开云体育

ctrl + shift + ? for shortcuts
© 2025 开云体育
  1. Kicad-Users
  2. Topics

Multiple parallel SMD capacitors - one footprint

 

On 25.10.24 17:57, Luke Vogel wrote:
> I'm drawing up a Low Pass Filter for a HF power amp.
> The topology of the filter requires some parallel capacitors to achieve the required capacitances.
...
> Below is the type of footprint I'd prefer to use ...
> My question is; What is the best way to achieve a clean PCB design and still have a comprehensive BOM?

If the depicted 4x1206 assembly is in your library as a single
component, then I'd just set its value to: 100pF + 47pF + 47pF + 22pF
and add a line of e.g. awk to split that up into 4 components on BOM
generation. (Your arithmetic has them all in parallel, so placement is
arbitrary within the assembly, I figure.)

Mind you, if hand assembling a small number of units, I'd also consider
just stacking them in a small layer cake, if soldering didn't become too
fiddly. It seems easier to avoid adding stray capacitance while tweaking
to the nearest pF, if unnecessary copper areas are avoided.

Erik
 

Thanks for the reply Erik,
?
The footprint is currently assigned to every group, and yes, the location of each member of the group is totally arbitrary.? The main center pad(1) is the active pad with the signal, the external pads(2) are grounded to zones either side of the signal trace.
I'm semi familiar with awk from my old linux days, but currently I'm using KiCad on a windows computer.
I also now use Google Sheets (rather than Excel), so I guess I could write a script to pull the values out and give them their own component line although it would be a fair bit of mucking around, especially if I want to make this foolproof!? Ultimately I'd like to put this project in the public domain so I'd like to avoid anything that could be too hard for the average project builder.

I'm not sure I understand what you mean by "small layer cake" ... are you suggesting soldering components on top of each other rather than using the PCB pads?
?
I was hoping for a bit of a simple work around to achieve this.
?
cheers
Luke
?
 

On 25.10.24 23:41, Luke Vogel wrote:
> I was hoping for a bit of a simple work around to achieve this.

TLDR; Skip to last paragraph.

If soldering all four capacitors in a neat little stack is OK for your
public to perform, then I'd make a library component without pads. Then
you could place one ordinary capacitor with pads, and place additional
padless capacitors in the same place.

That also solves the BOM issue, as all capacitors are there separately,
without special treatment or any effort.

The neat little stack also minimises stray capacitance where you are
tweaking to the nearest pF. Spreading components in a broader plane
cannot be as effective in that regard.

And if someone skews a capacitor a few degrees in the stack, then it
won't matter after the lid is on the box.

If there are many to do, then maybe pre-assemble each "216 pF" unit by
soldering them while held against a flat surface, in tweezers or
similar. Alternatively, the three sides of a cut off corner of a
cardboard box might align all and allow holding with a toothpick.
Subsequent soldering onto pre-tinned pads might then be achieved without
much loss of aesthetic appeal.

Mind you, if the extra stray capacitance of planar placement (maybe +5 pF?) is OK in practice, then I'd just place the capacitors with abutting pads, linked with half a mm of track, and cancel
any clearance errors in the DRC. Then you don't have to? do anything.

Erik
 

When I need to stack surface-mount components I use a bit of "blue painters tape" on my bench with the sticky side up. I place a few large washers or similar on the ends of the tape to keep it from sliding around or curling.

Then, just stick the parts to the tape in the proper alignment and solder them together. I've even built an "order 7" elliptic filter that way. :-)

Steve

On 10/26/24 04:49 AM, dvalin via groups.io wrote:
On 25.10.24 23:41, Luke Vogel wrote:
> I was hoping for a bit of a simple work around to achieve this.
TLDR; Skip to last paragraph.
If soldering all four capacitors in a neat little stack is OK for your
public to perform, then I'd make a library component without pads. Then
you could place one ordinary capacitor with pads, and place additional
padless capacitors in the same place.
That also solves the BOM issue, as all capacitors are there separately,
without special treatment or any effort.
The neat little stack also minimises stray capacitance where you are
tweaking to the nearest pF. Spreading components in a broader plane
cannot be as effective in that regard.
And if someone skews a capacitor a few degrees in the stack, then it
won't matter after the lid is on the box.
If there are many to do, then maybe pre-assemble each "216 pF" unit by
soldering them while held against a flat surface, in tweezers or
similar. Alternatively, the three sides of a cut off corner of a
cardboard box might align all and allow holding with a toothpick.
Subsequent soldering onto pre-tinned pads might then be achieved without
much loss of aesthetic appeal.
Mind you, if the extra stray capacitance of planar placement (maybe +5 pF?) is OK in practice, then I'd just place the capacitors with abutting pads, linked with half a mm of track, and cancel
any clearance errors in the DRC. Then you don't have to? do anything.
Erik
 

To achieve what you want, I just use 4 footprints. No problem for BOM. But in PCB, I just hide all the value text. Then add a field at one of them says "4x100nF". Just like that.?

You can also add a text or leader. But it won't move with the component. A field can move with component.

1 - 5 of 5