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Flex Talk: Additive and Subtractive—When Opposites Attract

The majority of my career has been spent working in the PCB industry. Like many, I landed here “by accident.” Fresh out of college with a brand-new economics degree, I was looking for a position in finance and was offered a position in accounting and human resources in a small flexible circuit manufacturing company. Honestly, I did not even know what a flex circuit was, so they were truly taking a risk with me. My initial training there was not in accounting; it was in manufacturing, spending time working on the product, and learning the processes so that I would be better able to understand what I was doing when I did start working in accounting. The manufacturing processes I learned during that time were the traditional subtractive etch processes, essentially starting with copper laminate and etching away the unwanted copper to create the designed circuit pattern. 
 
Not to date myself, but back in those days a design with a 125-micron (5-mil) line and space on flex or rigid materials was pushing technology limits. Today, PCB fabricators are manufacturing designs with 25-micron (1-mil) line and space. In fact, most of us are using smartphones with PCBs with 30-micron trace and space. These extremely high-volume, commercial designs are produced using mSAP or modified semi-additive technology. This process starts with a very thin layer of copper foil and uses an additive rather than a subtractive chemistry process to create the circuit pattern. Fabricators offering mSAP technology typically serve very high-volume requirements, and the process requires significant capital investment.
 
Market dynamics in the electronics industry are quickly changing. Today, it is not only high-volume applications that are being driven to the need for 25-micron trace and space. Military, aerospace, medical, automotive, and industrial designs are also being driven to smaller and smaller feature sizes. With the currently available subtractive etch process, the design solution often requires multiple layers of stacked microvias and multiple lamination cycles. This solution adds considerable cost to the PCB and often introduces reliability and yield concerns. 
 
Today, there is an alternative that has been installed in three U.S.-based PCB fabrication facilities: the A-SAP™ process, which is Averatek’s semi-additive process. As a broad overview, this process starts by etching away all copper from the laminate, applying a liquid metal ink (LMI™), which enables an extremely thin layer of electroless copper—a photolithography process defines the circuit pattern, and electrolytic copper builds the circuit pattern. This process is currently capable of achieving line widths and spaces at 15 microns with advanced processing enabling even finer feature sizes. 
 
Additive processes bring exciting new options to the PCB design community and PCB fabricators. Using finer lines and spaces could allow designers to route with fewer layers, shrink the overall size and weight of the PCB, reduce the need for multiple levels of microvias, or—looking at it from another perspective—enable the designer to increase the functionality of an existing PCB footprint. That is a lot to think about and wrap your head around. And as is true with any new technology, exploring and implementing A-SAP™ technology to its full potential will push designers and fabricators to look at things in a creative new way. 
 
From a fabrication perspective, even though additive processing is the opposite of subtractive processing, the A-SAP™ process fits well with the traditional subtractive etch processes and equipment. The process requires the addition of a relatively simple series of tanks and heaters but utilizes the fabricators’ existing copper plating chemistries and photolithography equipment. Once the circuit pattern is created, the panels proceed through fabrication as would any subtractive etch panels. 
 
Not only does this additive process fit well with existing PCB fabrication processes and equipment, but additive layers also can be used selectively and combined with subtractive etch layers in a PCB design. Reviewing one example, a highly complex 12-layer design with stacked microvias and multiple lamination cycles can be re-imagined and re-designed. A-SAP™ layers could be used on four layers and combined with another four layers using subtractive etch technology, significantly simplifying the design. Four layers are removed, reducing material costs, and only a single lamination process would be required. 
 
Reviewing a second example, the original design is 10 layers, 75-micron trace and space, and three lamination cycles. The goal in this example is to significantly reduce the overall size of the PCB. Using the A-SAP™ technology, the design was adjusted to 25-micron trace and space on all layers. Overall, this reduced the layer count from 10 to eight, still with three lamination cycles, but it had a significant impact on the number of PCBs per fabrication panel. The original design allowed 70 parts per panel. With 25-micron trace and space, this was increased to 400. That is a significant decrease in size to meet packaging goals and is also a significant decrease in the overall cost of the design.
 
Yes, opposites do attract—at least with additive and subtractive PCB fabrication. As the rapidly changing electronics market drives designs to smaller packages with increasingly sophisticated technology, fabricators have the opportunity to implement new additive technology to meet the needs of PCB designers in new and exciting ways. It will be interesting to watch and learn as the early adopters of this technology work with their customers to develop design and fabrication best practices.
 
This column originally appeared in the July 2020 issue of PCB007 Magazine.