USB 4 高速信號佈局關鍵因子 (三 ) : Via Stub

Via Stub

• Via 本身有電容(Capacitive)及電感(Inductive)效應. 倘若用 PATH (Plated Through-Hole) 貫串性Via 會產生stub (殘端)效應, 產生反射波,影響SI(Signal Integrity).
• General Rules :

       ▶  10GBd NRZ (Nyquist Freq. <= 5GHz)   allow  < 30 mils stub

       ▶  20GBd NRZ (Nyquist Freq. <= 10GHz)  allow < 15 mils stub

       ▶  20GBd NRZ or PAM4 (Nyquist Freq. > 10GHz)   allow < 10 mils stub

    

Via with long stub

Via with short stub

  

Via Stub- Back Drill

• 利用Back-Drill (背鑽)來移除stub, 使得差動阻抗不會有所偏差,因此可以減除信號的反射波,由此方式大大改善SI 的品質.