Design of small distance QFN package PCB crosstalk suppression analysis

- Sep 15, 2018-

One, the introduction

With the development trend of high-speed and high-density circuit design, QFN packaging has been applied to 0.5mm pitch or even smaller pitch.The crosstalk problem of PCB wire-span-out area introduced by the device with small distance QFN package is more and more prominent with the increase of transmission rate.For high-speed applications of 8Gbps and above, attention should be paid to avoid such problems and provide more margin for high-speed digital transmission link.In this paper, the method of suppressing crosstalk introduced by small spacing QFN package in PCB design is simulated and analyzed to provide reference for such design. 

2. Problem analysis

In PCB designs, QFN encapsulated devices typically use microstrip lines to fan out from the TOP or BOTTOM layers.For small distance QFN encapsulation, attention should be paid to the distance between microstrip lines and the length of parallel running lines in the fan out area.Figure 1 is a size marking diagram of a 0.5 pitch QFN package. 

Figure 1. 0.5 pitch QFN package dimensions labeling diagram

Figure 1. 0.5 pitch QFN package dimensions labeling diagram

Figure 2 is a typical 6-layer PCB design with a 1.6mm thickness using a 0.5mm pitch QFN package:

Figure 2 QFN encapsulated PCB design TOP layer routing

Figure 2 QFN encapsulated PCB design TOP layer routing

Line width/line distance of difference line: 8/10, line distance from reference layer: 7mil, plate: FR4.

FIG. 3 PCB differential routing spacing and lamination

FIG. 3 PCB differential routing spacing and lamination

From the above design, we can see that the spacing between the difference pairs and the line spacing within the difference pairs in the fan out region is equal, which will increase the crosstalk between the difference pairs. 

Fig.4 shows the simulation results of the near-end crosstalk and far-end crosstalk of the difference scheme designed above. In the figure, D1~D6 are difference ports. 

Design of small distance QFN package PCB crosstalk suppression analysis

FIG. 4 port definition of difference mode and crosstalk simulation results

It can be seen from the simulation results that even in the case of short parallel travel line, the near-end crosstalk of difference port D1 to D2 exceeds -40db at 5GHz, reaches -32db at 10GHz, and the far-end crosstalk reaches -40db at 15GHz.For applications of 10Gbps and above, the crosstalk at this place needs to be optimized to control the crosstalk below -40db. 

Iii. Analysis of optimization program

For PCB design, a more direct optimization method is to adopt the tightly-coupled difference routing, increase the spacing between the difference pairs, and reduce the distance between the difference pairs. 

Figure 5 is an example of crosstalk optimization using tight coupled difference lines for the above design:

Design of small distance QFN package PCB crosstalk suppression analysis

FIG. 5 distribution diagram of the tight coupling difference

FIG. 6 is the simulation results of near and far crosstalk of the difference scheme designed above:

FIG. 6 tightly-coupled difference port definition and crosstalk simulation results

FIG. 6 tightly-coupled difference port definition and crosstalk simulation results

As can be seen from the optimized simulation results, the use of tight coupling and increasing the spacing between difference pairs can reduce the proximal crosstalk between difference pairs by 4.8~ 6.95db in the frequency range of 0~20G.Remote crosstalk decreases about 1.7-5.9db in frequency range of 5G~20G. 

Design of small distance QFN package PCB crosstalk suppression analysis

In addition to spacing the difference pairs and reducing the parallel distance when wiring, we can adjust the distance between the line layer and the reference plane of the difference line to suppress crosstalk.The closer you are to the reference layer, the easier it is to suppress crosstalk.On the basis of tight coupled routing, we adjusted the distance between the TOP layer and its reference layer from 7mil to 4mil. 

FIG. 7 layout adjustment

FIG. 7 layout adjustment

According to the above optimization, the simulation results are as follows:

Design of small distance QFN package PCB crosstalk suppression analysis

Figure 8. Crosstalk simulation results after adjustment

It is worth noting that the impedance of the difference line also changes when we adjust the distance between the line and the reference plane, and the difference line needs to be adjusted to meet the requirement of the target impedance.When the distance between SMT pad and reference plane of the chip decreases, the impedance will also decrease. Therefore, the impedance of SMT pad shall be optimized by hollowing out on the reference plane of SMT pad.The size of the excavation needs to be determined by simulation according to the lamination. 

Figure 9. QFN pad impedance optimization after lamination adjustment

Figure 9. QFN pad impedance optimization after lamination adjustment

As can be seen from the simulation results, after adjusting the distance between the routing line and the reference plane, tight coupling and increasing the spacing between difference pairs can reduce the proximal crosstalk between difference pairs by 8.8~ 12.3db in the frequency range of 0~20G.Remote crosstalk decreased by 2.8~9.3dB in the range of 0~20G. 

Design of small distance QFN package PCB crosstalk suppression analysis

Design of small distance QFN package PCB crosstalk suppression analysis

Four, conclusion

Through simulation optimization, we can reduce the near-end difference crosstalk caused by small distance QFN encapsulated on PCB by 8~12dB, and the far-end crosstalk by 3~9dB, providing more margin for high-speed data transmission channel.The crosstalk suppression method involved in this paper can be considered comprehensively when making PCB routing rules and lamination, and avoid crosstalk risk caused by small spacing QFN package in the early stage of PCB design