Whether you are a novice or an experienced designer, you are likely familiar with flexible PCB. Thanks to their flexibility and slim design, flexible PCB play a crucial role in various applications. However, their lack of rigidity can pose challenges in design. To address this issue, designers often incorporate stiffeners. What exactly are stiffeners, what are their benefits, and how should they be properly applied? In this article, TechSparks will explore these questions to help you better understand and utilize flexible PCB stiffeners.
Understanding Flexible PCB Stiffeners and Their Functions
When flexible PCB circuits are excessively bent or subjected to edge stress, pads or traces can easily crack. To prevent this risk, an additional layer of material is added at weak points (such as edges) to provide greater mechanical support and this is known as a flexible PCB stiffener. Stiffeners are not part of the original PCB composition but are added as a supportive component, typically in the final steps of flexible circuit manufacturing. Here are the main functions of flexible PCB stiffeners:
- They significantly increase the overall mechanical strength by being distributed at key points on the flexible PCB, providing extra support and protection.
- They limit unnecessary bending and twisting of the flexible PCB when not in use, ensuring stable electrical performance during operation.
- By reducing mechanical stress and strain, they effectively extend the lifespan of the flexible PCB.
- They make it easier to solder components to the stiffer parts of the circuit board, thereby improving assembly success rates.
Of course, the design and placement of stiffeners are not arbitrary. To achieve the best results, it is recommended to place stiffeners on the same side of the circuit board as the inserted components and to align the holes in the stiffener accurately with the plated through-holes on the PCB. This not only aids in soldering but also ensures that component leads can smoothly pass through the holes.
Types of Flexible PCB Stiffeners
We’ve mentioned some tips on improving stiffener effectiveness during design. However, other factors also need consideration, such as increasing the stiffener thickness at high-stress bending points, ensuring compatibility between the stiffener and flexible PCB manufacturing processes, and using stiffener materials compatible with the flexible PCB substrate and coverlay. The most common types of flexible PCB stiffeners include PI (Polyimide), FR4, and stainless steel, each with its own advantages and characteristics.
For instance, PI (Polyimide) is perhaps the most widely used type of flexible PCB stiffener. It has features like solder resistance and high adhesive strength, typically used to increase the thickness of ZIF (Zero Insertion Force) connectors. The ideal application position for PI stiffeners is usually on the back of the inserted gold fingers. Sometimes, simply selecting a PI stiffener portion that is not laminated or attached to the FPC body can easily insert the flexible printed circuit connector fingers into a ZIF socket or connector. Common PI stiffener thicknesses include 0.001 inch (25μm), 0.002 inch (50μm), 0.003 inch (75μm), and 0.005 inch (125μm).
FR4, on the other hand, is a material very familiar to circuit designers, often used as the substrate layer for rigid PCB and as a stiffener material for flexible PCB. Its biggest advantage is its low cost compared to other stiffener materials, yet it still provides adequate support for SMT assembly and reflow soldering processes. It can also be used to replace SMT carriers by adding an FR4 frame to the border of a panelized PCB. Common FR4 stiffener thicknesses range from 0.003 inches (0.08 mm) to 0.125 inches (3.18 mm).
For more advanced applications with higher quality requirements, TechSparks strongly recommends considering stainless steel stiffeners. As the name suggests, stainless steel as a stiffener material offers advantages such as corrosion resistance, sturdiness, and formability. However, its main drawback is its high cost. In flexible circuits, stainless steel stiffeners are often added to terminals to strengthen the flexible printed board and facilitate assembly. Common stainless steel stiffener thicknesses range from 0.1 mm (4 mil) to 0.45 mm (18 mil).
How to Apply Stiffeners on Flexible PCB
There are various methods to apply stiffeners on flexible PCB, with the most common being pressure-sensitive adhesive (PSA) and thermal bonding. PSA is a simpler technique that only requires applying a small amount of pressure during the process to firmly bond the stiffener to the flexible PCB, much like applying a sticker. Although this method has relatively lower reliability, it is sufficient for IPC Class 2 products such as televisions, laptops, and consumer electronics. Most importantly, it is very inexpensive and can significantly reduce production costs.
In contrast, thermal bonding requires both heat and pressure to attach the stiffener to the flexible PCB, resulting in higher bond strength. Although it is more expensive, the high reliability and durability it offers make it worthwhile for IPC Class 3 products such as military and aerospace applications. However, this method results in longer process times, so production schedules need to be reasonably arranged when using this method.
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