What is Flex PCB coverlay?

Flex PCBs can be found everywhere around us nowadays. This type of boards can be found for example in televisions, smartphones, cars and more. The compact size and high flexibility make Flex PCB a popular type of board for various applications. An important part of the Flex PCB is the so-called "coverlay", which we will introduce in this article.

Coverlay - meaning

Coverlay is a type of special material used to encapsulate or cover exposed layers of flex PCB circuits. It typically consists of a polyamide substrate and an adhesive. The adhesive is used to bond the polyimide to the flexible circuit and seal the circuit. Once applied, this "coverlay" is aligned under specific thermal and pressure conditions with the surface of the circuit. As the name implies, the purpose of this layer is to cover the surface of the flex PCB to protect and insulate the circuit.

The "coverlay" function:

• Insulation: to ensure proper circuit function and prevent short circuits between different layers, it is important to prevent current flow between PCB   surfaces. The "coverlay" serves this purpose because it has insulating properties.
   
• Flexibility: Complex shapes and limited space require flexibility and bending properties. Because the "coverlay" itself is flexible, it helps to maintain this flexibility throughout the flex PCB.
   
• Circuit component protection: The reliability of a flex PCB is proportional to the protection of the circuitry from mechanical damage, dust or chemicals during bending and movement. The "Coverlay" serves as a protective layer in just these cases.
  "Coverlay" is most commonly applied, laminated or bonded over circuit patterns on flex PCBs. Selecting the appropriate material and process for the "coverlay" is a key step to ensure the performance and reliability of the flex PCB.

Mil is a measure equal to 1/1000th of an inch. There are 1000 miles per inch, 1000 miles = 1 inch. A mil is a unit of measurement used in the imperial system that represents one thousandth of an inch. I compare it with the Central European unit of measurement of 1 mil = 0.0254 mm.

Choosing the right "coverlay" for your flex PCB

The "coverlay" for a flex PCB can be selected in different combinations of adhesive and film thickness. A commonly used combination is a 1:1 ratio (1 mil of film and 1 mil of adhesive).

The combination of film and adhesive depends on the following factors:

• Minimum bend radius: strict bending requirements may require a thinner "coverlay".
   
• Thickness of the outer layer of finished copper: At least 1 mil (25 µm) of adhesive is required per ounce (35 µm) of finished copper
   
• Dielectric withstand voltage
   
• Cost

Differences between "coverlay" and solder mask for flex PCB

Both the "coverlay" and the solder mask perform the same primary function of protecting or insulating the external circuitry of the flex PCB. However, each material has different properties, functions and suitability for certain design requirements.

So what are the differences between "coverlay" and solder mask?

•  "Coverlay" is more suitable for the flexible parts of a printed circuit. In contrast, a solder mask is more suitable for rigid and rigid parts of a flex PCB.
   
•  Solder mask is liquid based, while "coverlay" is a combination of adhesive and kapton.
   
•  The track width for the coverlay holes can be close to 3 mil. However, using 3 mil for the solder mask may result in slivering or non -  registration problems. It is preferable to use 4 mil for the solder mask.
   
• "Coverlay" cannot be used for parts with smaller pin spacing, while solder mask can be used.
   
• The gap for "coverlay" should be at least 10 mil. The size for the solder mask should be at least 4 mil. The groove prevents the molten solder from overflowing from one pad to another nearby pad.

In conclusion, coverlay plays a crucial role in protecting and ensuring the reliability of flex PCBs. Its insulating properties, flexibility, and ability to safeguard circuits from mechanical damage and environmental factors are essential for the proper functioning of flexible printed circuit boards. The differences between coverlay and solder mask are significant, with each material being suited for different applications. Selecting the appropriate coverlay, considering factors such as material thickness, bending requirements, and dielectric strength, is vital to achieving optimal performance of flex PCBs in the demanding conditions of modern technology.