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Material Requirements of High-Density Flexible PCB

by: Jan 10,2019 3260 Views 0 Comments Posted in Flexible PCB

High-Density Flexible PCB


Recent market trends show that high-density interconnection technology, or HDI, is growing more than twice as fast as conventional lines. HDI circuit usually means that the pitch of the circuit is less than 8mils (200um) and the aperture is less than 10mils (250um). It is estimated that the most significant applications driving the growth of HDI flexible printed circuit boards are in mobile communications and computer devices like PDA. Other high-growth applications include tape flexible PCBs for chip packaging, suspension flexible PCBs and interconnections for hard drives, flexible PCBs for flat-panel displays, and flexible PCBs for inkjet printer cartridges.


HDI applications require finer lines and smaller plated through-holes, so thinner conductor layers and substrates are required for flexible printed circuit boards. However, the physical properties of this kind of substrates often fail to meet the technical requirements.


1.     Dimensional Stability of Flexible Materials

In the past, one of the perceived disadvantages of using flexible materials and rigid substrates was that flexible materials varied greatly in size during fabrication, which is because that the PI film, the most commonly used substrate in flexible circuits, shrinks during the manufacturing process as the stress generated in the lamination of the film and the substrate is removed. This shrinkage affects the alignment of the circuit layer and the cladding film, as well as the alignment of the mounting components.

Thin materials will affect the performance and yield of FPC, and the dimensional stability of materials is the main reason affecting the yield in the production of high-density flexible printed circuit boards. If we want to get the high density circuit products with high reliability, we must address the problems of materials, structure, and fabrication that may arise during the manufacturing process.

2.     Resistance to Electron Migration of Binders of Flexible Material:

In flexible circuits, electron migration is a phenomenon that leads to the destruction of circuits. Some types of binders of flexible circuits, when exposed to moisture, high temperature or bias voltage, the copper ions in the circuits can move through the binders, thus forming a line between the positive and negative poles. When ions start to line up between copper wires, the wire starts out as a twig or lightning bolt. In some extreme conditions, it can cause a short circuit when the wire reaches a certain thickness between the two wires.

With the increase of voltage and line density, the problem of circuit reliability caused by electron migration increases obviously. Therefore, the application of HDI represents the increase of potential risks, so it requires circuit designers to be aware of these potential problems and take appropriate measures to prevent the occurrence of these risks.

3.     Thin and Low Profile Copper Foils:

One of the main ways to achieve HDI flexible circuits is through the use of thin, fine grained and low-profile copper foil, which is typically 1OZ thick as standard copper foil for conventional flexible circuits. For graphics of normal density, 1OZ copper foil can provide suitable performance, while for HDI applications, most manufacturers will use 1/2 OZ, 1/3 OZ and 1/4 OZ copper foil.

4.     Control of Fluidity of Cladding Adhesive:

The main reason for using a coating film in an HDI circuit is to make a small window and to ensure that the window is not filled with binder during lamination to prevent the exposed copper from being covered. Therefore, the flow performance of the coating material used for HDI must be strictly controlled.

For example, too little fluidity can create holes in fine lines, reducing electrical insulation. And if the liquidity is too large may overwrite the window. Not all flexible binders have this property, so we must choose the appropriate coating film and according to the binder flow properties to choose the appropriate lamination process.


In today's rapidly changing market competition, it is clear that flexible circuits will lead the trend of The Times, so we must adjust the design ability, choose the right materials and manufacturing process to ensure that we are in a favorable position to compete in the market.

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