Electronic – How practical are these land patterns for connecting components in a SMPS


Was reading this app note from Linear Technology: PCB Layout Considerations for Non-Isolated Switching Power Supplies and on page 7, there is the following image.

undesired and desired layout for connecting chip components to a PCB for a switch-mode power supply

Are the desired connections practical?

Are thermal reliefs only necessary if there is an uneven distribution of copper (a trace on one side and a polygon on the side side) or are thermal reliefs necessary for any "large" body of copper?

If I use the desired connections in my layout, would I have any assembly problems (assuming everything else is fine)?

Best Answer

The advice is practical and is critical for getting top performance from a switch-mode power supply.

For hot air rework, our techs have not had any problems. For our SMT automated line, they don't have any problem with this. The entire board is pre-heated anyway.

For hand soldering, you may need a larger tip to get more heat on the board with no thermal reliefs. Our techs haven't complained but they can solder much better than me and may resort to hot air rework anyway.

Note that you do not need to do entire SMPS this way. This is just needed on the high current switching paths and GND return paths, especially the input and output capacitors and MOSFETs. I use to normal layout techniques for the parts related to the feedback divider, signal lines, control lines.

As an aside, I have one SMPS design where I did not follow the recommended layout and I have a sub-harmonic oscillation and the output does not meet my calculated full output power because the switching gets erratic at max load. That product is shipping; the SMPS works good enough. On a different SMPS design, I followed the recommendations of the FAE after he reviewed my layout (same company as that app note) and that design switches very cleanly and max output performance is as I calculated.