“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 of these 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 truly, one wouldn’t design a flexible circuit without needing to utilize that technology for some reason. That 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 3 tips for reducing cost and shortening lead-time when working with flex.
Understand your 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 are capable of producing 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 .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 .10” drill to be standard and increased costs are incurred at .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 and 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 adhesiveless 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 .5 mil to 6 mil. Sounds great, right? Absolutely! But while all of 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.
In general terms, laminates with ½ or 1 ounce copper and 1 or 2 mil polyimide will be less expensive than other combinations. BUT, 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. Please 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 over-looked.
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 all of these have in common? I believe the best way to reduce cost and lead-time is 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!