EIPC Summer Conference 2016, Day 2: Strategies to Maintain Profitability in the European PCB Industry

Estimated reading time: 23 minutes

In the as-received state, the immersion tin finish on a PCB had a uniform white appearance, with a thin oxide layer at the surface and a thin Cu₆Sn₅ intermetallic layer at the interface, both of which were beneficial in Nichols’ opinion. After solder paste printing and during pre-heating prior to reflow of the first side, the oxide layer was removed by the flux. During the peak temperature phase of the reflow process, both immersion tin and solder were melted and there was rapid growth of Cu₆Sn₅ intermetallic, in the form of “scallops,” and slow growth of Cu₃Sn intermetallic. After the first reflow, soldered pads were very well covered with solder and the non-soldered pads still appeared clean and white clean. During reflow of the first side, similar intermetallic growth occurred on the as-yet unprinted side, possibly reaching the surface, and the oxide layer increased in thickness. To compensate for this, it was recommended to print more paste on the second side. During pre-heating, the flux had more oxide to remove, and at peak temperature, because there was less free tin, there was reduced spread of solder. After the second reflow the non-soldered pads were still visually clean, but slightly darker in colour.

Nichols went on to discuss possible causes of de-wetting on the second side after successful first-side reflow: residues on the copper surface, contamination of the tin surface or evaporation of volatiles from solder mask during the first reflow. In his experience, solder masks were often not completely cured free from volatiles on as-received PCBs, and he strongly recommended precautionary UV bumping to give additional reliability.

A novel electroless nickel / immersion palladium / immersion gold universal finish for PCBs was described by Professor Karl Ryder from University of Leicester, a leading expert on applications of deep eutectic solvents in metal finishing. He explained that deep eutectic solvents were a class of ionic liquid in which organic cations combined with halide anions and complexing agents to yield a purely ionic material with remarkable solvent properties. The specific example used in the university’s research work was known as Ethaline 200, composed of ethylene glycol and choline chloride in 2:1 molar ratio, and this was relatively inexpensive and environmentally benign. To demonstrate the unusual solvation properties of deep eutectic solvents with metal ions, he showed a series of solutions of copper ² salts, which would be blue in aqueous solution, covering a full rainbow spectrum of colours.

The benefits of deep eutectic solvents had been demonstrated in metal finishing applications such as electropolishing, electroplating and immersion plating, as well as in metal recycling and energy storage, and as fluxes that enabled soldering direct to difficult-to-wet surfaces such as electroless nickel. And a previous EU 7^th Framework project called IONMET had demonstrated that silver could be successfully deposited on copper as a solderable finish for PCBs. A current project, MACFEST, co-funded by Innovate UK, aimed at producing a high-reliability solderable and wire-bondable universal PCB finish, with good planarity and long shelf life. Deep eutectic solvent technology was being employed to improve functionality and to reduce safety and environment concerns. The first 15 months of the 24-month project had been completed. Immersion palladium had been deposited onto a proprietary electroless nickel base layer from Ethaline at 80°C to a thickness of 70–100 nanometres in 30 minutes. The palladium deposit had been over-plated with gold from a second Ethaline-based formulation, with the gold present as chloride or thiosulphate, at 50°C for 9–15 minutes. Bright uniform deposits had consistently been achieved from chemistry free from acid and cyanide. This ENIPIG (electroless nickel, immersion palladium, immersion gold) finish had shown excellent solderability, with no evidence of the black pad or mud-cracking effects which could occur on the nickel surface when traditional aqueous chemistries were used for gold deposition.

Alex Stepinski is well-known for planning and engineering the first captive PCB manufacturing facility in North America in many years, at Whelen Engineering in Charlestown, New Hampshire. Their fabrication process is characterised by a high level of automation and the innovative re-thinking of many of its fundamental operating principles. His presentation focused on the reduction of chemical system costs, where fresh chemicals represented only about 40% of the total cost associated with a new chemical process. This percentage could be even less if equipment was used that had not been optimised for the process. Typical breakdown of the remaining 60% in the North American market was: regulatory costs and permits 15%, wastewater treatment costs 15%, drag-out losses, 10%, chemical maintenance costs 10%, energy costs 5%, and fume losses 5%. So where did he achieve savings?

Starting with emissions management: “Fumes are good chemistry being wasted in the process of generating system under-pressure to prevent ambient exposure of personnel and equipment to toxic chemicals.” Stepinski discussed various techniques both for minimising fume generation and for recovering chemistry, water and solvents from fume extraction systems. The ultimate solution was to totally mitigate emissions with a hermetically sealed system, which could give very rapid return on investment in the case of high total-dissolved-solids chemistries.

Turning to dragout management: “Chemical drag-out is good working chemistry being wasted by mixing it with water needed to rinse the panels for the subsequent process step”. He discussed drag-out minimization and recovery techniques and demonstrated that approximately 50% of dragout could be recovered simply by using concentrate rinses for make-up of the process bath. This could be increased to 80% if off-line thermal and membrane systems were used at the point source.

Finally, Stepinski considered developments in central wastewater technology: “Central wastewater treatment should only be used for purification of trace contaminants, and for the treatment of spent chemical concentrates. 90% of drag-out should never reach this process.” Traditional systems separated regulated contaminants from wastewater and discharged the decontaminated wastewater, with metal hydroxide waste as a by-product. Modern systems took the process a step further and deionised the wastewater for re-use. A hybrid reverse osmosis / high capacity ion exchange system was the most cost-effective solution currently available. Furthermore, thermal distillation of concentrate waste could result in a zero liquid discharge.

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