A question that most of our friends would like to know: how fast can you run on a PCB hard board: 10Gbps, 25Gbps, 56Gbps or even 112Gbps! What speed can you run on a PCB soft board: Er…
Yes, in the application of hard board and soft board, maybe more friends pay attention to the design and application of hard board, and Mr. High Speed has successfully increased the high speed signal rate of hard board from 10G to the current design of 112G. In contrast, the article on the soft board is relatively less, of course, this is also the application environment and the proportion of the industry is related. However, as the application environment has changed over the years, more and more designs have been used to transmit high-speed digital signals on flexible boards. There are a lot of friends in our seminar or exhibition to see the design of some of the hard and soft board we show, can not help but ask how high speed can the soft board transmission? At one time, Mr. High Speed did not have a very positive answer to answer everyone, so Mr. High Speed made a piece of hard and soft board to see what the performance of the high speed signal soft board is like.
However, we should introduce some knowledge of the soft board, say some more serious introduction first. Rigid-flexboard, also known as Rigid-flexboard, is the result of the combination of soft board (FPC) technology and traditional Rigid board. It has the flexibility of the soft plate, the flexibility of bending, and at the same time has the rigid region of the hard plate to realize the device mounting. It is of great help to save product internal space, reduce finished product volume and improve product performance.
At the beginning, the structure of the hard-soft plate is relatively simple, the number of layers in the soft plate area is not much, and the number of layers in the hard plate area is relatively consistent. As the product function is more and more complex, the design requirements of the hard and soft board are increasingly complex, the number of layers in the soft board area began to increase, the number of layers in the hard board area has become unstable, in the design of high reliability of the hard and soft board, the preferred structure 1 and structure 2; Structure 3 is selected when impedance shielding is required; For high density demand, structure 4 was selected; The soft plate has inserted and pulled golden fingers, and structure 5 is selected. Double sided soft and hard structure structure 6 (not recommended); Structure 7 and 8 can be selected for specific installation requirements.
Of course, there are a lot of knowledge related to the process, the length of the relationship, here is not one of the introduction, we will have the opportunity to talk about it, this paper we still focus on the performance of SI!
Oh, that’s right. We designed a test board to verify the performance of a high speed signal on a soft board, and it looks like this.
Above we have verified many different flexible board routing structures, including the above mentioned structure 1,2,3,4.
But we’re not going to talk too much about comparisons today, we’re going to talk about the general structure that you’re most interested in. We selected the software trace at the bottom of this test board for testing, which is a soft inner board trace, as shown below:
This is the simplest soft plate structure that you can think of, and it is the practice of solid copper. What is the soft plate of solid copper? Here I sell a pass first, and I will explain it later.
We tested the loss of the soft board, and found that in the 20GHz frequency band we tested, the whole line was very linear. From the linearity, it was basically the same as the signal on the hard board, and the loss was also a good situation for this length.
Of course, as mentioned earlier, this is a soft plate structure design in the case of solid copper. Solid copper means that where copper is laid on a large scale, for example, the floor surface is the same as that of a hard plate, which is covered with solid copper. But from the use of the point of view, for the soft plate is unfavorable, because the copper proportion, the worse the bending. So, in order to compromise the bending of the soft plate, the industry has developed another approach, which is the mesh copper soft plate structure, just like the wire of our test board.
Yes, the holes in the ground plane you can see from the picture are without copper. The test board we made is 20mil*20mil mesh copper. In terms of bending property, this mesh copper has been significantly improved, not to mention SI performance.
However, from the analysis of some of our SI theories, it is bound to have a certain impact on the performance of our high-speed signal. As a result, the reference plane of the route is not complete, and the impedance will continue to mutate, which does not seem to satisfy the basic principle of ensuring the integrity of the reference plane for high-speed signals. But we will not be too discouraged, because from its test results, it is not bad to have no friends at all. It is also very linear up to at least 10GHz, and the loss is not significantly worse.
So the soft board can still be used in high-speed signal, basically go to the 10Gbps signal without any problem. If the design is a little better, I believe it is possible to go straight up to 25Gbps. However, there will be a lot of factors affecting the design of the soft board, such as processing error factors, the design difficulty of the hard and soft interface position, or the use of factors, will lead to some of the instability of the soft board, so we still need to plan well before using the soft board scheme!
