What are Flexible Printed Circuits?
Flexible printed circuits are continuing to grow in popularity and necessity for PCB designers around the world. What is a flexible PCB? In simple terms, it is simply conductive traces that are bonded onto a variety of flexible substrates. Flexible substrates can be made from a polyimide (most common) or even a polyester material. Depending on your application, flex circuit manufacturers can use an adhesive to bond the conductive to the substrate or they can use an adhesive-less bonding process. An adhesive-less bonding process is more expensive but needed if your flexible circuit is going to be in higher temps for longer period of times.
As devises become smaller and smaller, flexible printed circuits are ideal because of their characteristics. They are thin and flexible, so they are able to be in tight spaces. For example, if you take a part an iPhone or any newer smart phone, you will find a variety of flexible printed circuits. Flexible circuit’s traces can be as thin as <.0001 to="">.010”. The thickness of the polyimide or polyester can range from .0005” to .010”. The options for surface finish for a flexible PCB are the same as a rigid PCB. ENIG and Gold are popular since flexible circuits are widely used for connecting to some type of connection on another board. Exposed copper will oxidize and covering them with an ENIG or Gold finish will prevent oxidation from happening.
Similar to rigid boards, flex circuits can come in 1, 2, 4, 6, and 8 layers of copper, but it is rare to go any higher than that for a flexible PCB. As the layer counts go up, the price does as well. One thing to note is that flexible circuits are almost always more expensive than a rigid board when prototyping. It is safe to say that a flexible circuit is about 1.5 to 2 times more expensive than a rigid board. Even a single layer flexible circuit is more expensive than a 2 layer rigid.
Sometimes, when you are designing a flex circuit, you need to put an FR4 stiffener in a particular spot of the flex where you are going to be assembling medium to larger size components. If you do not put a stiffener in those locations, your components could pop out when handling the circuit. Stiffeners also help the assembly house assemble components because it helps keep the flex circuit flat.
When designing your first flexible circuit, you do not need any special PCB design software or plugin that specializes in flexible circuits. Most likely, you will be able to use your current design software. For flexible circuits, instead of a solder mask, a cover layer is used, but there is also a flexible solder mask that can be used. A cover layer is usually more common and less expensive. Add a new layer for your stiffener and label is accordingly. Have an overall outline of your entire flex, so the flexible circuit manufacturer knows which layers need to be flexible, rigid, and/or need a stiffener. This is when fab notes really come in handy. Specifying your desired stack-up, surface finish, material, impedance, etc.
Assembling a flexible circuit is done the same way that you would assemble a rigid circuit. You can either hand solder or use a pick and place machine. Contract manufacturers tend to like smaller panels for flexible circuits because depending on the thickness of your stackup, the panel won’t lay exactly flat. This can cause problems for them when they are trying to put solder paste on the panel with a stencil. Only part of the panel will line up correctly and it turns into a pain. Smaller panels are easier to handle, but it ends up driving up the overall cost of your flex circuit because more panels are needed.
The potential for flexible circuits are endless and it is really only limited by the imagination of the engineering team. If you are new to designing flexible circuits, don’t be shy, call a few flexible manufacturers and start asking questions. Finding a knowledgeable fabrication shop can save you money in the long run.