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For those of you following my column or who have attended one of my sessions on flex and rigid-flex, you know that the number one message I try to drive home is to work with your fabricator early in the design process, especially when you are new to designing with flexible materials. I can tell countless stories about flexible circuit missteps:
- “The flex that didn’t flex”
- “The flex conductors that cracked when flexed”
- “The flex that took eight weeks longer and cost an additional $2,000 than necessary because uncommon materials were specified”
The list could go on and on.
I can also tell countless stories about designers working with fabricators early in the process resulting in the most robust circuit, successfully dynamically flexing, on time and within budget. The key to success is working in partnership with your fabricator. Did you notice that I reiterated that key point twice, just in my opening statements? That is how strongly I feel about that message.
As I have shifted my focus to additive electronics, specifically circuitry that spans the gap between subtractive etch and IC scale, this message becomes even more pertinent. While there are certain high-profile, high-volume designs that have been fabricated with semi-additive PCB processes for several years, the technology is now also serving the low-volume, high-mix market domestically and offshore. As with any shift in technology in the PCB industry, this comes with a learning curve for both the design community and the fabricators, as we all learn how to best apply and take advantage of these new capabilities.
As I diligently work to disseminate information and to help shorten this learning curve for both designers and fabricators, I am routinely asked for “design rules” for SAP and mSAP. That is a fair request; after all it is what we have been requesting from fabricators for decades. Only this time, it is a little trickier. There is technology capability involved with creating a PCB trace and space with semi-additive or modified semi-additive processes that is dependent on each fabricator’s equipment. These processes change the manufacturing constraint for trace formation from the etching process to the photolithography process; feature size capabilities will differ depending on equipment capabilities and photoresist. In fact, SAP technology can go far below the 25-micron trace and space that we see in marketing materials and if your fabricator has photolithography equipment that goes to 5 microns, they can produce at that feature size.
Semi-additive and modified semi-additive processes also have very little direct impact on other PCB fabrication processing steps that come before or after the trace and space formation processes. Drilling is a good example. If your fabricator can only offer a 6-mil mechanical drill with subtractive etch processing, they can still only offer that feature size after installing a semi-additive process, unless they also make a capital investment in drill equipment at the same time. Materials is another good example. If your fabricator is currently running flexible materials, rigid-flex, and exotic materials, these are all compatible with semi-additive processes, but as we have all experienced, not every fabricator specializes in these materials. That is independent of the circuit formation, semi-additive processes.
I think that is fairly intuitive. Here is where things get a little trickier and we start entering the territory of moving through a learning curve. Semi-additive PCB fabrication techniques don’t require every layer to be done with semi-additive technology; in fact, it is common to mix layers of subtractive etch with semi-additive layers. One thing to consider is what we are terming “layer pairs.” As an example, if layer two is using semi-additive processes, the related layer on that core should also use semi-additive processes. It is not strictly required, but the fabrication process will be much simpler, resulting in improved yield and reduced cost. This is also related to fabricator capabilities and preferences—reinforcing the need for designers and fabricators to work collaboratively with this new technology.
Let’s look at another example. The starting point is an eight-layer design with five signal layers. As a starting point, we took the manufacturing capability to 35 microns, a very comfortable feature size for semi-additive processing. This resulted in a reduction to a four-layer design with two signal layers and two power ground layers. Sounds impressive, doesn’t it? It is and would certainly be a win for design simplification and manufacturability. But, working collaboratively, we realized that the impedance control in that particular stackup would be impacted with traces with that tight pitch. After further review, it was decided to use 50-micron traces and 22.5 micron spacing to meet the impedance requirements. The routing was still accomplished in those four layers. Because we were having a detailed dialogue through this process, it was also suggested that reducing this design from eight layers to six layers, adding in additional power and ground layers, would still result in a simplified construction while at the same time potentially yielding power benefits that had previously not been available. Interesting to ponder, isn’t it?
One certainty I can bring to this discussion is that semi-additive processes can significantly simplify complex designs. Some are working to simplify existing designs; others can create a new design utilizing these new manufacturing capabilities. Designers have a learning curve to navigate. Fabricators have a learning curve to navigate. Collaborating with and supporting both as the industry adopts these new technologies is sure to shorten the learning curve for all.
This column originally appeared in the September 2021 issue of Design007 Magazine.