HASLEN: An Obituary to Black Pad
“HASLEN: An Obituary to Black Pad”--a bold title, bound to attract attention and provoke controversy. The term "Black Pad" has been used since the late '90s to describe a type of solder joint failure associated with electroless nickel immersion gold (ENIG) PCB finishes, where solder separates from from the electroless nickel surface leaving a black or dark grey deposit. Many theories have been considered as to the cause, but recent research suggests the effect originates from hypercorrosion of the nickel-phosphorus deposit during the immersion gold plating process.
A webinar presented by ITRI Innovation introduced a new solderable finish known as HASLEN, combining features of two established technologies to deposit solder directly on to electroless nickel by hot air solder leveling, and made possible by novel fluxes based on deep eutectic solvents. The HASLEN finish was claimed to reduce cost and to offer overall improvements in the longevity and reliability of PCB assemblies when compared with ENIG.
A background perspective was provided by Nick Hoo, Materials Technology manager at ITRI Innovation, who summarised the characteristics of currently available solderable finishes for PCBs and focused on the properties of ENIG: Its excellent solderability and solderability retention on storage and after multiple reflow, good planarity, low substrate dissolution, and low intermetallic growth during soldering or in the solid state because of the nickel diffusion barrier between tin and copper in the final joint. Set against these attributes were the disadvantages of high cost and loss of high frequency signal integrity, and issues such as black pad, nickel passivation, gold coating porosity and nickel-copper migration.
Hoo shared the opinion that black pad was a consequence of excessive corrosion of the nickel coating during the immersion gold process, although the term appeared to have become a catch-all for problems encountered with ENIG coatings. He referred to an observation, made during the EU-funded ASPIS project when ionic liquid technology had been investigated as a basis of alternative plating methods, that the properties of certain ionic liquids offered scope for development as soldering fluxes for electronics applications.
The University of Leicester, a collaborator in the ASPIS project, has been for many years the centre for research in ionic liquids, resulting in the development of many specialist metal finishing applications. Dr. Andrew Ballantyne described how a class of ionic liquids known as deep eutectic solvents (DES), which were low cost and environmentally benign, exhibited an exceptional ability to dissolve metal oxides without the use of acids. Used as the basis of soldering fluxes, these materials could effectively clean metal surfaces and prevent metal oxidation during soldering, and had been shown to be effective on a range of metals, some of which had been considered unsolderable using conventional fluxes.
In co-operation with ITRI and Merlin PCB Group, and with funding support from IeMRC, extensive solderability testing had been carried out on a range of PCB finishes using DES fluxes, with consistently good results. Certain of Merlin’s customers specified PCBs with a lead-free hot-air solder-levelled (HASL) finish. It was difficult to achieve complete coverage on 24” x 18” panels by a single 3-second immersion in SN100C solder at 270°C using commercial fluxes, and a second immersion was routinely necessary.
As an experiment, a DES flux was used and wetting immediately improved such that complete coverage was achieved in a single immersion. It had been seen in the laboratory that nickel-plated tongs, normally unsolderable, used to immerse test coupons in a solder pot, actually became soldered to the test coupon when DES flux was used! Upon further investigation, it was found that electroless nickel could be soldered straightforwardly with the DES flux and the next step was to try hot-air solder-levelling on PCBs finished in electroless nickel. The results were remarkably good and further optimisation resulted in a practicable solderable finish given the name HASLEN--hot-air solder-levelling on electroless nickel. Compared with standard lead-free HASL, HASLEN retained its solderability through multiple reflow cycles much more effectively as a consequence of the electroless nickel barrier layer preventing the growth of Cu6Sn5 intermetallic. Compared with ENIG as a solderable finish, apart from a substantial cost saving, HASLEN offered a definite means of avoiding black-pad effects, by eliminating the potential for corrosion during immersion gold plating.
Merlin Quality Director Dennis Price took up the story and gave the practical viewpoint of a technically-aware PCB fabricator who had noted that copper dissolution during lead-free hot-air solder levelling was a significant problem, more so with SAC alloys than with SN100C, to the extent that they had increased their electroplated copper thickness to compensate. A customer who regularly purchased Polyimide burn‐in boards was experiencing inaccurate DC resistance readings after the boards had been operating at elevated temperature. The effect was believed to be the caused by Cu6Sn5 intermetallic formation consuming free tin from the solder on the test pads. Merlin had shown that the problem could be resolved by firstly applying an ENIG finish then processing through lead‐free HASL. This was a costly workaround and ran the risk of gold contamination of their solder pot if used regularly.
During their initial laboratory soldering trials with DES fluxes, Merlin had observed that oxidised electroless nickel could be wetted perfectly and the trial was scaled up to evaluate the fluxes on full size manufacturing panels on their tin-lead and lead-free HASL machines.
Initial results were positive: Various shapes and sizes of surface mount pads wetted easily and completely with a uniform thickness distribution. An issue with excessive dross pickup on solder mask was overcome by increasing the viscosity of the flux. HASLEN appeared to be a technically acceptable and commercially viable process. In conjunction with University of Leicester and ITRI, Merlin had canvassed opinion regarding the best use of HASLEN. The initial consensus was that it would be primarily suited to defence, high temperature and harsh environment products. Merlin were currently manufacturing standard HASLEN test boards for customer evaluation, and were happy to supply sample boards to engineers interested to explore the potential of the HASLEN solderable surface finish.
The webinar certainly provoked an energetic question and answer session, ably moderated by the ubiquitous Bob Willis. It remains to be seen how quickly the HASLEN finish is evaluated, qualified, and specified by the electronics industry.
