背景介绍

背景主要包括了 多层PCB的优势，以及引出其中stub和带状线的问题；接下来解释了via stub所带来的问题，resonance effect；然后介绍了部分文献所提供的一些模型和考虑的方面；最后介绍了本文的侧重点，即通过时域仿真和测量来描述via stub的谐振效应，并给出了综合考虑信号过孔和地过孔的原因，即这种方法能够形成较好的wave guided properties，并且能够产生low radiation losses。

A multilayer printed circuit board is a type of low-cost interconnect structure which commonly includes through via holes and striplines. Resonance effects due to a stub in a transition from a through via hole to a stripline in multilayer PCBs can significantly worsen electric performance of the transition at higher frequencies. In particular, it was indicated on this effect in 1 where differential via holes were considered and effectively modeled as a cascade of capacitances and inductances. In this letter, we present a study of the resonance stub effect in via structures including ground vias embedded in a multilayer PCB by a three-dimensional electromagnetic field simlator based on the time-domain analysis and measurements at frequencies up to 20 GHz.

Via stub的结构

这里研究两种stub结构，如下图所示，左图为stub输出在第10层金属上，有图为stub输出在第3层金属上。整体结构为12层金属，图中参数包括H=2.4mm" role="presentation">H=2.4mm$H=2.4mm$，介质参数为ϵ=3.8" role="presentation">ϵ=3.8$ϵ=3.8$，金属设置为理想导体。

However, being different in their lengths calls for the establishment of the resonance in the transitions at different frequencies (for the longer stub, the resonance will be shifted to the lower frequencies.)

图1. 两种不同的via stub结构 如图2所示，采用地过孔产生回流路径，保证电流连续，地过孔与信号的距离为l" role="presentation">l$l$。（”this via configuration corresponds to a cell of a high-density via structure under a large-scale integration chip.”）信号过孔焊盘直径dpad=0.5mm" role="presentation">dpad=0.5mm$d_{pad}=0.5mm$，焊盘厚度tpad=0.055mm" role="presentation">tpad=0.055mm$t_{pad}=0.055mm$，反焊盘大小dcle=0.8mm" role="presentation">dcle=0.8mm$d_{cle}=0.8mm$，孔径(rod diameter)drod=0.8mm" role="presentation">drod=0.8mm$d_{rod}=0.8mm$。地过孔与信号过孔结构一样，只是连接了所有的地平面层。
带状线的尺寸为w=0.127mm,h1=0.23mm,h2=0.13mm,t=0.035mm" role="presentation">w=0.127mm,h1=0.23mm,h2=0.13mm,t=0.035mm$w=0.127mm,h_1=0.23mm,h_2=0.13mm,t=0.035mm$。
图2. 过孔和带状线结构示意

仿真结果

不同长度via stub的仿真结果如图3所示，其中l=1.0mm" role="presentation">l=1.0mm$l=1.0mm$，较短via stub的长度H1=0.455mm" role="presentation">H1=0.455mm$H_1=0.455mm$，较长via stub的长度H1=1.91mm" role="presentation">H1=1.91mm$H_1=1.91mm$。上述两种结构的仿真数据如下图所示，对于较长stub的情况，其谐振频点在13.7 GHz，而较短stub的谐振频点超过了讨论的范围。同时，图3也讨论了有无地过孔对于S参数的影响。

Moreover, comparisons of presented data show that ground vias can improve the performance of through hole via to stripline transitions, but outside the stub resonance range.

图3. 不同长度、有无地过孔的via stub仿真结果 一方面，stub谐振效应在较长stub情况下会严重降低转接的性能，另一方面，地过孔形式的stub会形成一个高Q值的带阻结构。

Ground vias can shape a stub resonance structure of a high-Q-factor which can be applied to develop compact bandstop filters.

实验结果

为了实现前述仿真的情况，我们采用了另外一种更加实际的测试结构。测试PCB包含了铜导体层和FR4层，FR4的ϵ=3.8, tanδ=0.016" role="presentation">ϵ=3.8, tanδ=0.016$ϵ=3.8,tan\delta =0.016$。测试结构包含了两个相同的信号过孔，距离为l2" role="presentation">l2$l_2$，而其他参数包括信号过孔焊盘直径dpad=1.1mm" role="presentation">dpad=1.1mm$d_{pad}=1.1mm$，反焊盘大小dcle=1.6mm" role="presentation">dcle=1.6mm$d_{cle}=1.6mm$，孔径(rod diameter)drod=0.7mm" role="presentation">drod=0.7mm$d_{rod}=0.7mm$，其余参数可以参考前文。每个信号过孔被地过孔围住，地过孔参数为dpad=1.35mm" role="presentation">dpad=1.35mm$d_{pad}=1.35mm$，drod=1.0mm" role="presentation">drod=1.0mm$d_{rod}=1.0mm$，l1=5.08mm" role="presentation">l1=5.08mm$l_1=5.08mm$。
单个过孔转接的插入损耗和回波损耗可以使用参数提取方法进行，这里假设SMA-带状线转接具有一致性。测试了l2=200mm" role="presentation">l2=200mm$l_2=200mm$和l2=400mm" role="presentation">l2=400mm$l_2=400mm$两种情况，这里需要保证所有测试均从过孔开始。
图4. 测试结构示意 不同线长的测试结果如图5所示，仿真和实测结果较为符合，验证了stub effect的影响。
另外，这里的谐振频率为9.0GHz，比图3的谐振点低，这是由于焊盘和过孔的结构尺寸发生了变化，从而导致等效的L和C发生了变化。这可以用来调节stub的谐振频率，或者如果想要得到较好的传输特性，那么等效的LC需要足够小。

One can notice that these properities of via structures can be used to control the resonance frequency of the stub. Also, a transition from a through via hole to a stripline or a set of these transitions having effective capacitance and inductance of the stub as small as possible is a way to minimize the stub resonance effect in interconnect circuits at the considered frequency band.

图5. 去嵌的S参数结果

个人总结

1. 作为via stub早期的研究，该论文系统展示了via stub的结构，以及via stub在不同长度下的影响。目前，已有部分论文有着更深入的研究，keysight的一个视频也有所涉及；
2. 该论文的缺陷在于前面仿真的结构和后面的测试结构不一致，这并不能很好地说明谐振点不一致的原因；
3. 对于去嵌这部分，可以参考L-2L去嵌技术，貌似是一样的操作。

论文网址

1. Resonance stub effect in a transition from a through via hole to a stripline in multilayer PCBs , 2003, MWCL
2. keysight-demystifying vias in high speed PCB designs
3. keysight-附录2录像