What pad sizes to use? - Page 1
Forum section: electronics / beginners

author: Stupid Bear
author_rank: Contributor
timestamp: January 21, 2023, 08:54:53 pm
content: I'm designing footprints in Altium for components specifically common ones like resistors, capacitors, diodes, etc which I thought would adhere to some size standards but it seems there is a discrepancy between them. For example the IPC wizard created them one size while the datasheets have different suggested sizes and the libraries I find online have other dimensions. They will all technically work but how do you know which is the right one? I would like to have one master library that I could pull from for my projects than would work for most common components. How important is it to be IPC compliant or these just min/max requirements.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 21, 2023, 10:47:11 pm
content: As you say, they will all work for hand assembly.Each board builder will have slightly different processes which work best with slightly different pad sizes. You can't really answer the question without talking to whoever is building your boards about their preferences for the specific components you are using. If your production volume doesn't motivate them to discuss this with you, you will just have to pick the one you like and hope it works well enough.There are other factors which can enter into it. Perhaps your need for density and routing space will motivate you to modify pad sizes, knowing that you will take a hit in board yield. In any case there isn't really a best for everything answer.

author: Stupid Bear
author_rank: Contributor
timestamp: January 22, 2023, 02:34:33 pm
content: As far as the IPC specs goes... is there a need to abide by them? Do certain industries/applications demand them or are they just formalities? If we don't have to use them do people just tend to use whatever pcb library they have or do they just try to get everything from one mfg do that they can go by that companies datasheets or is it mostly dependent on who builds the boards?

author: Siwastaja
author_rank: Super Contributor
timestamp: January 22, 2023, 05:23:29 pm
content: There is no right or wrong, there is just "good enough" for optimum production yield and that also somewhat depends on the exact process - for example, solder paste used, stencil thickness, reflow profile and so on.And the sweet spot is sometimes quite wide, so you don't see a decrease in yield until you go too far and then it can degrade quickly.Whenever the footprint as calculated from the IPC recommendations (e.g., by the wizard) differs from the manufacturer recommendation, it's not a bad idea to take some kind of average. This has never failed me.The only failures I have had have been due to gross errors by using totally botched footprints made by someone else. For example, some really old version of Altium I had to use years ago shipped with some stock libraries which were blatantly incorrect. Most other CAD packages do the same. It's some kind of sadistic act I guess, to test the young players and make them see why it is important to make your own libraries.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 22, 2023, 05:31:04 pm
content: There is no "one size fits all". In an old PCB program (I used about 30 years ago) they had different "density levels". They had pretty big pads that were supposedly easy, and smaller pads for higher density boards, which need tighter process control for reliable automated PnP & soldering.

author: berke
author_rank: Frequent Contributor
timestamp: January 22, 2023, 05:39:39 pm
content: Quote from: Stupid Bear on January 22, 2023, 02:34:33 pmWill your boards go in a car, plane, satellite, some medical equipment or otherwise run anything dangerous or mission-critical?

author: rdl
author_rank: Super Contributor
timestamp: January 22, 2023, 08:13:44 pm
content: This pdf might be helpful.http://alternatezone.com/electronics/pcbdesign.htm

author: MarkS
author_rank: Unknown
timestamp: January 22, 2023, 11:20:00 pm
content: My personal view on this is to follow the manufacturer's recommendations in the data sheet. Sometimes it's arbitrary, but often it's not, especially when dealing with SMD components. The size of the pad will affect solderability and thermal conductivity, often negatively and in ways you would not expect.