In a flex or rigid-flex PCB construction, many elements have to be considered that can be seen as important to the overall design such as copper weight, layer count, and many other things. However, among all the portions of a design, the one that usually gets forgotten is what bonds and holds it all together, the adhesives and pre-pregs that serve to bond the stack-up.
In flexible PCB stack-ups, adhesives play a core part, where only the extreme and rare instances of a stack-up contain no adhesives. While in rigid-flex designs, adhesives are used in flexible portions, while also using prepregs to bond in the rigid portions and to keep the layers together.
What Are Adhesives?
In flexible circuits, adhesives are nearly the most critical part of any stack-up. While some stack-ups can exist without adhesives, it would be difficult to operate without any. From the coverlay, to the stiffeners, to extra pressure adhesives applied to the exterior of the circuit board, and even to adhesive-based cores, adhesives play a major role in forming the stack-up of a flexible circuit.
Rigid-flex PCB with pressure sensitive adhesive.
When it comes to adhesives for a flexible circuit, they usually come in three different varieties.
Acrylic Adhesives
The first are acrylic adhesives. These are versatile adhesives and have proven to be among the most used adhesives in many offshore manufacturing locations.
Epoxy Adhesives
Next are epoxy adhesives. These are less commonly seen than acrylic, however they serve a purpose in that they can be less reactive to some chemical environments than acrylic can be. The downside is that due to the nature of epoxy adhesives, they can sometimes be regarded as slightly stiffer than acrylic adhesives, and the lower demand can mean increases in cost or lead times depending on materials used.
Polyimide-Based Adhesives
The last of the three adhesives are polyimide-based adhesives. These can withstand higher temperatures than the previous two and they’re highly chemical resistant. The downside is that the demand is so low for this adhesive that many PCB manufacturers cannot support them, and that aside from low demand, the process to manufacture them requires specialty lamination procedures that go beyond what many circuit board fabrication shops can perform.
Adhesives in Rigid-flex Circuits
In rigid-flex circuits, the role of normal adhesives remains the same, due to the flexible portion of the part. However, this changes when thinking of the rigid section, where adhesives are recommended to be removed. This recommendation comes from IPC guidelines, suggesting that due to the differing coefficient of thermal expansion of the rigid materials and flexible materials, it is best to make sure no adhesives extend into the rigid section where a through hole would travel through it.
This can be quite a limiting factor, leading to innovations that allow for multi-layer flexible sections within rigid-flex PCB designs to be bonded in some sections with adhesives, whereas in the rigid portions of the stack-up, they are bonded with a near equivalent thickness of prepreg. This allows a circumvention to include adhesives in rigid stack-ups and results in a more reliable build.
As for what prepregs are used, the main restriction is that prepreg’s must be what’s referred to as “no-flow” or, depending on manufacturer definition, sometimes referred to as “low-flow”. These terms refer to the propensity of the prepreg to flow during lamination processes, where it is most desired in a rigid-flex PCB stack-up for the prepreg to exhibit little to almost no flow characteristics, to now flow out into the flexible section and interfere with the bending capability of it. These types of prepregs are achieved by taking the standard formulation and changing it with the addition of a polymer and, in some cases, by inducing a form of pre-lamination with the manufacturer that can reduce the flow characteristics of the selected prepreg dramatically.
Summary
Bonding within rigid-flex PCBs and flexible circuit board applications can be quite a complicated process. While the terminology can be unexpected, and the different adhesives to be difficult to remember, it is always important to remember what the application the part is meant for and the environment it is set to be in.
This is the single most important factor and can heavily influence the material choice when designing the circuit board. While this can sound like a daunting task, involving the circuit board manufacturer from the start can greatly help in deciding how to approach the stack-up to a design, what materials should be used, and which materials they can support.
Key Takeaways
- Adhesives are a critical component in flexible and rigid-flex PCB construction, playing a key role in bonding stack-ups, attaching coverlays and stiffeners, and ensuring structural integrity.
- Three primary adhesive types, acrylic, epoxy, and polyimide, offer varying benefits, with acrylic being the most used, epoxy providing chemical resistance, and polyimide offering high-temperature durability but requiring specialized manufacturing.
- Rigid-flex circuits should minimize adhesives in rigid sections due to differences in thermal expansion, instead using no-flow or low-flow prepregs to maintain stability and prevent interference with bending.
- Prepreg selection is crucial for rigid-flex PCB designs, as these bonding layers must exhibit minimal flow during lamination to avoid seeping into flexible sections and affecting their mechanical properties.
- Consulting a PCB manufacturer early in the design process ensures proper adhesive and prepreg selection, helping to optimize the stack-up for reliability, manufacturability, and the intended application environment.
