Temperature Limitations of Rigid Flex Circuits
Rigid flex circuits have been widely adopted in electronics for applications that require both structural integrity and flexibility, like automotive electronics or wearable devices. These hybrid PCBs offer space efficiency, and they can be bent or folded into tight spaces to fit where other types of PCBs cannot. They also help simplify assembly processes by eliminating the need for extra connectors and cables. However, the rigidity of a rigid flex circuit can sometimes be challenged by its thermal conditions, which can cause reliability issues and performance degradation over time.
The first step to overcoming these challenges is knowing what they are. The temperature limitations of rigid flex circuits are typically due to the materials used in their construction, which have different maximum operating temperatures. Choosing the right materials and constructions, as well as implementing heat management techniques like using heat sinks, thermal vias, and optimizing trace widths, thicknesses and copper plane configurations, can significantly reduce these limitations.
One of the biggest factors that affects the maximum operating temperature of a flexible circuit is its adhesive. The typical flexible epoxy-based adhesive has a maximum continuous operating temperature of only around 140 degrees Celsius. Once a flex circuit exceeds this limit, the adhesive will begin to degrade, causing the flex layers to separate from each other. This can lead to a number of different issues, including signal integrity degradation and electromagnetic interference (EMI) problems.
Understanding the Temperature Limitations of Rigid Flex Circuits
Other factors that affect the maximum operating temperature of a flex circuit include its design, fabrication, and assembly processes. Rigid flex PCBs are often fabricated with an adhesiveless cladding process, meaning that the copper circuit layers are plated directly onto the underlying polyimide core. While this offers greater flexibility and better mechanical performance, it is also more difficult to solder. This is because the copper can create air-pockets during soldering, which can negatively impact the heat flow in the circuit. To avoid this, fabricators have to use an alternate low-temperature eutectic solder that can withstand the higher melting point of the copper.
In addition, the type of copper circuit layer and its thickness can impact how much heat it can carry through a rigid flex circuit. The resulting thermal conductivity of the circuit can then be a factor in its maximum operating temperature. The thickness of the copper, as well as how much it is spaced apart from other layers, can also influence how efficiently a rigid flex circuit can dissipate heat.
As the maximum operating temperature of a rigid flex circuit increases, it is important to consider all of these factors and determine whether or not this type of PCB will be suitable for the application at hand. It is not advisable to cut corners in terms of design, fabrication and testing, as this can result in costly reliability issues down the road. Ultimately, it is best to choose a reputable rigid flex circuit manufacturer that can work within the appropriate temperature limits for your unique application.
