All About Flex: Flexible PCB: What’s in a Name?
November 12, 2015 | Dave Becker, All FlexEstimated reading time: 3 minutes
Flexible PCB is a common term that is synonymous with flexible circuits. While the term “PCB” is generally used to describe rigid printed circuitry, “flexible PCB” is a little contradictory because “boards” aren’t really flexible. Some companies, like All Flex, design and manufactures flexible PCBs, but not rigid PCBs.
There are many similarities between the two, but also significant differences.
Design
The shortest distance between two points is a straight line, so a circuit trace on a flex circuit should look like a circuit trace on a rigid circuit board, right? Actually there can be considerable differences between designing a rigid PCB and a flexible PCB. Because of the flexibility feature, there are unique issues affecting robustness. For example, since flex can bend, one needs to be sure that bending does not crack or break solders joints near a bend location. For this reason, design features like radiused corners and filleted pads are common on flexible circuits. Another issue is that the substrate used for flex is not as dimensionally stable vs. its rigid cousin. Allowances for dimensional changes may be required for flexible PCB design. Most PCB design software is tailored to rigid printed circuit materials and features. Tradeoffs occur when this same software is used to design flexible PCBs[1].
Fabrication
Rigid PCBs and flexible PCBs have very similar fabrication steps and often use the same equipment and equipment manufacturers. For example, photo imaging the base circuitry layer can use virtually identical technology. The differences arise because of the difference between handling a rigid board versus a flexible substrate. Some ultra-thin circuits are extremely flimsy and need special material handling systems. While a PCB is relatively easy to handle, a thin flexible circuit may need special clamping, fixturing or vacuum systems to hold it in place while processing. Some very high-volume flexible PCB manufactures process circuits in a continuous reel (reel-to-reel). Reel to reel processing uses significantly different material handling mechanisms vs. systems processing discrete panel sizes.
Tooling
Both PCBs and flexible PCBs use similar tooling such as CAD files, photo tools and electrical test fixtures. Perhaps the biggest difference is around the cutline definition. Flexible circuits provide unlimited configuration opportunities that often results in very odd shapes and sizes. Circuit boards often are rectangular or may have some notches or curved outlines routered. Because of this nuance, steel rule dies and hard tool dies are probably used more often for flexible PCBs than for boards, at least for ultra-high-volume applications. Laser definition of the circuit outline is also a common flex circuit process. Other differences in tooling involve fixturing that may be required to handle flexible substrates.
Coverlay/Covercoat
Flexible PCBs require different dielectric materials such as flexible photoimagable soldermask or laminated coverlay (also called coverfilm) vs. the non-flexible PCBs. This creates materials with unique properties and challenges. Materials need to bend and flex, with some applications requiring millions of cycles, without cracking or delaminating. Several design features need to be considered with dynamic flex applications, including: copper grain direction, circuit pattern layout, coverfilm thickness, and trace geometries. The most common flexible dielectric is laminated film dielectric bonded to the substrate with high temperature and pressure, a solution rarely used for rigid PCBs.
Applications
Flexible PCBs constitute about 10–15% of all PCBs produce globally, so there are far more applications that use hardboards than flex. The biggest advantage for flex is that it can reduce the size, weight and amount of hardware used for many electronic packages. It is best described as an “electronic interconnect packaging solution.” The concept of an “origami flex circuit” also comes to mind. If you have an application that requires multiple planes of interconnections, or there are active bending or flexing requirements for parts of the package, then a flexible PCB may be your best choice.
References
Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.
Suggested Items
The Chemical Connection: Reducing Etch System Water Usage, Part 2
05/02/2024 | Don Ball -- Column: The Chemical ConnectionIn my last column, I reviewed some relatively simple ways to reduce water usage in existing etch systems: cutting down cooling coil water flow, adding chillers to replace plant water for cooling, lowering flow rate nozzles for rinses, etc. This month, I’ll continue with more ways to control water usage in your etcher. Most of these are not easily retrofittable to existing equipment but should be given serious consideration when new equipment is contemplated. With the right combination of add-ons, it is possible to bring the amount of water used in an etch system to almost zero.
Designer’s Notebook: What Designers Need to Know About Manufacturing, Part 2
04/24/2024 | Vern Solberg -- Column: Designer's NotebookThe printed circuit board (PCB) is the primary base element for providing the interconnect platform for mounting and electrically joining electronic components. When assessing PCB design complexity, first consider the component area and board area ratio. If the surface area for the component interface is restricted, it may justify adopting multilayer or multilayer sequential buildup (SBU) PCB fabrication to enable a more efficient sub-surface circuit interconnect.
Insulectro’s 'Storekeepers' Extend Their Welcome to Technology Village at IPC APEX EXPO
04/03/2024 | InsulectroInsulectro, the largest distributor of materials for use in the manufacture of PCBs and printed electronics, welcomes attendees to its TECHNOLOGY VILLAGE during this year’s IPC APEX EXPO at the Anaheim Convention Center, April 9-11, 2024.
ENNOVI Introduces a New Flexible Circuit Production Process for Low Voltage Connectivity in EV Battery Cell Contacting Systems
04/03/2024 | PRNewswireENNOVI, a mobility electrification solutions partner, introduces a more advanced and sustainable way of producing flexible circuits for low voltage signals in electric vehicle (EV) battery cell contacting systems.
Heavy Copper PCBs: Bridging the Gap Between Design and Fabrication, Part 1
04/01/2024 | Yash Sutariya, Saturn Electronics ServicesThey call me Sparky. This is due to my talent for getting shocked by a variety of voltages and because I cannot seem to keep my hands out of power control cabinets. While I do not have the time to throw the knife switch to the off position, that doesn’t stop me from sticking screwdrivers into the fuse boxes. In all honesty, I’m lucky to be alive. Fortunately, I also have a talent for building high-voltage heavy copper circuit boards. Since this is where I spend most of my time, I can guide you through some potential design for manufacturability (DFM) hazards you may encounter with heavy copper design.