In a previous blog, we discussed a couple of the top burning questions that bubble up when designers are first looking at this new technology and the ability to now design with 25-micron trace and space. First, how does the Semi-Additive Process work compared to the typical subtractive etch process? Second, can you mix subtractively etch processes and semi-additive processes on the same circuit layer? Here is a link to the blog post that explores those questions in detail.
Continuing with these most common questions, we are often asked if this printed circuit board process can produce plated through holes. That is an excellent question. For those unfamiliar with the history of semi-additive processes in the PCB space, a number of semi-additive or fully additive techniques have been explored that did not have the capability to produce plated through holes. The processes that the circuit board industry is adopting today DO have the ability to plate copper in the hole, whether that be thru holes or micro via layers, producing reliable plated through holes.
Yes, is a simple answer at the surface level, but designers quickly dig a little deeper. Can you do a cap-core construction? Do you need to design in layer pairs, so the core is designed to be fabricated with SAP PCB layers on top and bottom? When working with power and ground planes can you mix subtractively etch fabrication processes with the semi-additive process on the same PCB layer? Can you use SAP PCB layers on the outer layers of the circuit board or only on the inner layers? It is a little like opening Pandora's box.
To over-simplify once again, the answer to those questions is also YES. With very few exceptions, design stack-ups and fabrication process combinations that are being done with subtractive etch process techniques can all be done with semi-additive process techniques for PCBs. The primary difference with semi-additive PCB processing is the way that the circuit trace pattern is created on the layer. From there, with only a few exceptions, the fabricator follows the same process flow. Drilling, solder mask, lamination or laminations, surface finish plating, electrical test, and inspection are accomplished with little to no deviation from subtractive processing.
So, can you take your 12-layer, 3-lamination cycle design and simply swap in a few SAP PCB layers to decrease your layer count? Sure, that is possible, and everyone will probably feel pretty good about reducing the layer count and simplifying the design.
Here is my challenge to the printed circuit board design community as we all tackle the learning curve of the semi-additive PCB process capabilities. How creative can you be when finding ways to best apply the ability to design with 25-micron trace and space? This is our opportunity for fabricators and designers to work together to design WITH manufacturing, not FOR manufacturing.
I am often asked to supply design rules for this technology and frankly, I hesitate to provide those. First, I agree, there are specific things that are very important to communicate. As with subtractive etch processes, each PCB fabricator will have slightly different capabilities. How small of a trace and space can be designed? What is the tolerance of those traces in both width and thickness? (This will be explored in detail in a future blog) What surface finishes can be used? What materials can be used? In reality, the capability of each fabricator will drive those answers far more than the capabilities of the PCB SAP processes.
For example, the A-SAP™ process has the ability to create circuit patterns as fine as 5 microns, yes 5 microns, not 50! But the fabricators who are currently fabricating with this process are limited to approximately 25-micron feature size based on their imaging equipment and photo-resist preferences.
Second, in my opinion, providing design rules at this point in the development cycle will discourage the creativity that I think is needed to fully take advantage of this new tool in the designer’s toolbox. This is our chance to simplify designs and reset the technology curve that we have all been beating our heads against. The limits of subtractive etch PCB processing has pushed the industry to increasingly complex PCB designs. After all, as electronics packaging shrinks and those BGA’s become tighter and tighter pitch, the only available solution to the PCB designer has been to increase layers, increase the dependence on micro via layers, and increase lamination cycles. These all come with a cost and reliability impact.
So…..let’s get creative. Is there a way to reduce or eliminate the need for via-in-pad-plated-over (VIPPO) technology? Hint…your fabricators will love you! Can you re-think the design to keep SAP layers on layer pairs, greatly simplifying the fabrication process? Could you use subtractive etch technology on the outer layers? Again, this greatly simplifies the fabrication process. This is our time to really dig in, learn the fabrication process and what the challenges are, and do our best to work with PCB fabricators rather than designing for fabrication. This is our opportunity to work WITH manufacturing and leverage their experience and expertise to simplify designs for our increasingly complex electronic requirements.
Please keep reaching out with your questions about semi-additive PCB processes and fabrication capabilities!