Author: Huang Gang, a member of Yibo Technology Expressway Media
A question that most friends would like to know:
How high a rate can a PCB hard board run: 10Gbps, 25Gbps, 56Gbps or even 112Gbps are all OK!
How high a rate can a PCB soft board run: Uh...
yes, in the application of hard and soft boards, more friends may pay attention to the design and application of hard boards, and Mr. Gaosuo has successfully increased the high-speed signal rate of hard boards from 10G to the current 112G design in recent years. In contrast, Mr. Gaosuo has relatively few articles on soft boards. Of course, this is also related to the application environment and proportion in the industry. However, as everyone's application environment has changed in recent years, more and more designs have been applied to soft boards to transmit high-speed digital signals. Many friends have seen some of the designs of soft and hard boards we displayed at our seminars or exhibitions, and they can't help asking how high a rate the soft board can transmit? Mr. Gaosuo did not have a very definite answer to answer everyone at one time, so Mr. Gaosuo made a hard and soft board himself to see what the performance of the high-speed signal soft board is like.
However, as usual, we should introduce some knowledge about soft boards. Let's start with some more serious introductions. Rigid-FlexBoard, also known as Rigid-FlexBoard, is a product of the combination of flexible board (FPC) technology and traditional hard boards. It has the flexibility and bendability of soft boards, and at the same time has the rigid area of hard boards to realize device mounting. It is very helpful to save internal space of products, reduce the volume of finished products, and improve product performance.
The initial structure of the rigid-flex board is relatively simple, with few layers in the soft board area and relatively consistent number of layers in the hard board area. As product functions become more and more complex, the design requirements of the rigid-flex board are becoming more and more complex. The number of layers in the soft board area begins to increase, and the number of layers in the hard board area becomes unfixed. In the design, for high-reliability rigid-flex boards, the preferred structures are structure 1 and structure 2; when there is an impedance shielding requirement, choose structure 3; when there is a high-density requirement, choose structure 4; when the flexible board has plug-in gold fingers, choose structure 5; when the double-sided rigid-flex structure chooses structure 6 (not recommended); for specific installation requirements, choose structure 7 and structure 8.
Of course, there is a lot of knowledge related to the process, but due to the length of the article, I will not introduce them one by one here. We will talk about them in detail in the future. In this article, we will focus on the performance of SI!
By the way, what I just said is that we designed a test board to verify the performance of high-speed signals on the soft board, which looks like the following.
We have verified many different soft board routing structures above, including structures 1, 2, 3, and 4 mentioned above.
However, we will not talk too much about comparison today, but will talk about the common structure that everyone is most concerned about. We selected the following soft routing of this test board for testing, which is an inner soft board routing, as shown below:
This is the simplest FPCB structure that you can think of, and it is made of solid copper. What is a solid copper FPCB? I will keep you in suspense for now, and will explain it later.
We tested the loss of this FPCB trace and found that within the 20GHz frequency band we tested, the entire trace is very linear. From the perspective of linearity, it is basically the same as the signal on the hard board, and the loss is also very good for this length.
Of course, as mentioned before, this is a soft board structure design under the condition of solid copper. The so-called solid copper means that in places where copper is laid on a large scale, such as the ground plane, it is the same as the hard board, and is covered with solid copper. However, in terms of usability, it is not good for soft boards, because the higher the proportion of copper, the worse the bending property. Therefore, in order to compromise with the bending property of soft boards, the industry has developed another approach, that is, a soft board structure with grid copper, just like the trace on our test board.
That's right, the holes in the ground plane you see in the picture are without copper. Our test board is made of 20mil*20mil mesh copper. This mesh copper has obvious improvement in terms of bendability, not to mention SI performance. Friends who have attended our seminars or exhibitions have tried it themselves, and it is indeed much softer.
However, softness is softness. From the perspective of some of our SI theories, it will inevitably have a certain impact on the performance of our high-speed signals. Because this will cause the reference plane of this trace to be incomplete, the impedance will continue to change suddenly, and it seems that the most basic principle of ensuring the integrity of the reference plane for high-speed signals is not met. But everyone will not be too discouraged, because from its test results, it is not so bad that no one has it. At least within 10GHz, it is still very linear, and the loss has not deteriorated significantly.
So the soft board can still be used for high-speed signals, and basically there is no problem with signals up to 10Gbps. If the design is better, I believe it is possible to directly upgrade to 25Gbps! However, there are still many factors that affect the design of the soft board, such as processing error factors, design difficulty at the interface between soft and hard, and usage factors, which will lead to some instability of the soft board. Therefore, you still need to plan well before using the soft board solution!
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