Flex Talk: Demystify Flexible Stack-ups

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The sheer number of flexible laminate materials and constructions can be a bit daunting for those new to flex and rigid-flex design. I sat down with Jeff Martin from Omni PCB for his insight into flexible laminates and his advice when working on a flex stackup. 

Tara Dunn:  Hi, Jeff, thank you for taking time to speak with me today. Before we dive into today’s topic, can you give a brief introduction to what you are doing now and what your background is with flex and rigid-flex? 

Jeff Martin: Hi, Tara. I am currently the chief technology officer for Omni PCB. I provide our customers with a high level of negotiation and project management support, technical consulting, and industry insight. I have been in the printed circuit board industry for 25-plus years. A number of those years were working as a front-end engineer and project manager for flex and rigid-flex manufacturing. 

Dunn: That is a long history with flex materials. For the many people who are new to designing with flexible materials, can you give us a basic understanding of flexible laminates?   

C_Dunn_Jeff_Martin.jpgMartin: Absolutely. Flexible laminates come in a variety of materials. Probably the most used and what people typically think of when starting to design a flexible circuit is polyimide dielectric. Breaking that down, the laminate will consist of a layer of rolled-annealed copper bonded to the polyimide and this is done in two different ways. Adhesive-based cores are made up of a polyimide core, a layer of adhesive and a layer of copper which is bonded together using heat and pressure. The adhesive commonly used is either epoxy or acrylic. A typical adhesive thickness is one-half to 1 mil. Adhesiveless based cores have the copper directly attached to the polyimide core without the use of any adhesives. Adhesiveless cores are typically recommended for applications that have a thickness constraint and also when working with rigid-flex constructions.   

Dunn: If the adhesive thickness is one-half to 1 mil, what thicknesses are available for the copper and polyimide? 

Martin: Although you can likely get nearly any combination of dielectric and copper thickness, the laminates will typically have some combination of half-ounce to 2-ounce copper, with anywhere from half-mil to 5-mil polyimide. When you factor in options for adhesiveless construction or adhesive-based, with or without flame retardant formulations, there is a wide variety to select from. 

Dunn: I imagine that can be overwhelming for those new to flex design and trying to select the best materials. Are there certain materials that are more common than others and likely to be in stock with a fabricator?     

Martin: This one is a little difficult to answer because it really does vary from fabricator to fabricator, but in general constructions using half-ounce and 1-ounce copper and 1 and 2 mils of adhesive are probably the most stocked materials. If a fabricator builds a lot of rigid-flex, they may be more likely to stock adhesiveless materials. It is risky to assume that materials may be in stock, so working with your fabricator as you are creating the stackup to understand what is in stock is very beneficial. If you are able to use materials in stock, that can greatly reduce your lead time and if you identify early in the process that there is a material that does need to be ordered, that material can be pre-ordered while the design is being completed. 

Dunn: I agree, working with your fabricator early in the design is always a good idea. Flexible circuit stackups also need to include any polyimide or FR-4 stiffeners that are being used. Can you explain the function of each? 

Martin: Sure. Stiffeners are used to create a support area where the flex will not bend and will support components and connectors. An unclad piece of FR-4 is a common solution. Another common type of stiffener is polyimide. This, too, can add support to component areas and is also often used to build up the termination area with ZIF terminations, thickening that specific area without increasing the overall thickness of the flex.   

Dunn: We have been speaking primarily about flexible circuits, what are the key things to keep in mind regarding materials and stackups for rigid-flex designs?  

Martin: First, as I mentioned earlier, check with your board manufacturer to see what material they stock. This will help when you are considering your stackup. With rigid-flex it is very important to keep acrylic adhesive outside of any rigid area that will have plated through-holes. So, selecting adhesiveless flex materials, and using bikini cut with the coverlays is highly recommended. From there, keep in mind the form, fit and function of the circuit. How will it mount? Where will it flex? How often does it flex? What will it plug into? And so on. 

Dunn: I often share my experiences with the “flex that didn’t flex.” Do you have any flex war stories and lessons learned you would like to share? 

Martin: I honestly do not have a recollection of one.   

Dunn: Wow, that is a great track record after working with flex and rigid-flex for 25 years. As we wrap up, please share your best advice regarding flexible circuit stackups.   

Martin: As I have mentioned a few times, I think my best advice to people new to flexible materials, or even those experienced flex designers, is to get in touch with your board house early in the process. Find out what materials they stock, all the way down to the adhesives they stock, and work with them to dial in the stackup. They are the experts and that simple step can prevent a lot of headaches later in the design. 

Dunn: Great advice. I couldn’t agree more. Thanks, Jeff, it has been great talking with you today and I am sure this will be informative for designers just getting started with flexible materials.

This column originally appeared in the January issue of Design007 Magazine


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