PCB components and corresponding pads

Discussion in 'The Projects Forum' started by bjsterilite, Jun 13, 2011.

  1. bjsterilite

    Thread Starter New Member

    Jun 13, 2011
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    Hey guys,

    I was looking for some help on choosing the what kind of pads i should use with components on a PCB. This is my first time designing a PCB of any sort. This project comes from a tutorial on a website for mods and its basically a circuit to control a set of LEDs that will be animating 3 sheets of plexiglass.

    The problem I'm having is that because I'm not too familiar with components I didn't know what size pads to place on my pcb.

    EDIT: I forgot to add a part
    My bill of materials is:
    1 x 4017 Decade/Counter
    1 x 555 Timer
    3 x 3mm White LEDs
    3 x 10k Ohm Resistors
    3 x NPN Transistors
    1 x Electrolytic(+) Capacitor 1uf
    1 x 120 Ohm resistors
    1 x ~47k Potentiometer
    1 x SPST Switch
    1 x USB Connector Cable(B1)

    [​IMG]
     
    Last edited: Jun 13, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You don't say what software you are using to generate the schematic, and then the PCB design.

    I've been using Cadsoft Eagle for a few years. It's complex, but has a pretty rich set of features. The freeware version is limited to 2 layers, and a board of ~3" x ~4". It comes with quite a few libraries for parts. A 2-layer PCB and a 3" x 4" board is enough for LOTS of hobbyist projects.

    The first place to look for pads is the manufacturers' datasheets for the components themselves. Standard parts like DIP IC's are usually contained in a library for your chosen layout softwere.

    You won't find pad layouts for devices like switches that are designed for mounting on a panel; you will either need to use a connector of some sort, or simply use wire pads so that you have something to solder to.

    If you decide to try Cadsofts' Eagle, then I highly suggest that you go through Sparkfun's tutorial, here:
    http://www.sparkfun.com/tutorials/108
    It will save you a lot of time.
     
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  3. bjsterilite

    Thread Starter New Member

    Jun 13, 2011
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    Sorry about that. I'm using ExpressPCB and Express SCH.

    Is there a rule of thumb I should follow when I find the datasheet?
    Also, in ExpressPCB the pads they offer are round pads with round holes, square pads with round holes, SMT rectangular pads and something called BGA pads. I really am not sure which ones to use as wire pads. And what part of the pad is making the connection, the center hole? I'm sorry for all the questions, I'm just so confused and thanks again
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Additionally, I don't see what I would call a "parts list." I see a list of descriptions.

    You can't start a layout until you know what every part looks like.

    This is the very tedious part (like we don't have enough of those, right?) even before you start the layout. You need to get a specific part number and perhaps a data sheet on each and every part, switch, connector, any and everything that goes onto your PCB. When I an doing a design (on my computer, of course) I make a top job folder, and one of the sub folders is named "Docs" which is a nice place to save all of them.

    So pick a distributor you like (I like Digi-Key) and flesh out all those parts into something you can buy with a full part number, and a back order number of zero (meaning they have what you need on their shelf). Download the data sheets (another reason to like DK) into your Docs folder while you are at it.

    Now you are almost ready. Look again at your schematic and visualize what goes on the board and what doesn't: does that switch actually mount on the board, or does it mount to the panel and connect somehow to the board? Somehow how? Just wires to pars, or a connector, is it a clever PC mount switch you can put on the board? How far and where exactly on the board?

    You may end up with several schematics, one just showing PCB mounted parts, and another overall schematic showing "things not on the board."

    (When I dropped off my first bill of materials the buyer quietly walked it back to my boss the ask WTF cause my list looked like yours.)
     
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  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    We cross-posted.

    I happen to like ExpressPCB and use it when I can. It is much faster then getting it all up in Orcad and Allegro.

    Some parts (surface mount especially) show a recommended footprint on the data sheet (or somewhere on their website, it can be a challenge to find them).

    For DIP packages, the pads are pretty standard. ExpressPCB even has pre-made component groupings. It helps a lot to understand these groups as their software can link the schematic to the PCB and show you which pins are supposed to connect to other pins.

    As far as square or round thru-hole pads go, typically the square pad is reserved for pin number 1, and round holes for the others. Just makes assembly easier that way.

    The very long list of footprints can be upsetting until you find the SEARCH function. ;)
     
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  6. nigelwright7557

    Senior Member

    May 10, 2008
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    Your CAD package should have corresponding pcb components for the shcemtaic components already in the library.

    For a 4017 I would use a 70 thou pad with a 40 thou hole.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    What size & shape pad is rather subjective.

    Some like to use round pads for most everything. Cadsoft Eagle IC libraries use oval pads for DIPs. With any software package, you need to verify the dimensions of packages that someone else made; I'd found a number of Eagle libraries that had holes too small for DIPs (they should be 32 mils aka 0.032" or 32 thousandths of an inch.) I'm not certain why nigelwright7557 likes to use holes that large. It would make IC insertion somewhat easier, but you'd lose some of the pad that you may very well need.

    If you don't need to run traces between the pins of your IC's, you could get away with 70 mils or even larger; but I've been using 50 mils for the width of the oval pads; that gives room to run a trace between the oval (actually, more of a stretched octagon) pads. 70 mils would make that kind of a run rather tight for a homemade PCB.

    You can make whatever pad layouts you need with ExpressPCB, but I have found the software to be somewhat limited in functionality; Eagle has a much longer learning curve, but has quite a few more features.
     
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  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I spent time learning and using ExpressPCB and ExpressSCH; both were easy to learn and easy to use, and as long as I ordered PCB's from the providers of the software, all went well. But then I decided I wanted PCB's from other sources and discovered that Gerber files were required (which ExpressPCB doesn't provide.) So, I tried to learn Eagle and failed. Then, I bought the starter edition of DipTrace which is also easy to learn, more capable than ExpressPCB, and does produce industry standard Gerber files.
     
  9. jerseyguy1996

    Active Member

    Feb 2, 2008
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    I will give my impressions of ExpressSch and ExpressPCB as an amateur (hobbyist) who also just started designing my own PCB's. As a previous poster said, one of the best things you can do is take the time to assign each part a part number, a part name, and a part ordering number (I use mouser). I learned the hard way that it is much better to completely draw a schematic first and label all of the pins on each part, even if you are just copying a schematic from somewhere else. When it comes time to draw the PCB, the software will highlight pins that are connected on the schematic to make it easier to run the proper traces. I make the pad diameter and pad hole diameter just a bit bigger than the standard pad size used on the library components because it makes it less likely that my drill bit will rip the pad off of the board when I drill my holes. There is a bit of a learning curve associated with using any software but ExpressSch and ExpressPCB is pretty quick. It took me one board to get up to speed on all of the little shortcuts. My boards still aren't very pretty but they do work. When I get home I will look up the pad size that I use for typical through hole components and post it here. One other thing. How do you intend to etch the boards or are you going to send out for them to be professionally etched? I use the toner transfer method and I have found that I need to leave a reasonable amount of space between traces to keep the toner from bleeding across and creating shorts.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It has been a few years since I did this so the policy may have changed, but I got gerbers from ExpressPCB. The terms were I was limited to designs I had already purchased from them, and they charged me about what another set of boards would have cost. At the time I did this the company I worked for had no in-house PCB design capability, and free tools were just not worth it.

    It was a win-win as we were going into volume production (where ExpressPCB cost too much) and they prefer small but $$$ orders.
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I have heard that this option was available, but didn't try it and didn't know what they would charge. For some situations, it's a viable option. Still, for about "what another set of boards would have cost," I bought the starter version of DipTrace and can now get Gerbers for free on every PCB I design. (Yes, I am cheap. :))
     
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