Guest Post: Ruminating Rigid-Flex – Flex Circuit Materials (Part 1)

Ben Jordan - Altium

Ben Jordan – Altium

Today, designers face increasing project requirements for densely populated electronic circuits including pressures to reduce manufacturing times and costs, writes Ben Jordan of Altium, in the first of a three-part series. To meet these requirements, design teams have increasingly turned to 3D rigid-flex circuits to meet their project’s performance and production requirements. However, designing for flex or rigid-flex presents a number of challenges to electronic and mechanical design teams, that require careful consideration.

See also:

Ruminating Rigid-Flex – Flex Circuit Materials (Part 1)

Ruminating Rigid-Flex – Fab documentation (Part 2)

Ruminating Rigid-Flex – Dos and Don’ts of Flex Circuit Circuit Design (Part 3)

Rigid-Flex can have many benefits, and many designers are at least considering it today who previously did not have to. More PCB designers are facing higher pressures to build ever more densely populated electronics, and with that comes additional pressure to reduce costs and time in manufacturing.

Well, this is really nothing new of course. It’s just that the scope of these pressures that the engineer and PCB designer have to respond to is continuously broadening. Rigid-Flex PCB technology offers a solution that is viable for many product designs facing these challenges.

Yet there are aspects of rigid-flex technology which could be pot-holes in the road for newcomers. So it’s wise to first understand how flex circuits and rigid-flex boards are actually made. In this three part blog series which unfold over the next several weeks, we will take a look at how flex circuits and rigid-flex boards are made and design issues imposed by the materials used, and the fabrication processes employed by most rigid-flex PCB manufacturers. We will also examine Fab Documentation for Flex Circuits and Rigid-Flex Boards and the do’s and don’ts of flex circuit design. From there we can find a clear path to discover the best practices for rigid-flex PCB design.

Let’s begin with examining the Flex and Rigid-Flex Fabrication Process.

Flex Build-Ups

At first glance, a typical flex, or rigid-flex board, looks straightforward. However the nature of these requires several additional steps in the build-up process. The beginning of any rigid flex board is always the single or double-sided flex layers. The fabricator may begin with pre-laminated flex with foil, or may begin with unclad PI film, and then laminate or plate up the copper for the initial cladding. Laminating the film requires a thin layer of adhesive, whereas adhesive-less cladding requires a “seed” layer of copper. This seed layer is initially planted using vapor deposition techniques (i.e. sputtering), and provides the key to which chemically deposited copper is plated upon. This one or two-sided flex circuit is drilled, plated through, and etched in much the same steps as typical 2-sided cores in rigid boards.

Flex Fab Steps

The steps below show Flex-Circuit creation for a typical double-sided flex circuit.

Adhesive/Seed coating applied

Either an epoxy or acrylic adhesive is applied, or sputtering is used to create a thin copper layer for a plating key.

 Adhesive coating

Copper foil added

Copper foil is added, either by lamination to the adhesive (the more mainstream approach) or chemical plating onto the seed layer. Newer fabrication processes by materials vendors allow adhesive-less lamination of rolled annealed copper as an alternative.

 copper foil laminated

Drilling

Holes to vias and pads are most often mechanically drilled. Multiple plated flex substrates can be drilled simultaneously by combining them from multiple reels on drums, drilling between work plates, then rolling out to separate reels on the other side of the drilling machine. Pre-cut flex panels can be combined and drilled between rigid blanks in the same way rigid cores are drilled as well, though it requires more careful registration and the alignment accuracy is reduced. For ultra-small holes, laser drilling is available, though at much added cost because each film has to be drilled separately. This would use Excimer (ultraviolet) or YAG (Infrared) lasers for higher accuracy (microvias), and CO2 lasers for medium holes (4+ mils). Large holes and cutouts are punched, but this is a separate process step.

 drilling

Through-hole plating

Once the holes are made, copper is deposited and chemically plated in the same way as rigid PCB cores (commonly referred to as Cuposit). Through-hole plating in flex circuits is recommended to be at least 1 mil in plating thickness to add mechanical support to the pad or via, whereas a typical low-cost rigid PCB may only have ½ mil cuposit.

 through-hole

Etch-resist printing

Photosensitive etch resist is coated onto the film surfaces, and the desired mask pattern is used to expose and develop the resist prior to chemical etching of the copper.

 etch-resist

Etching and stripping

After exposed copper is etched, the etch resist is chemically stripped from the flex circuit.

 etch and stripping

Coverlay or Covercoat

Top and bottom areas of the flex circuit are protected by coverlay layers which are cut to shape. There may be components actually mounted on sections of the flexible circuit, in which case the coverlay is also acting as a solder mask. The most common coverlay material is additional polyimide film with adhesive, though adhesive-less processes are available. In the adhesive-less process, photoimageable solder mask (the same as used on rigid board sections) is used, essentially printing the coverlay onto the flex circuit. For coarser cheaper designs screen printing is also an option with final curing of this covercoat film by UV exposure. Basically, the difference is that coverlay is a laminated film, whereas covercoat is an applied material coating which then needs to be cured.

 coverlay

 

An example of flex-circuit with Coverlay

Figure 2.1: An example of flex-circuit with Coverlay – notice that the openings in the coverlay are generally smaller than the component pads (source: GC Aero Inc.).

Also, cutouts for component or connection pads in the coverlay leave at least two sides of the pad land to anchor under it. You can see this clearly in Figure 2.1 above.

Cutting out the flex

A blanking die for cutout of Flex Circuits

Figure 2.2: A blanking die for cutout of Flex Circuits (Source: Haoji Stamping Tool & Die Co.,Ltd.)

The final step in creating the flex circuit is cutting it out. This is often referred to as “blanking”. The high-volume cost-effective approach to blanking is by a hydraulic punch and die set (like the one shown in Figure 2.2), which involves reasonably high tooling costs. However, this method allows punching out of many flex circuits at the same time. For prototype and low-volume runs, a blanking knife is used. The blanking knife is basically a long razor blade, bent into the shape of the flex circuit outline and affixed into a routed slot in a backing board (MDF, plywood or thick plastic such as Teflon). The flex circuits are then pressed into the blanking knife to be cut out. For even smaller prototype runs, X/Y cutters (similar to those used in vinyl sign making) could be used.

Lamination and Routing

If the flex circuit is to form a part of a rigid/flex combined stack-up (which is what we are interested in), the process doesn’t stop there. We now have a flex circuit that needs to be laminated in between the rigid sections. This is the same as an individual drilled, plated and etched core layer pair, only much thinner and more flexible due to the lack of glass fiber. As noted previously though, a less flexible layer could be made with PI and glass depending on the target application. Because this is being laminated in between rigid sections, it ultimately has to be framed in a panel that mates with the rigid board panel sections as well. Flex circuits that are not being combined with rigid sections are adhered temporarily to a rigid backing board of MDF or FR-4 style materials.

Laminated Stackups

The flex circuit is laminated into the panel along with the rigid and any other flexible sections, with additional adhesive, heat and pressure. Multiple flex sections are not laminated adjacent to each other unless you are designing multi-layer flex. This generally means each flex section has a maximum copper layer count of 2, so that flexibility is maintained. These flex sections are separated by rigid pre-pregs and cores or PI bonding sheets with epoxy or acrylic adhesives.

Essentially, each rigid panel is separately routed out in the areas where the flex is going to be allowed to, well, flex.

Here is an example process of laminating into a rigid-flex board, with two, 2-layer flex circuits embedded between three rigid sections. The layer stack up would look like that shown in Figures 2.3 & 2.4.

separate cores

(a) Separate Cores, Pre-pregs, and Flex-circuits with ‘bikini’ coverlay, before lamination in to the assembly panel.

 drilled

(b) Drilled, plated and etched cores, pre-pregs, and flex-circuit sections laminated into the assembly panel.

 

Final Rigid-Flex PCB

(c) Final Rigid-Flex PCB. Normally component assembly and reflow would be done in the panel.

Figure 2.3: How the Etched, plated, coverlayed and blanked flex panels are combined with the glass-epoxy rigid panels.

 

Detailed Stack Diagram

Figure 2.4: Detailed Stack Diagram including plated-through holes for each flex section, as well as final through-plated holes in the rigid section (click to expand).

In the example stack up shown in Figure 2.4, we have two pre-etched and cut flex circuits, each double sided and plated through. The flex circuit has been blanked into a final assembly panel including boarders for framing – this will keep the flex circuit flat during final assembly after lamination with the rigid panel sections. There are certainly some potential hazards with inadequate support of flex circuit elbows and large open sections during assembly – especially in the heat of a reflow oven.

While this example does show adhesive layers it’s important to note that many designers are shying away from using adhesives, due to unacceptable z-axis expansion in reflow. However FR-4 pre-preg and thermosetting epoxies effectively achieve the desired result and are for all intents and purposes here considered ‘adhesive’ layers. Additional adhesion can be achieved through treatment of the copper on the flex layers to improve the ‘tooth’ into the laminated prepregs. Adhesive-less double-sided flex laminates are shown here. These are entirely polyimide film with a bondable polyimide coating which the copper foil is bonded to. DuPont Pyralux and Rogers Corp. R/Flex are examples of popular adhesive-less laminates.

The coverlay is also applied – like stickers laminated on with adhesive, or by a photo-printing process as mentioned earlier. Once the final flex and rigid panels in this 6-layer stackup are placed together, they are laminated with the outermost (top and bottom) final copper foil layers. Then another drilling for top-to-bottom plated through holes is done. Optionally, laser drilled blind vias (top to first flex, bottom to last flex) could also be made, again adding expense to the design. The holes are plated through from top to bottom, and blind vias if there are any, and the final outer layer copper patterns are etched. The final steps are the printing of the top and bottom solder mask, top and bottom silkscreen and preservative plating (such as ENIG) or hot air leveling (HASL).

Careful attention needs to be paid to layer pair planning and documentation for drilling and through-hole plating, because blind vias from a rigid surface layer down to an opposing flex-circuit layer will have to be back-drilled and add significant cost and lower yield to the fab process.

Physical Constraints

Multiple Flex Sub-Stacks

While it’s possible to build just about any stack-up with rigid and flex sections, it can get ridiculously expensive if you’re not careful to consider the production steps and the material properties involved. One important aspect of flex circuits to remember is the stresses within the materials occurring as the circuit bends. Copper, being a non-ferrous metal, is known to suffer work-hardening, and fatigue fractures will occur eventually with repeated flex cycling and tight radii. One way to mitigate this is to only use single-layer flex circuits, in which case the copper resides at the center of the median bend radius and therefore the film substrate and coverlay are in the greatest compression and tension, as shown in Figure 2.5. Since the Polyimide is very elastic this is not a problem, and will last much longer under repeated movement than multiple copper layers will.

RA copper in single-layer flex

Figure 2.4: For highly repetitive bending circuits, it’s best to use RA copper in single-layer flex to increase the fatigue life (in cycles before failure) of the copper in the circuit.

Along the same lines, having multiple separate flex circuits is often necessary, but it’s best to avoid having bends at overlapping sections where the length of the flex sections limits the bend radius.

Adhesive Beads

There are times when you need to consider using strengtheners where the flex circuit exits the rigid board. Adding a bead of epoxy, acrylic or hot-melt will help improve the longevity of the assembly. But dispensing these liquids and curing them can add laborious steps to the production process, increasing cost. As always with PCB design, there are trade-offs.

Automated fluid dispensing can be used, but you need to be really careful to collaborate with the assembly engineers to make sure you don’t end up with globs of glue dripping under the assembly. In some instances the glue must be applied by hand which adds time and cost. Either way, you need to provide clear documentation for the fabrication and assembly folks.

Stiffeners & Terminations

Extreme ends of flex circuits typically terminate to a connector if not to the main rigid board assembly. In these cases, the termination can have a stiffener applied (more thick Polyimide with adhesive, or FR-4). Generally then, it’s convenient to leave the ends of the flex embedded within the rigid-flex sections as well.

The Panel

The rigid flex circuit stays together in its panel for the assembly process, so components can be placed and soldered on to the rigid terminations. Some products require components to be mounted also on flex in some areas, in which case the panel has to be put together with additional rigid areas to support the flex during assembly. These areas are not adhered to the flex and are routed out with a controlled-depth router bit (with “mouse-bites”) and finally punched out by hand after assembly.

Final Rigid-Flex panel example

Figure 2.5: Final Rigid-Flex panel example. Notice that this one has front and back board edges, and flex circuit, routed out. The rigid sides are V-grooved for snapping off later. This will save time in assembly into the enclosure (source: YYUXING Shenzhen Electronics Co., LTD.).

For more information you can download this free guidebook at http://go.altium.com/rigid-flex-pcb-design-guidebook.html

Ben Jordan got his start in electronics as an 8 year old, when his big brother got him his first soldering iron with a multivibrator LED flasher kit. Ben holds a Bachelor of Engineering with First Class Honors from the University of Southern Queensland, and has worked as an AE, FAE and in Marketing and management roles at Altium since 2004. Ben has more than 20 years experience designing electronics, PCBs, and embedded computing and FPGA hardware and software, and has research interests in signal processing, audio electronics, and PCB design.

Tags: Altium, PCB, Rigid-Flex

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