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Flex Talk: Something New for Everyone

Just the other day, I was recording a podcast with Altium discussing flexible circuit cost drivers. During that discussion, I was asked a question about what I see as a trend in the market. My first thought was that I am seeing an increase in frequency of questions coming from people that are just new to flex and rigid-flex design. There are enough idiosyncrasies with flex that people are a little unsure and are reaching out with questions. Around this same time, I had been contemplating what would be a good topic to write about for the “New Technology” theme of the March 2018 issue of PCB007 Magazine. The light bulb went off; there is such a range of experiences one can have with flex and rigid-flex that even more experienced users can feel like they are working with it for the first time, when in fact they’re not. Imagine what it’s like for people who are totally unfamiliar with it! So at the end of the day, brand new to flex and flex-rigid or not, most people who use it feel like they are working with new technology.

Single- and Double-Sided Flex

Single-layer flex, flex with one layer of copper, is a place many new to flexible circuits start. This is used when all conductors can be routed on one layer of copper. This may be replacing wire, solving a packaging problem or even be used for aesthetic reasons in a package that will be visible to the end user. When circuitry can’t be routed on a single layer, or shielding is needed, the progression is to move to double-sided (two copper layers) flex, or even multilayer flex. 

If single- and double-sided flexible circuits are a new technology for you, material selection can be daunting. There are many material options to consider, but the predominant material is rolled annealed copper/polyimide laminate. Within this material type, there are two different options. Adhesive-based material, with either acrylic adhesive or flame-retardant adhesive, or adhesiveless material. Many single- and double-sided designs will use the adhesive-based materials. These materials are often less expensive than the adhesiveless version. Laminates are typically provided with ½ oz. to 2 oz. copper and ½ mil to 6 mil polyimide. The most commonly spec’d materials tend to be ½ and 1 oz. copper with 1 mil or 2 mil polyimide and, because they are the most common materials, pricing tends be lower and fabricators will often have this material in stock. Adhesiveless materials are most often recommended for higher layer count flex designs and rigid-flex construction.

Rigid-Flex

Rigid-flex construction consists of a flexible section and rigid section on the same board. What differentiates this construction from flex with a stiffener is that plated through-holes extend through both the rigid and flexible sections. This construction is most often used when the design requires dense surface mount pads on both sides of the circuit. 

If rigid-flex is a new technology for you, there are a few key things to keep in mind. The term “bikini cut” is important. It is recommended to keep the adhesive within 0.050” of the edge of the rigid portion of the design. Adhesiveless flex materials should be used and coverlay should not extend into the plated through areas. There is a Z-axis mismatch between the rigid materials and the adhesive that can impact the reliability of the design.

The simplest version of a rigid-flex construction is to keep all plated through-holes in the rigid area of the designs. It is certainly possible to create a rigid-flex with plated through-holes in the flex regions as well, but this type of design requires additional processing, adding cost to the design.

The flex layers can also be bonded or unbonded. If there are several flex layers or flexibility is a concern, one common solution is to eliminate the adhesive between selected flex layers, providing more flexibility to the overall design (Figure 1). Often, this is confused with bookbinder rigid-flex construction. (Source: The Printed Circuit Designer's Guide to... Flex and Rigid-Flex Fundamentals)

Bookbinder Rigid-Flex

Bookbinder construction has been around for decades but seems to be regaining popularity in the market. A bookbinder rigid-flex is similar to a hard-covered book. The flex layers are staggered, each flex layer gaining length as it is stacked on the bend so that when the flex area is bent, it does not buckle and create stress on the flex layers. Bookbinder construction is both labor and engineering intensive and there are only a handful of fabricators that specialize in this construction. 

If bookbinder rigid-flex is new to you, attention should be given to the variables that need to be considered to allow the proper fit. It is advisable to add extra length if air circulation is required to keep the flex cool in a high current application rather than tightly nest the layers. It is also important to plan for the mechanical space this bulge will require in final assembly. Moving along the technology scale would be dual bend bookbinder rigid-flex, which includes multiple bookbinding areas that do not all bend it the same direction causing a hump on both sides of the board. 

Additive Process, Sub-1-mil Lines and Spaces

Using an additive process, rather than a subtractive etch process to form the circuitry, opens up significant advantages in the HDI and fine line market. The process I am most familiar with uses a special catalytic precursor “ink” that can be imaged to create the patterns or areas where conducting metal is to be deposited. The ink controls the horizontal dimensions of the line width and spacing and the vertical dimension is controlled using an additive plating process that deposits metal only on the patterns defined by the ink.

If this additive process is new technology for you, this is your chance to use your imagination and think outside of the box. Vias can be drilled prior to the metallization process and are then plated at the same time that the surface conductors are formed, eliminating several process steps. This process can deliver fine lines down to 5 microns in width. There is a significant advantage to RF designs with this process. Because the traces are formed with an additive process, the trapezoidal effect from the subtractive etch process is removed. This process also offers the option of using metals other than copper, which is critical for applications with biocompatibility concerns.

Whether you are new to single- and double-sided flex, moving into rigid-flex construction, thinking of using bookbinder technology, or investigating an additive process, working with new technology can be both exciting and challenging. My best advice when working with flex and rigid-flex is to involve your fabricators as early in the design process as possible. They work with this technology every day, have an enormous wealth of knowledge, and are happy to share and guide designers as they learn and adjust to new technology.

This column originally appeared in the March 2018 issue of PCB007 Magazine.