Bypass Caps and Power Planes

Making PCB Layouts, Manual routing, Auto-routing, Copper pouring, Updating from Schematic, Manufacturing Output
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doug12745
Posts: 14
Joined: 10 Sep 2018, 17:24

Bypass Caps and Power Planes

#1 Post by doug12745 » 06 May 2019, 13:47

I have a general PCB layout question and hope that maybe the more experienced forum members might share their thoughts and experiences on this.

My question: I am trying to place bypass caps across VCC and GND on my IC's. My PCB consists of two signal layers (top & bottom), and internal power and ground planes. Ideally, according to a couple of app notes I read, the bypass caps should be placed as physically close to the IC power and GND pins as possible. I have manually placed traces between the bypass caps and locked them before performing an autoroute. However, the Diptrace Autorouter seems to go ahead and directly place vias from the IC pins to the PWR & GND planes anyways. This seems to cancel out the intent of having the bypass cap feed the IC through the cap.

It seems that the Vcc and GND pins on the ICs should be placed on a separate net which then connects to the bypass cap which is then connected to the planes. A plane cutout beneath the IC's Vcc and GND pins just moves the via outside the cutout and connects that to the pin. I've seen some comments on other forums (StackExchange, etc) which suggest that having power planes eliminates the need for bypass caps as the planes provide enough capacity.

Any suggestion or comments on how others view this?

doug12745
Posts: 14
Joined: 10 Sep 2018, 17:24

Re: Bypass Caps and Power Planes

#2 Post by doug12745 » 07 May 2019, 13:27

I found my answer in an app note from Texas Instruments (scaa082a.pdf). This document is very comprehensive and I recommend it as a reference for PCB layout people.
ByPassCaps.jpg
  • Poor placement of bypass caps occurs when the IC draws current through vias from the power and ground planes rather from the bypass capacitor itself.
  • Good bypass placement allows the IC to draw from the bypass cap directly, and the cap itself is charged from the power and ground planes through vias.
The problem I was having is that DipTrace autoroute was punching vias from the IC power and ground pins to the internal power planes while also still connecting my bypass cap to the same IC power pins (thus negating the effect of a bypass cap altogether). This is similar to the top example. Electrically (DC) the two layout examples are the same, but from an AC response, they are quite different.

I manually rerouted these sections of my PCB to obtain the bottom configuration. I removed the vias placed by autoroute from the power pins and ran traces directly to the pins from the bypass cap.
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