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Flex Talk: Knowledge is Power

“What can I do to help drive cost from my design?” This is a question that I am asked routinely. That question is often followed by, “Can I get these faster?” Both questions are even more predominant when talking about flexible circuits or rigid-flex. Flexible circuits are often thought of as a high-priced solution and one truly wouldn’t design a flexible circuit without needing to utilize that technology for some reason. That reason may be space, weight, packaging, flexing requirements or even aesthetics.

I think that most will agree that a quality product that is available when you need it is the primary concern when launching a new design. But, that said, designing the most cost-effective solution to meet your needs is always going to be critical. Today, I want to share my top three tips for reducing cost and shortening lead-time when working with flex.

Understand Fabricators’ Capabilities

In today’s fast-paced electronics world, designers and engineers rarely have time to visit a board shop for a facility tour to better understand the circuit board manufacturing process.

In a perfect world, everyone would have a chance to understand not only the basic process steps that these custom-built products go through, but also understand the complexities that are involved with specialty products such as sequential lamination, microvias, flex and rigid-flex and even flex and rigid- flex with sequential lamination and microvias.

In today’s market, there are many companies that manufacture flex and rigid-flex. There is also a significant difference in capabilities across the market. Some manufacturers specialize in single-sided and double-sided flex, some in multilayer, some in rigid-flex. Within each of these specialties, there are companies that work with leading edge technology and some that do not. All can produce quality product.

But when looking at ways to ensure you are not adding cost to your design, regularly working with your fabricator and understanding their capabilities and sweet spot in the market and then matching those capabilities with the requirements of the design can have a significant impact.

Here are a couple of examples. First, you are working with two different designs. One is a single-sided flex with 0.010” line/space. The second is a complex, 16-layer rigid-flex with stacked microvias. Your approved supplier list consists of three fabricators who offer flex: Company A manufactures primarily single- and double-sided designs, Company B manufactures both flex and rigid-flex, but typically works with designs that are 10 layers or less and Company C specializes in complex rigid-flex. It can get a little tricky. It is very likely that the company that will have the best lead-time and pricing for a complex rigid-flex will not have the best pricing for the simple flex. If cost isn’t a factor, it can be easier to order both from the same fabricator, but if cost is a factor, then finding the best fit for each technology level is going to be most cost-effective.

The second example has to do with understanding the capabilities matrix for each supplier. It is important to understand for each supplier that you work with, what is considered standard, advanced and emerging technology. Using drilled hole size as an example, certain manufacturers consider a 0.10” drill to be standard and increased costs are incurred at 0.008”.

With others there is no increase in cost until you reach .006” drill. This in no way reflects on the quality of the product at each manufacturer, but more reflects their comfort level and their specific cost drivers at a certain level of technology. Once you understand where those thresholds are, you can thoughtfully weigh the cost vs. benefit of moving beyond the standard technology.

Select common materials that are in stock

There are many different types of material available for flexible circuits, and that number grows exponentially when you consider rigid-flex construction. To simplify, using the standard copper/polyimide laminates as an example, the laminate is available in two types: adhesive-based and adhesive-less material. For both types, there are a vast number of combinations of materials. Copper is typically available in ¼ oz. to 2 oz. copper and polyimide thicknesses typically range from 0.5 mil to 6 mil. Sounds great, right? Absolutely! But while these options are available, it does not mean that they are all commonly stocked at a fabricator or that they are low cost. The best advice I can give when designing for cost and reduced lead-time is to work closely with your fabricator to develop a stack-up.

Figure 1: Example of a rigid-flex stack-up. Image source: I-007eBooks, The Printed Circuit Designer’s Guide to...Flex and Rigid-Flex Fundamentals.

In general terms, laminates with ½- or 1-ounce copper and 1- or 2-mil polyimide will be less expensive than other combinations. However, cost and lead-time will boil down to the materials that your selected fabricator works with most regularly. Please don’t spec an adhesive-based laminate just because it should be less expensive. If your fabricator manufactures with more adhesiveless materials (highly recommended for rigid-flex), they may be purchasing laminate in enough volume that pricing is reduced and that savings will be passed along to you. The same thing is true for lead-time; designing with materials that are in stock will eliminate the delays from material lead-time when the prototype is placed and lead-time is critical.

My recommendation is to work with your fabricator for a stack-up and be clear about your requirements. Let them know if materials are not critical and ask that they use commonly stocked materials. That eliminates all assumptions and will result in the lowest cost, best lead-time scenario.

Communicate clearly in the fab notes

Typically, 75% of flex and rigid-flex designs go on hold while being tooled at the fabricator. A significant portion of those questions that need to be asked stem from unclear fab notes. An unclear stack-up is a very common issue with rigid-flex. Make sure that you are clearly calling out which layers are flex and which are rigid. If you have asked for the stack-up prior to releasing the design, this is simple to include.

Flex and rigid-flex designs can make people unsure and the basics are sometimes overlooked.

Another requirement that can be easily overlooked on the fab notes is the UL requirement. There are many examples where after failing a burn test and investigating the cause, it is found that the UL requirements are clear in the assembly drawings, but not in the fab notes. Your fabricator will not necessarily default to UL materials in the absence of the spec and the contract manufacturer will routinely separate the fab notes from the assembly drawings when asking for a flex quotation. Always clearly state any quality requirements in both the assembly drawings and the fab notes.

What do these have in common? I believe the best way to reduce cost and lead-time is to work with your fabricator throughout the design process and communicate requirements clearly. They say experience is the best teacher and they work with new designs every day. Take advantage of that knowledge!

Tara Dunn is the president of Omni PCB, a manufacturer’s rep firm specializing in the printed circuit board industry. To read past columns or to contact her, click here.

This column originally appeared in the August 2017 issue of The PCB Magazine, click here.