What’s the difference between PI base material, PI coverlay, and PI stiffener?

As a special engineering material, PI has been widely used in aviation, aerospace, microelectronics, nano, liquid crystal, separation membrane, laser and other fields.It has flame retardant, high temperature resistance and low temperature resistance at the same time, long-term use temperature in -200℃~426℃.

Polyimide(PI) is very common in Flexible Board, it exist in base material, coverlay, even stiffener. Maybe some people feel confused about it, so today let discuss what’s the difference between them?

The difference in function:

  1. PI base material: For FPC base material, it have two types, adhesive base material or adhesivelesss base material. No matter for adhesive base material, or adhesivelesss base material, Polyimide(PI) is essential.
  2. PI coverlay: Its main function is used for circuit insulation.
  3. PI stiffener: Often applied to the area on the back of the FPC goldfinger. The PI stiffener is used to increase the thickness and hardness of the finger, which is easy to insert and remove.

The difference in thickness:

  1. PI base material: For the PI in base material, 1/2mil, 1mil are very common, its thickness even can arrive 4mil, 5mil(Dupont adhesivelesss PI material).
  2. PI coverlay: On coverlay, the PI thickness have two choice, 1/2mil or 1mil.
  3. PI stiffener: For PI stiffener, the thickness have many option according to customer’s need, such as 0.075mm, 0.1mm, 0.125mm, 0.15mm, 0.175mm, 0.2mm, 0.225mm, 0.25mm.

The difference in color choice:

  1. PI base material: No choice for option.
  2. PI coverlay: PI coverlay have three color, yellow, white and black.
  3. PI stiffener: It will change with the thickness change. The thicker it is, the darker it is. For example, 0.075mm PI stiffener looks like brown, 0.25mm PI stiffener more close to dark.

Wish above information is helpful for you. If you have any question, pls feel free to contact us.

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What is the difference between FPC coverlay and EMI shield?

Covering is the film that covers and presses on the surface of the flexible circuit, usually polyimide resin, used for solder mask, etc., and has little effect on signal shielding. The Electro-Magnetic Interference (EMI) shield is generally attached to the coverlay, and the price of the EMI shield is much more expensive than the coverlay. What is the difference between FPC coverlay and EMI shield?

FPC coverlay, known as CVL in the industry, it has the same main function as the solder mask of PCB: 1) Protect the copper foil from being exposed to the air and avoid oxidation of the copper foil; 2) Cover for the areas which it do not need to do surface treatment. 3) The solder resist function in the SMT. The usual colors are yellow, black and white in Best Tech.

FPC with yellow coverlay

The EMI shield uses mixed composite metal as the shielding material and has excellent OSP performance and corrosion solvent resistance. It is superior to similar products in the market in shielding and winding characteristics. The EMI shield is suitable for flexible printed circuits. It has high shielding, bending and heat resistance. The usual color is black in Best Technology.

EMI material

After pressing the cpverlay, the FPC will press a layer of EMI shield as a conductive layer to shield the external electromagnetic interference. EMI shield is mainly used in 3C products such as notebook computers, GPS, ADSL and mobile phones, because they all produce noise due to high-frequency electromagnetic interference, which affects the quality of communication.

FPC with EMI shield
Rigid-flex with EMI shield

If you want to know more design guild about the EMI shield, please feel free to contact us at sales@bestfpc.com

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Why the FPC can not use HASL surface treatment?

The hot air solder leveling is a technology developed for the coating of lead-tin on rigid printed circuit boards. Due to the simplicity of this technology, it is also applied to flexible printed board FPC. But we usually don’t suggest to use HASL and suggest to use ENIG.

In modern market, there are no FPC manufacturer will be use HASL surface to instead of ENIG. Although ENIG will be expensive than HASL. Attached the ENIG FPC and HASL FR4 PCB pictures for your reference, please kindly check it.


The hot air leveling is to immerse the board directly in the molten lead-slot, and the excess solder is blown off by hot air. This condition is very demanding for the flexible printed board FPC. If the flexible printed board FPC cannot be immersed in the solder without any measures, the flexible printed board FPC must be clamped to the middle of the screen made of titanium steel. Then, it is immersed in the molten solder. Of course, the surface of the flexible printed board FPC is also cleaned and coated with flux.

Due to the harsh hot air leveling process conditions, it is easy for the solder to be drilled from the end of the cover layer to the underside of the cover layer, especially when the bond strength between the cover layer and the copper foil surface is low, and this phenomenon is more likely to occur frequently. Since the polyimide film is easy to absorb moisture, when the hot air leveling process is used, the moisture absorbing moisture will cause foaming or even peeling of the cover layer due to rapid heat evaporation, so it is necessary to dry and prevent moisture before the FPC hot air leveling management.

As we all know, the Gold is a better finish for the corrosive environment if it is clean going into the field. HASL will typically use a very corrosive flux to prepare the surface and if this is a lead free HASL or leaded it does not matter, both are susceptible to corrosion.

These must be very clean going into the fielded environment or just the residue from the HASL process can set up corrosion cells. The plating process for the gold finish is not as difficult to clean and the residues are typically very low even with RO water rinsing.

If you would like to learn more details or you have any different opinions, please feel free to contact us and let us know at sales@bestfpc.com

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