PCBs with Gold Traces and Conductors, Now on Flex Materials
One of the many benefits of using polyimide flexible material is that it is bio-compatible, making it critical in many medical electronics applications. Using a PCB with gold traces and conductors on polyimide instead of copper conductors provides a fully biocompatible option. Wait, what? Gold conductors, not gold-coated copper conductors? Yes!
Someone recently commented that it took a significant amount of digging and internet searching to find a fabricator capable of producing gold conductors on polyimide. That makes sense. Traditional subtractive etch processes start with a polyimide/copper laminate and etch away the unneeded copper to create the circuit pattern. The chemistries that enable these processes are specifically formulated for copper, and with most fabricators, that has been the only metal available for circuitry. But as the industry begins to adopt additive or semi-additive PCB fabrication techniques, fabricators can build circuitry with much finer features, and the ability to form gold conductors becomes much easier.
A-SAP™: Semi-additive Fabrication of a PCB with Gold Traces
Let’s take a step back and look at the A-SAP™ technology and how a semi-additive fabrication process works. As the name implies, the circuitry is “added” to the laminate rather than created by “subtracting” the copper from the laminate. Starting with a traditional polyimide/copper laminate, the copper on the panel is COMPLETELY etched away, leaving the bare laminate. LMI™ (liquid metal ink) is then applied to the laminate. Once that is cured, an extremely thin layer of electroless copper is applied to the panel. A photoresist is then applied, and photolithography techniques define the circuit pattern. Electrolytic copper builds the traces to the required thickness, and a flash etch removes the remaining very thin electroless copper layer. The panels follow the same process steps as the subtractive etch processed panels would finish the standard PCB fabrication process.
This process has recently begun to be licensed to PCB fabricators interested in meeting the increasing market demand for circuit boards with very fine feature sizes. A-SAP™ allows designers and fabricators to work with trace width and space as small as 15 microns, which can significantly simplify the routing and fabrication processes. For example, with design constraints at 75 microns, which is the common limitation for subtractive etch processing, a designer was forced down the path of 12 layers, stacked microvias, and three lamination cycles. This construction is complex to build, often has lower yields, and carries significantly higher costs. That same design was re-thought using 25-micron trace and space, resulting in a much simpler design. Four subtractive etch layers were combined with four A-SAP™ layers, reducing the total layer count to 8 and reducing the required number of lamination cycles from 3 to 1.
This technology is relatively new to the commercial market, and many are excited about the possibilities that this brings to PCB design: smaller form factor, reduced layer count, less dependence on stacked microvias, increased functionality within a set circuit size.
Gold on Flexible Polyimide
Along with the ability to form very fine feature sizes and simplify complex routing requirements. Because this semi-additive approach completely removes the copper from the laminate, flexible circuit designs using polyimide dielectric are no longer constrained to copper conductors. Platinum, palladium, nickel, silver, and gold PCB traces are possibilities. Do you have an application that could benefit from the properties of one of these other metals? Does that get your imagination going? I hope so. This is an exciting new capability.
Duke University published a study about the use of gold conductors and polyimide in a neural probe application. Several material combinations were included in the study, two combinations of polyimide, copper conductors and ENIG, LCP with gold conductors, and polyimide with gold conductors. The polyimide with gold conductor combination far outperformed the other options.
The A-SAP™ process is compatible with PCB fabrication techniques and is being licensed by fabricators. The real power behind A-SAP™ is the ability to form feature sizes as small as 15 microns. The best way to apply this technology is to continually evolve as the industry collectively starts to utilize technology and develop best practices. My best advice when working with A-SAP™ is to work with your fabricator from start to finish in the design process. With very few “design rules,” this collaborative approach will ensure that using A-SAP™ in your manufacturing run is cost-effective.
The added benefit of this new process is the ability to re-think the conductive metal used for circuitry. It is something that is just starting to be explored in a commercial environment rather than the lab environment. The clear benefit of gold on polyimide for biocompatibility is something that I imagine more and more medical applications will explore as the technology develops and commercial capacity is available. As with the fine feature sizes, work collaboratively with your fabricator throughout the design process. Both designers and fabricators will be following a learning curve with these new capabilities. This is a great time to apply even more creativity to the design process and take advantage of PCBs with gold traces.