PCB Layout - Footprint Generation Guidelines?

Almost all part manufacturers now provide a recommended copper geometry for soldering. General rules, as stated, can be good starting points if you have no other data.

We found that these geometries often need to be modified for the specific solder process used. In an older wave solder process, manufacturers geometries often produced solder whiskers, high stress on parts (bad for SM ceramic chip caps!), and other defects. We analyzed the solder defects that were common in our particular process and, with some careful "common sense" changes, modified the pad & hole geometries to virtually eliminate solder problems.

My recollection is that most of the stock manufacturers geometries worked well for a newer reflow process. ALL of this changed and we had to go through the whole tweaking process again when we switched to no-clean flux and once AGAIN when we switched to no-lead solder. Painful, but near zero solder defects was worth the effort.

MMP,

A company called PCB Matrix (owned by Valor, who just got bought by Mentor), has a free "Land Pattern Calculator" program that is very good. It will help you develop PCB footprints based on the manufacturers package drawings/dimensions and tolerances. It was developed using the IPC-7351B spec as a guide, which is generally accepted to be the "bible" for PCB land patterns.

It's not the easiest software to use, but for the price, you can't beat it

LP Calculator Link

Give it a try, I think you will find it to be very useful.

Tom D. (PCB Designer)

P.S. I mentioned that Mentor just bought the parent company of this software for a reason, since they just aquired this software, they may end up starting to charge for it, so it would be good to get it while you can.

Thanks for the info. I'm a little irritated because their website now amounts to a 'bait and switch', as it still claims all this stuff is free, etc., etc., but because of the information you mentioned about the acquisition, they now respond to your registration request with : "Thank you for registering with Mentor Graphics. All 30-day evaluation licenses and free product downloads are temporarily suspended. A new download center is currently under construction and scheduled to open on June 30. You will receive an email notification the week of June 30 of where you can proceed with your request for a 30-day evaluation license for LP Wizard 2010 SPac1." I'd be interested in trying it out if you have the install files and are willing to share, but otherwise I probably won't bother. I really appreciate your help, though!

Wow, I guess I'm a bit too late with my recommendation! I guess I should not be surprised with the way they are taking things........... too bad.

Tom

This site http://landpatterns.ipc.org/default.asp has a IPC7351 calculator listed as a download of IPC-7351A Land Pattern Calculator – Version V2010(zip). The download works, I don't yet know anything about the workings of the file. The site has a useful looking link to IPC-7351A Land Pattern Naming Convention that still works. Other links on the page take you to the "try for 30 days then buy" page that you have already been on. I don't know if the download works as desired, but you may want to grab a copy quickly before they decide to take it down.

I also don't know if the download is "free" or "free for 30 days". Not being told that it is only good for 30 days makes me think that it is the old version that was free forever.

No one yet mentioned that for surface mount footprints you also have to take into consideration the solder paste stencil thickness. The thicker the stencil the more solder paste will be on the pcb. While this may not be critical for large and well spaced pins, it is however a real concern when dealing with 0402 parts and 7thou pads/spacing. It's wise to check with your stencil manufacturer or specify the stencil thickness in your manufacturing specifications.