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42 PCB007 MAGAZINE I JUNE 2020 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™ lay- ers could be used on four layers and combined with another four layers using subtractive etch technology, significantly simplifying the de- sign. Four layers are removed, reducing mate- rial 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 im- pact 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 de- crease 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 ad- ditive and subtractive PCB fabrication. As the rapidly changing electronics market drives designs to smaller packages with increasing- ly sophisticated technology, fabricators have the opportunity to implement new additive technology to meet the needs of PCB de- signers 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. PCB007 Tara Dunn is the president of Omni PCB, a manufacturer's rep firm specializing in the PCB industry. To read past columns or contact Dunn, click here. 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 lami- nation 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 Aver- atek's semi-additive process. As a broad over- view, 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 photolithog- raphy 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 ad- vanced processing enabling even finer feature sizes. Additive processes bring exciting new op- tions to the PCB design community and PCB fabricators. Using finer lines and spac- es 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 subtrac- tive processing, the A-SAP™ process fits well with the traditional subtractive etch processes and equipment. The process requires the addi- tion of a relatively simple series of tanks and heaters but utilizes the fabricators' existing copper plating chemistries and photolithogra- phy equipment. Once the circuit pattern is cre- ated, the panels proceed through fabrication as would any subtractive etch panels.