Eagle cad library parts generation question

Discussion in 'General Electronics Chat' started by osx-addict, Jul 17, 2012.

  1. osx-addict

    Thread Starter Member

    Feb 9, 2012
    122
    9
    Hi all.. So, I've got a few parts I've added to my own library that I will be using but which were missing from the standard libs.. The latest part I'm trying to add is a Vishay VO14642AABTR which is a surface mount SMD-6 part and the first real one I've not been able to copy from elsewhere. So, all of the tutorials suggest adding the requisite number of SMD pads a particular part needs and then using a calculator & the eagle inspector to place each pad where it needs to be respective to the other pads around it.

    So, I used the inspector (the "i" button) to set the pad sizes per the data sheet to 1.52mm x 1.78mm. No problem with that. Each pad center has a spacing of 2.54mm (.1").. The exact distance between pads is .76mm edge to edge and the edge-to-edge gap from one side of the part to the other is 8mm min.

    So, I get out the calculator and once I've got one pad at say +2.54 (this would be the right column on the part) on the x-axis then I need to go -10.54 to the left to find the location of the pad on that side of the part (-2.54+-8).. I think that's about right.. Unfortunately Eagle make it hard to verify without changing the grid size.. and I keep forgetting that it likes to measure from the center of the pad with regards to the X/Y position..

    It'd be cool to have a calculator that you could punch in the dimensions for each pad and how they're related spacing wise and have it calculate the exact locations within the Eagle grid system so you only have to put them in..

    Is this more or less the way that you all do this (by hand)? Any tricks you wish to share? I suspect I'm probably over thinking this..
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
    2,341
    1,022
    I refer to the package dwg and use or determine part's center, put it on a grid point then radiate dimensions off of that. And I use the ol' Little Professor calculator to figure out pad locations etc. from that point. If all of the pins/pads fit on a grid, I'll set the grid to that for ease of placing. If not, I just bang in the dims and let it go at that.

    You've probably noticed that 0,0 in the pattern editor is the reference point when placing the part on the PCB. Put that in a convenient place e.g. pin 1 for through-hole DIPs, center for SMTs.
     
  3. osx-addict

    Thread Starter Member

    Feb 9, 2012
    122
    9
    Thanks John.. So, for SMT's you use 0,0 as the center of the SMT part or for one of the pins? I was assuming you want that cross-hair in the center of the part and will assume that is what you meant..
     
  4. JohnInTX

    Moderator

    Jun 26, 2012
    2,341
    1,022
    Pretty much. Note that you can draw the layout pattern anywhere on the screen relative to 0,0. When you are happy with it, you can group-move the whole thing, locating 0,0 where you want on the component. If you do it that way, be SURE to move ALL layers. Or, better, just start at 0,0 and radiate away from that.

    I use component center for SMTs since that keeps the components themselves on grid and easier to manually place on the layout. The pads aren't drilled so no worries about grid there. Seems to make the pick and place programmers happy as well.

    For through-hole, I try to keep the holes on grid since that makes placing the component and routing traces around holes easier to keep on grid. The component's origin has to be somewhere; pin 1 seems logical.

    For non-grid through hole stuff, I org the part more or less in the dimensional center so when I rotate it, it tends to stay in one place on the screen.

    Mechanical parts that interface with other mechanical parts (board chassis connectors etc.) get org'd at whatever the reference point on the mechanical drawing is to make it easier to ensure that everything lines up per the chassis dwgs, etc.

    These are general rules. Where I eventually set the org depends on what else has to refer to the part's location on the board.

    There was a time where component orientation/gridding etc. had a big impact on total board cost when it was fabbed and stuffed. Its probably less of an issue now with modern fab/assembly equipment (at least on the modest boards I do) but old habits die hard.
     
    Last edited: Jul 17, 2012
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