At first glance, the answer to the question “How many layers can a flexible circuit have before it can no longer bend?” sounds like a simple, black-and-white answer. However, this answer proves to be more complex than expected and involves a great deal of specifications before it can be answered.
An answer to that question ends up depending on a variety of options, such as material thicknesses, impedance requirements, and copper thicknesses. Each of those options can affect the thickness, and thus the flexibility of the flexible circuit, turning a flexible circuit with few layers into something unbendable or turning a flexible circuit with six layers into something manageable.
Material Thicknesses
At first glance, saying that the material thicknesses used where flexibility is derived from can impact the flexibility seems to be a simplistic thought. Simply changing material thicknesses can turn a 6-layer flexible circuit into something bendable or a 1-layer design into something closer to rigid than flexible. However, there are a few notable distinctions where material can be changed to subject the flexible circuit to thinner or thicker stack-ups.
Coverlay and Adhesive
The first is coverlay and adhesive. To achieve a thinner board, thinner coverlay and adhesive combinations can be used, which can have a small effect on overall thickness. The adhesive portion of the combination is commonly varied, usually due to copper weight. For heavier copper weights, more adhesive is used to flow in between the traces and pads. However, coverlay and adhesive thicknesses normally only affect thinner flexible circuits, instances where the absolute minimum thickness is required.
Polyimide Core
The next major impact on the thickness of the circuit is the polyimide core itself. When it comes to the construction of flexible circuits, the two types of cores used are adhesive-less and adhesive-based. Just as the name suggests, adhesive-based cores are cores that have a central polyimide portion that is then sandwiched between two plies of adhesive that adhere the copper layers to the core.
At a minimum, usually, the standard for these cores is over double that of adhesive-less cores for the typical minimum thickness. As per the name, adhesive-less cores are similar, but lacking the adhesive layers within, thus allowing the cores to be much thinner, albeit at a higher price.
For 1-layer constructions, the main driving force for the balance of the construction is the polyimide core. With the core being adhesive-less, the thickness is even thinner, resulting in an incredibly low profile and low minimum bend radius.
Impedance Requirements
Generally, impedance can be controlled by three factors: copper weight, dielectric thicknesses, and line widths/spaces. Among these, the copper weight and dielectric thicknesses are what can control the thickness and flexibility of the overall board.
Regarding the core, it follows similar rules to the previous section on material thicknesses, except where core thicknesses are chosen solely based on achieving impedance values. From that, trying to achieve impedance can end up causing a flexible circuit to dramatically thicken and reduce flexibility. The best option when considering impedance is to get proper calculations done to understand the dielectric thicknesses so that the board's flexibility and thickness can be understood.
In its current configuration, a 2-layer design is a standard stack-up. However, if a single-ended 50-ohm impedance were to be added on either side, the core would immediately increase in thickness to be able to best match impedance requirements.
Copper Weight
When designing a flexible circuit, a primary concern that always comes up is copper weight. Copper weight can drive the minimum line widths and spaces, amperage allowances, impedance, and in the scope of this post, the flexibility and thickness of the flexible circuit.
While less important as layer count decreases, copper weight must be considered when trying to achieve a flexible circuit. For just a 4-layer design, just a slight increase from one-third ounce copper to one-half ounce copper across all layers can increase the thickness by nearly 0.001”. For something more dramatic like one-half ounce to one ounce, the thickness can increase by more than 0.002”, nearly 0.003” in thickness. This can dramatically reduce the flexibility of the flexible circuit.
By increasing all copper weights to 1oz, the thickness of the entire design increases by 2 mils, a drastic amount that can severely affect the board.
Summary
The question of “How many layers can a flexible circuit have before it can no longer bend?” ends up being a more complex question than it would seem at first glance. There are three main factors going into it: material thickness, impedance requirements, and copper weight.
When considering those alongside the number of layers and the end use case, the question ends up having an answer of “it depends” rather than a definitive number. In order to know just how flexible a board needs to be, the ends justify the means, the end use case defines the flexibility, so the number of layers ends up being just as variable as all other parts of the stack-up that can define thickness.
