Distance between centre pads

[Winsu]

Hi All,

I'm a bit confused. I think this is a simple thing but I can't get my head around and be 100% sure.

I have designed a footprint for a Electrolytic capacitors. It is basically 2 pads( rectangles). I think that the distance between both pads centres( of the footprint) must be the same than the distance between the centre of both pads of the package( capacitor).

By doing that and making sure that footprint pads are bigger and they absorb packcage tolerance it should work.

Does it make sense?

Cheers,

 

[Relayer]

Hello Winsu,
The pad centers are the points of reference when distance measuring. They should be the same
as the physical capacitor itself.
Unfortunately pad distances vary with higher working voltages and larger capacitance values.
You should design your capacitor parts to include the different distances and pad sizes.
i.e. The higher the capacitance and distance between the pads: then you need to make the pad
sizes larger, as well as the appropriate hole size.
I hope the above helps.
Regards,
Relayer

 

[senilicus]

The center of the pad does not have a relation with the center of the terminal on the component.

When calculating a pad size, there are five important dimensions.

• The contact area of the terminal on the pcb
• The heel size
• The toe size
• the side size
• the height of the part

So, do not mess up things by using a random oversize value that you made up. Use IPC-7351 and IPC-J-STD001 as a standard for you footprint design.


Go to https://www.pcblibraries.com and download the "FREE Pro" version of their "PCB Library Expert" software. It comes with the IPC standard integrated in the software.

 

[Winsu]

Hi Senilicus,

I have download the software and generated the footprint. I guess it is how it should be but I feel that the package will be sitting to close to the inner pad foot print. Is there any reason why it is like that?. In built library is more generous with copper. I have attached a picture to illustrate.

Thanks.

 

[senilicus]

It all has to do with the values for the heel. have a look at the attached spreadsheets. It might explain it a bit better.