Question E6E09:
Which of the following component package types have the least parasitic effects at frequencies above the HF range?  

Choices:  
A: TO-220  
B: Axial lead  
C: Radial lead  
D: Surface mount    

Correct Answer is  D: Surface mount  

Surface-mount is correct because it has the shortest leads and smallest loop area, giving the lowest parasitic inductance (ESL) and stray capacitance at VHF/UHF and above. Short, wide connections on a PCB keep unintended reactances small, so components behave closer to their nominal values at high frequency.

Why the other choices are incorrect:
- TO-220: Long, thick leads and a large metal tab create significant lead inductance and capacitance to the heatsink/ground. Very poor at RF.
- Axial lead: Long wire leads on both ends and a long body length create large loop area and high ESL; very sensitive to lead trim length.
- Radial lead: Still uses lead wires and stand-off height; less loop area than axial but much more ESL/stray C than surface-mount.

Useful formulas and rules of thumb:
- Lead inductance (rule of thumb): about 1 nH per mm of lead length.
- Inductive reactance: $X_L = 2\pi f L$
- Capacitive reactance: $X_C = \dfrac{1}{2\pi f C}$
- Self-resonant frequency (approx.): $f_{SRF} \approx \dfrac{1}{2\pi \sqrt{LC}}$
  - Longer leads increase $L$, lowering $f_{SRF}$, so parts stop behaving ideally at a lower frequency.
- Example: a 5 mm lead ≈ 5 nH. At 500 MHz: $X_L \approx 2\pi (5\times10^8)(5\times10^{-9}) \approx 15.7\ \Omega$ — a big error for many RF circuits.
- Practical tips: use SMD (0603/0402 when possible), keep traces short and wide, minimize loop area, and use solid ground and short vias.