Lightweight and high-speed transmission technology for soft PCB substrate

- Nov 03, 2018-

Flexible PCB substrates (FPC: Flexible Printed Circuit) are mostly used in notebook computers in pursuit of legerity. Flexible PCB substrates have the characteristics of light, thin and Flexible. In addition to the components such as batteries and hard drives that can be avoided in the computer basket, they also play the functions of connecting host Board, USB, LAN and other auxiliary PCB substrates.

As the data transfer speed of the notebook computer becomes increasingly high-speed, the system manufacturers require the sound waves of the flexible PCB substrate with high-speed transmission characteristics to continue to rise. Therefore, the flexible PCB substrate must be managed with characteristic impedance value.

Soft PCB substrate is divided into single-sided and double-sided two categories, but the notebook computer with signal layer and ground layer of soft PCB substrate, copper foil USES mostly composed of double soft PCB substrate, the main reason is the characteristic resistance of the resistance value management more easily, especially the single side PCB substrate, soft soft PCB substrate once in contact with the basket body, easily is affected by the rate of induced electric contact objects, causing the characteristic impedance changes, at this time will need to consider the soft PCB substrate components around issues such as layout, therefore management more complex than double soft PCB substrate.

Then this paper will discuss the soft PCB substrate lightweight, and support data transmission requirements for high-speed transmission technology.

Lightweight technique

The development of lightweight flexible PCB substrate is mainly achieved by greatly simplifying the structure of double-sided flexible PCB substrate used for traditional notebook computer. The specific method is to make an interface layer on the surface of single-sided flexible PCB substrate using conductive adhesive to form a quasi-double-sided structure.

Comparison of structure and characteristics between traditional double-sided flexible PCB substrate and lightweight flexible PCB substrate is summarized as follows:


On the occasion of double-sided flexible PCB substrate, the signal layer and the ground layer use copper Coating for electrical connection. As the signal layer and the ground layer are covered with copper foil as a whole, the double-sided flexible PCB substrate has a high copper content ratio.

In contrast to the situation of lightweight flexible PCB substrates, the protective layer of single-sided flexible PCB substrates can be set up in advance to fill a large number of conductive adhesive at the opening position, forming an electrical connection between the signal layer and the substratum, thus effectively omitting the copper film which is very bad in quality.In addition, compared with the double-sided flexible PCB substrate, the lightweight PCB substrate can achieve 50% of the lightweight target.

Thin and soft

Lightweight flexible PCB substrate is basically a single-sided structure and does not require copper foil lamination. Compared with double-sided flexible PCB substrate, it can achieve 28% thin molding effect, while its softness is up by 89% compared with double-sided flexible PCB substrate. Therefore, complex and subtle avoiding coiling can be made inside the notebook computer basket.

Lightweight transmission characteristics of FPC

Lightweight flexible PCB substrate has been used in notebook computer internal part of S - ATA Ⅰ comes into contact with the hard drive (communication speed of 1.5 Gbit/S), the current is gradually S - S - ATA Ⅰ ATA Ⅱ (communication speed of 3.0 Gbit/S), the future is expected to evolve to S - ATA Ⅲ (communication speed of 6.0 Gbit/S), therefore lightweight flexible PCB substrate required to support the next generation high speed transmission of countermeasures.

Lightweight soft PCB substrate Impedance characteristic Impedance, the figure shows Impedance characteristics of lightweight, flexible PCB substrate, with the length of the transmission line slowly rise, eventually forming the so-called transmission losses.

Lightweight soft PCB substrate waveform simulation analysis result, the figure shows the Eye chart (Eye - the pattern) did not come into contact with the S - ATA Ⅰ benchmark, so central hexagonal shape parts to form a good wave shape, compared with S - ATA Ⅱ occasions, Eye chart has access to the central hexagonal, symbol recognition at a much lower margin, this also verify the S - ATA Ⅰ praise highly of lightweight soft PCB substrate, facing in the future high speed data transmission, the required PCB substrate, the improvement of the transmission characteristics.

FIG. 6 is a microscope photograph of the junction layer of conductive adhesive and then discusses the transmission loss mechanism of lightweight flexible PCB substrate.

The resistance of the formation itself

Conductive adhesive are mainly composed, then resin mixed with metal particles and resin on the electrical belongs to the insulator, the main function is a fixed metal particles, in other words the conductive adhesive metal particles is not the conclusion, but through the contact to electrical flow, so the conductive adhesive composition of the impedance of the ground plane itself is very large, it can make the transfer loss significantly worse.