Elementary, Mr. Watson: Rules of Thumb—Guidelines vs. Principles for PCB Design
Beyond Design: High-speed Rules of Thumb
Global PCB Connections: A Technical Overview of Long-flex Printed Circuit Boards
All About Flex: Flexible PCB: What’s in a Name?
November 25, 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.
Share on:
Suggested Items
Unlocking Advanced Circuitry Through Liquid Metal Ink
10/31/2024 | I-Connect007 Editorial TeamPCB UHDI technologist John Johnson of American Standard Circuits discusses the evolving landscape of electronics manufacturing and the critical role of innovation, specifically liquid metal ink technology, as an alternate process to traditional metallization in PCB fabrication to achieve ever finer features and tighter tolerances. The discussion highlights the benefits of reliability, efficiency, and yields as a tradeoff to any increased cost to run the process. As this technology becomes better understood and accepted, even sought out by customers and designers, John says there is a move toward mainstream incorporation.
Fresh PCB Concepts: The Critical Nature of Copper Thickness on PCBs
10/31/2024 | Team NCAB -- Column: Fresh PCB ConceptsPCBs are the backbone of modern electronics and the copper layers within these boards serve as the primary pathways for electrical signals. When designing and manufacturing PCBs, copper thickness is one of the most critical factors and significantly affects the board’s performance and durability. The IPC-6012F specification, the industry standard for the performance and qualification of rigid PCBs, sets clear guidelines on copper thickness to ensure reliability in different environments and applications.
Book Excerpt: The Printed Circuit Designer’s Guide to... DFM Essentials, Ch. 1
10/25/2024 | I-Connect007The guidelines offered in this book are based on both ASC recommendations and IPC standards with the understanding that some may require adjustment based on the material set, fabricator processes, and other design constraints. This chapter details high-frequency materials, copper foil types, metal core PCBs, and the benefits of embedded capacitance and resistor materials in multilayer PCBs.
The Cost-Benefit Analysis of Direct Metallization
10/21/2024 | Carmichael Gugliotti, MacDermid AlphaCarmichael Gugliotti of MacDermid Alpha discusses the innovative realm of direct metallization technology, its numerous applications, and significant advantages over traditional processes. Carmichael offers an in-depth look at how direct metallization, through developments such as Blackhole and Shadow, is revolutionizing PCB manufacturing by enhancing efficiency, sustainability, and cost-effectiveness. From its origins in the 1980s to its application in cutting-edge, high-density interconnects and its pivotal role in sustainability, this discussion sheds light on how direct metallization shapes the future of PCB manufacturing across various industries, including automotive, consumer electronics, and beyond.
Connect the Dots: Designing for Reality—Pattern Plating
10/16/2024 | Matt Stevenson -- Column: Connect the DotsIn the previous episode of I-Connect007’s On the Line with… podcast, we painted the picture of the outer layer imaging process. Now we are ready for pattern plating, where fabrication can get tricky. The board is now ready to receive the copper traces, pads, and other elements specified in the original CAD design. This article will lay out the pattern plating process and discuss constraints in the chemistries that must be properly managed to meet the customer's exacting manufacturing tolerances.