Capacitors under Processor

[maziaar83]

Hi

We have teacher who always advised us to place capacitors(power supply noise filters) under Processors in a PCB.
Is there anyone who can tell me why?

thanks

 

[Rame]

Hi,

It is the decoupling capacitor your talking about..Decoupling capacitor of value 0.1 & 0.01uf is usually used and we need to remember that the process of placing a capacitor adjacent to the IC is to supply the transient switching current, rather it is the process of placing an L-C network adjacent to the IC to supply the transient switching current. The inductance comes from the capacitor itself (typically 1-2 nH for a SMT capacitor), the interconnecting traces (typically 5 to 20 nH according to the layout), and the lead frame of the IC (typically 4 to 15 nH according to the type of IC package).

From the above we see that the inductance can vary from a low of 10 nH to a high of 37 nH, all assuming a reasonably good layout. It is this inductance that limits the effectiveness of the decoupling network.

It is very important to remember this fact -- we are placing an L-C network between the power and ground, not a capacitor!

Hope this help you...

Regards

Ramesh