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Placement of decoupling caps.

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Odin

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What is the correct way to place and connect decoupling caps on a 4 layer board? Top and bottom layers for routing, L2=+5V L3=GND. SMD and DIL packages. I've put the packages on the top layer, and the caps (100 nF 1206 smd) under the packages, on the bottom layer.
The DILs power pins connects directly to the inner layers. How should I route the caps for optimal result? Directly down to the power layers, or to the IC pins? It's my first 4 layer board. I'm used to place decouplings as close to the ICs as possible and connected directly to the power pins.
 

Ubergeek63

Well-Known Member
what for? it depends on the frequencies involved.

your caps are not great for high frequency systems, nor for fine pitch parts.

while most sample layouts you see show 0402 or smaller on the bottom side directly under the pins, I have never considered that optimal since the tiny via has a higher inductance than the nonexistent trace to a 100pF decoupling cap

Dan
 

duffy

Well-Known Member
It's not as critical when you are using the inner layers for a power plane and ground plane - they have a distributed capacitance effect that inherently reduces noise.

If you can get all the surface mount parts on the top it will save you money in assembly charges.

It's my first 4 layer board.
Use thermal reliefs on any through-hole components to those inner planes or your technicians will kill you.
 
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Odin

Member
It's not as critical when you are using the inner layers for a power plane and ground plane - they have a distributed capacitance effect that inherently reduces noise.
I guess you're right. Maybe I'm too conserned about the details. It's no HF involved. A PIC, a few logic gates and some LEDs.

If you can get all the surface mount parts on the top it will save you money in assembly charges.
It's going to be smd components on both sides, I'm afraid. Too many parts, too less space.


Use thermal reliefs on any through-hole components to those inner planes or your technicians will kill you.
I'm using the supply layer function in Eagle 5.4 pro. It's uses the reliefs as standard.
 

Ubergeek63

Well-Known Member
I guess you're right. Maybe I'm too conserned about the details. It's no HF involved. A PIC, a few logic gates and some LEDs.

It's going to be smd components on both sides, I'm afraid. Too many parts, too less space.

I'm using the supply layer function in Eagle 5.4 pro. It's uses the reliefs as standard.
I would be interested in seeing. I am notorious among our production people for getting more onto a PCB than what belongs there :)

Dan
 
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