Along with the advancement of technologies happening in the electronics industry, there exist a lot of issues with it. Particularly in Rigid-Flex PCB Constructions, where the escalation of mechanical and electrical issues has worsened, the device’s dependability has been compromised.
In place of this, the IPC has developed a way to address this problem through a method termed Air Gap Construction; however, not a lot of people are aware of it.
Consequently, we’d like to discuss the definition of Air Gap, several mechanical and electrical issues, benefits, and ways to address the Rigid-Flex PCB concerns in this article.
Introduction to Rigid-Flex PCB’s Air Gap
What is Air Gap Construction in Rigid-Flex PCB?
As already mentioned, the Air Gap Construction was created to address the technical glitches a Rigid-Flex PCB encounters. In addition, all of the notions and concept of this technique was the responsibility of IPC since they were the ones who discovered this solution.
Simply put, Air Gap Construction’s concept is to divide the flexible layers into a desirable amount per set. In essence, the acceptable number of layers per set is less than or equal to three layers only. However, per the experts in the industry, two layers per set are the optimal approach.
Nevertheless, it doesn’t mean that the manufacturers cannot incorporate three layers per set as the IPC 2223 Standard states that if the application necessitates enhanced controlled impedance in the versatile section of the board; then, it is recommended to integrate three layers in each set. Thus, it can eradicate the presence of flexible adhesives in robust sectors.
As for its structure, it is relatively straightforward. Since there are a maximum of three layers in every set, the first layer can act as the stripline. On the other hand, the rest of the adjacent layers can act as a protector. However, if the application calls for only two layers, there only exists surface microstrip configuration. Overall, the frequently preferred count of layers in Given the potential that a greater layer count could reduce the flexibility of the Flexible region, the Rigid-Flex PCB Construction is two layers per combination. Due to this particular reason, the Air Gap Construction was created in this approach.
Detailed Description of Air Gap
Effects of a Rigid-Flex PCB with a Higher Layer Count
One of the issues that every circuit board face is having a high layer count; however, some of them can be addressed through lamination. In Rigid-Flex PCBs, this approach can be possible; however, it can pose significant risks of deteriorating the reliability of the device both Mechanically and Electrically. Hence, the experts have created a way to resolve such issues through the development of Air Gap Construction. Nevertheless, we’ll touch on each of the components that contribute to the reliability concerns of the Rigid-Flex PCB.
Mechanical Reliability Issues
Simply put, the primary concern in the mechanical aspect of the Rigid-Flex PCB is the bend quality of the device. Basically, the relationship between the layer count and flexibility of the Rigid-Flex PCB is inversely proportional to each other because of the increased strain that the copper circuitry is experiencing; as the thickness of the flexible layer increases, its bend capability decreases. During a passive system, a recommended minimum bend parameter for 1 to 2 stacks of the flexible layer is ten times the bendable depth.
As for a 4-Layer Flexible composite portion, this amplifies at approximately 20 to 30 folds. In case this guideline wasn’t followed to a point wherein the desired bend capability was exceeded; then, there increase in the risk of cracking that may lead to the opening of the circuit, then failure. Although the physical properties of the constituents involved in the production of Rigid-Flex PCB can’t be altered, this concern can still be addressed by changing the manufacturing approach per the established guidelines.
Electrical Reliability Issues
Typically, the issues in terms of the electrical aspects are based on the inappropriate materials utilized in the construction. In general, the primary materials for Flexible Surfaces are either Epoxy or Acrylic-Based Adhesives; while, FR-4 is for Rigid Surfaces. In the technical aspect, the materials incorporated in the flexible areas of the Rigid-Flex PCB have a higher value for its Coefficient of Thermal Expansion (CTE); thus, it can easily expand when heated.
On the contrary, FR-4 material has a minimal amount of CTE only, making it retain its structure even when subjected to extreme settings. As a result, if the Rigid-Flex PCB is sent to an extreme temperature condition, the flexible materials expand and shrink, while the FR-4 remains. Thus, it may lead to opening the circuit since the via plating will be highly affected by the expansion.
How Does Air Gap Eliminate the Problem of Via Reliability?
In previous years, Rigid-Flex PCBs were developed by integrating countless layers and sticking them all together with the help of an adhesive. Such sealants, however, have a high Coefficient of Thermal Expansion (CTE), making them rapidly extensible.
As a consequence, it can simply experience stress when exposed to an extreme environmental condition that can damage the via because of the expansion it performs. Nevertheless, due to the development of Air Gap Construction, the issue relating to the Via Reliability has no longer a problem because, in this approach, the adhesive section was completely removed.
Simply put, instead of using an adhesive to stick the Flexible Layers, the Air Gap construction utilizes a prepreg that separates the set of layers. It can be a way to solve the Via Reliability Concern; however, it can also enhance the adhesions of each portion in the sets. Even though this approach can be fairly expensive, this can save a lot of money in the long run as it increases the reliability of the overall structure of the Rigid-Flex PCB; thus, reducing the amount that can be used for future repair costs.
Example of a 6-Layer Rigid-Flex PCB with 4 Air-Gap Layers
How can Rigid-Flex PCB’s Mechanical Flexibility and Bend Capabilities be Enhanced by Air Gap?
In essence, two (2) fundamental aspects highly contribute to the improvement of the Bending Capability of the Rigid-Flex PCB through the help of an Air Gap Construction. First, we can achieve an optimum capability through an Air gap Construction because of the reduced thickness of the individual flexible layers; it can hit two birds with one stone since it can enhance the flexibility and eradicate the occurrences of an I-Beam effect. Second, depending on its unique width, every flexible combination is permitted to fold independently with little to no disturbance from the other flexible sets.
Each flexible sets are free to bend per its inherent curvature, yet as the air gap between them gets less, they might eventually bump against one another. According to a little discrepancy in ductility across pairs and a stringent “U”-shaped curve restriction, the flexible couple located outside of the curve may commence collapsing the flexible pair within the arc. Ultimately, it is recommended to address this scenario since any bowing of a flexible brace will go beyond the substance’s capacity and may lead to fractured connections; thus, may result in the circuit system failure.
Example of Severe U-Shaped Curvature with Buckled in the Flexible Couple
Summary
In conclusion, Air Gap Construction in Rigid-Flex PCB is highly significant in enhancing the capability and performance of the overall device. In essence, through the development of Air Gap Construction, the high-layer count issues, via reliability concerns, and bending reliability of the Rigid-Flex PCB were addressed. All of it was comprehensively discussed in the entire blog.
At MV Flex Circuit, we always wanted to help our consumers easily understand the technical concepts related to the electronics industry. In case there are still questions left in mind about Air Gap Construction; please do not hesitate to reach out, we’ll be delighted to answer all of the confusion one may have in their bags.
Finally, if you’d like to avail of our services, especially the Air Gap Construction for the Rigid-Flex PCB, kindly message us, so that we can discuss the specifics of the desired project. We’re a manufacturer that has over fifteen years of experience in the industry; hence, you can entrust us with any PCB needs you may have. What are you waiting for? We’re on standby to fulfill your needs, and satisfy your PCB desires with our exceptional service. Contact us immediately!
