the process, developing an understanding of plating behaviour along with bath evolution and replenishment procedures.
This project is co-funded by Innovate UK.
The ICT Annual Symposium
is to be held in M Shed on Bristol Harbour on 1st June 2016.
Registrations from 09.30, the proceedings from 10.00.
Alun Morgan will deliver the Keynote presentation on Vehicle Electronics
The seminar will finish by 15:00 to allow time for the museum
Registration is £95:00 and will also include Museum Entry
Tabletops are available at £50:00. www.instict.org
Wapping Road, Bristol
0117 352 6600
Supported by Ventec-Europe
NATIONAL ELECTRONICS WEEK
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