ICT Spring Seminar: Nickel Not Welcome Here

Estimated reading time: 6 minutes

After a miserable, dull, and damp English winter, a really pleasant nearly spring day with the sun shining and daffodils in bloom greeted delegates to the Institute of Circuit Technology Spring Seminar at Puckrup Hall near Tewkesbury, March 5, in Gloucestershire, UK.

Fellows, members, and associates of the Institute were welcomed by ICT Chair Mat Beadel, who enjoyed an interesting and informative seminar with a primary focus on alternative metal finishes, coordinated by ICT technical director Emma Hudson, who introduced and moderated the program.

“We are surrounded by communication links that are speaking with each other,” said Erik Pedersen, field application engineering and quality director at ICAPE, in his opening remarks. “The Internet of Things is everywhere. Cars are communicating with the surroundings. There is just an increasing demand for radio frequency and high-frequency PCBs.”

A common feature of these high-frequency PCBs is additional cost: higher development cost, higher manufacturing complexity, and a limited number of suppliers with the necessary capability, higher material cost, specialist surface treatments, and final finishes.

Regarding final finishes, electroless nickel immersion gold (ENIG) has for many years been accepted as the de facto standard. But the ferromagnetic nature of nickel, although of little effect at lower frequencies, has a negative influence on performance as frequencies increase because of insertion loss due to the “skin effect” concentrating the current closer to the surface of the conductor and into the ferromagnetic nickel layer.

Silver does not suffer from skin effect insertion loss and autocatalytic silver immersion gold (ASIG) has been developed as a replacement for nickel-based surface finishes on PCBs in high-frequency applications. Among its attributes, ASIG provides excellent surface protection equal to ENIG. It is a flat finish with very long shelf life and remains solderable through multiple soldering processes. It exhibits good ductility and does not crack under mechanical stress. Solder joint strength is very good because the intermetallic is a tin-copper interface. Additionally, ASIG provides a cost-effective surface for aluminium and gold wire-bonding.

The ASIG finish has been tested by the European Space Agency to demonstrate feasibility, manufacturability, reliability, and performance, and is approved for use in space activities.

ICAPE carried out its own study of insertion loss characteristics, comparing ASIG with ENIG, and Pedersen described the experimental details. PCBs have been manufactured in their South African subsidiary on FR-4 and RO4003C substrates in three designs representing transmission line, filter, and antenna test circuits. Three surfaces have been compared: bare copper, ENIG, and ASIG, and a total of 36 test circuits have been measured using a vector network analyser.

He discussed the test results in detail. These indicated that the ASIG finish gave significant insertion loss improvement on all three designs on both substrates. Additionally, the bandpass filter design on FR-4 substrate showed an improvement in bandwidth when ASIG was the finish.

Up next was an intriguing title, “Next generation electroplating: electrode-less and solvent-less metal deposition,” for the presentation from Daren Caruana, professor of physical chemistry at University College London. He described a novel one-step non-contact technique based on atmospheric-pressure plasma-deposition of various metals from aqueous-based inks. No pre-treatment or post-processing is required.

Caruana contrasted the reduction chemistries of classical electrodeposition, where the source of electrons is from an external power supply, and classical electroless deposition, where the source of electrons is a chemical reducing agent, with the plasma-deposition approach, where the high-energy conditions in an atmospheric helium plasma enable electrons to move independently within the plasma without being bound to the helium nucleus.

Consequently, these electrons are available to reduce, for example, copper ions from a copper salt to metallic copper within the plasma. The action of the plasma enables copper to be deposited on to surface as particles typically 200 nanometres diameter and sintered to form highly conductive metal tracks, even on delicate substrates. Furthermore, the action of the plasma on the surface promotes adhesion.

A range of metals was printed: copper, silver, gold, platinum, palladium, and nickel, in thicknesses of 2 to 30 microns and track-widths of 0.10 millimetres. The technique was demonstrated on various conformal objects as well as on flat surfaces.

The fluid dynamics and deposition dynamics were studied in great detail, together with the structure, composition, and physical properties of deposits.

The plasma-jet printing process is energy and material efficient, and Caruana considered that it offers a sustainable, cost-effective solution for printed electronics, with faster production times and greater flexibility than ink jet printing. A start-up company, spun off from the university, is offering printing as a service together with consultation on applications.

The final presentation of the evening came from the seemingly ubiquitous Alun Morgan, modestly admitting that he had not been the first (or even second) choice to speak on the subject, but in fact, the third choice to give this presentation. Because it was his first presentation on metal finishing, he begged the audience's indulgence as he introduced on behalf of his collaborators at LiloTree a product curiously named “Ni-less ENIG-Premium,” described as a revolutionary and unmatched solution for high-frequency high-reliability PCBs/ICs, offering optimum performance and better reliability of electronic assemblies, “a true universal surface finish.”

Applied by a straightforward four-stage process sequence—proprietary pre-treatment, copper barrier deposition, immersion gold deposition, and proprietary post-treatment—the key feature is the nano-engineered barrier layer on the copper surface, taking the place of electroless nickel, followed by a 50-nanometre deposit of cyanide-free immersion gold, significantly lower in thickness than that used in direct-immersion-gold or gold finishes on electroless palladium barrier layers. The LiloTree finish enables the formation of robust solder joints with thin and distinct intermetallics.  

Although Erik Pedersen had already discussed the consequences of the ferromagnetism of nickel, Morgan demonstrated actual changes in the reflectivity of a grain of nickel when an electrical field was applied, causing dielectric loss in the metal itself as domains re-aligned and absorbed energy. He discussed how conductivity and permeability affect skin depth, making the nickel “skin” about twice as thick as that of copper, and explaining how it results in signal loss. 

All this confirmed that nickel was not a desirable component of a conductor finish in high frequency applications.

Morgan reviewed experimental results demonstrating that the insertion loss of “Ni-less ENIG-Premium” is almost exactly identical to the insertion loss of bare copper over the 0–100 GHz frequency range.

It was demonstrated that LiloTree’s nano-engineered barrier layer on the copper surface prevents diffusion of copper into the gold, leaving the surface corrosion-free even after six reflow cycles. The finish also withstood accelerated corrosion testing and gave excellent results in solderability testing. Solder joints had out-performed equivalent ENIG examples in pull, shear, and drop-shock testing.

The eco-friendly surface finish was cyanide-free and complied with RoHS and REACH regulations. Additionally, with sustainability becoming a critical consideration in the electronics industry, the use of nontoxic materials was a significant benefit. It was concluded that “Ni-less ENIG-Premium” was a cost-effective, reliable, and sustainable surface finish for high-frequency printed circuit boards.

Mat Beadel thanked the presenters and all who had attended, and invited everyone to join him in a convivial networking buffet—a real opportunity for the UK industry to swap stories and share experiences.

May I sincerely thank the multi-talented Alun Morgan for sharing his excellent photographs.