It might be factually correct, while also being completely impossible.
Question: I am looking to add a flex circuit supplier to our vendor base and requested its technical roadmap. After review, it appears almost exactly as the two vendors we currently have. Is this a coincidence, or do most (or all) flex suppliers have the same technical capabilities?
A technical roadmap is basically a document that outlines what a company can and cannot do from a technical standpoint (e.g., minimum trace/space, layer count, pad and via size, overall circuit size, etc.). I have never been a fan of technical roadmaps. Virtually every flex (and rigid) PCB supplier is compelled by their customer base to provide their capabilities, and therefore also their limitations. The problem with roadmaps is twofold. First, every supplier advertises the absolute best it has ever done in every single category. This is true even if it only did it one time, on one circuit, and in a beaker. This is not a fair representation of what the fabricator can or cannot do on production quantities. The second problem is the real answer for what a supplier can or cannot do is “it depends.” Let’s look at a few examples.
Trace/space feature size. Most flex suppliers will advertise their minimum trace/space feature size at 50µm to 75µm. But these numbers are meaningless without also knowing what copper thicknesses were used to establish these limitations. If they were able to etch 50µm to 75µm features on 9µm or 12µm copper, I say welcome to the club. If they did these feature sizes on 2+ oz copper, they have something to brag about. Unfortunately, this important detail is often missing from roadmaps.
Via/pad sizes. It is practically industry standard that any supplier with a laser drill can form 75µm to 100µm blind vias and hit 250µm pads. But what happens when the circuit size increases to where the dimensional instability of the flexible material starts to wreak havoc on the locations of those vias? Drilling and imaging equipment with scaling capabilities certainly helps, but to be really beneficial, they need to be used in conjunction with sequential lamination construction. Depending on the number of lamination steps, a price tag is associated with that capability.
Layer count. As layer count increases, so does the importance of very high etching yields on internal layers. Consider the following example. Say you have an eight-layer circuit, and the supplier can get eight parts per panel. In this example, let’s say your supplier averages one etching reject per layer. Theoretically, there will be panels where they may not get a good part. Murphy’s Law dictates the internal etching rejects will be randomly placed on each layer and will almost never line up at a single circuit location. So, maximum layer count is a function of the feature sizes on each layer in the circuit. The smaller the feature sizes, the lower the etching yields. The more layers in the stack that have these small features (i.e., non-plane layers), the lower the overall yield. And as you probably guessed, the lower the overall yield, the higher the price.
Circuit size. This is one of my personal favorites. Most flex suppliers have at some point built relatively long (22" to 24") circuits. These are typically just one or two layers and have relatively large features. This is where proclaimed maximum circuit size on the technical roadmap can come back to bite you. Since you have indeed made a circuit 24" long, you include that on the technical roadmap. No problem, right? Then you get a Gerber package to quote where you find a 24" long and 5" wide six-layer circuit filled with 50µm lines/spaces on four of those layers. You contact the customer to let them know you will have to no-bid, and they may have difficulty finding a source anywhere that can support the application. The inevitable reply: “But the technical roadmap on your website shows this is within your capabilities.”
Each feature on a technical roadmap is a standalone statement of that one capability only. When you mix and match these leading-edge capabilities, all bets are off. Again, I am not a fan of technical roadmaps because they are not indicative of the true capabilities of a supplier, or the supplier’s ability to stack multiple challenging features into a single design. Rather than relying on a published technical roadmap where you can learn about the best a supplier has done, give your flex vendor a call and provide them with a two-minute “big picture” scope, including the important details. Most vendors will be able to tell quickly if you are on the right track or wandering into never-never land.
Mark Finstad is senior application engineer at Flexible Circuit Technologies (flexiblecircuit.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. He and co-“Flexpert” Nick Koop (This email address is being protected from spambots. You need JavaScript enabled to view it.) welcome your suggestions. The authors will speak on flex design and manufacture at PCB West Virtual 2020 in September (pcbwest.com).
