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Bending Radius of Lead when designing component footprint

Discussion in 'Datasheets, Manuals and Component Identification' started by argh, Oct 16, 2014.

  1. argh

    argh

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    Oct 9, 2014
    Hi people

    I'm currently doing a project which involves designing footprints of components needed for a PCB. A lot of capacitors, resistors and diodes are involved. An example of one component I'm making to footprint of is this:

    http://docs-europe.electrocomponents.com/webdocs/0d9a/0900766b80d9a182.pdf (please skip to page 3).

    My component is PR01. Should I put down the distance between the drill holes as L2, or should I add more space between them, and how would I know how much space to use? The issue was raised by someone that if I put the holes right at the end of the body, I'm going to damage the component comes soldering.

    Here is the link for the resistor that I'm buying. Clicky.

    Thanks
     
  2. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    When you go to install the resistor, you will bend the leads perpendicular to the body of the resistor with a pair of long-nose or needle nose or round-nose pliers. In doing so, you will place a small radius in the bend that will extend the separation between the two leads just a little. It will definitely be longer than L2, but the actual separation is up to you. If you start the bend too far away from the resistor body you get excess separation between the two leads and that is usually undesirable. I say "usually" because sometimes you do need a bit more separation, but that depends on how the board is laid out. There may be multiple traces that need to cross under the lead and the resistor body, but if the total width of those traces is large, then a longer "overpass" of the component lead is necessary. You may avoid such situations with careful layout design, but sometimes there is just no other way without using multiple circuit board layers, which is expensive for hobby or one-off designs.

    The one thing you don't want to do is try to bend the lead right up against the end of the resistor body. That leaves NO room for a radius and could eventually cause the lead connection to the resistor body to fail from heating/cooling stress cycles. It also means there is no room for error in specifying the distance between the two resistor leads. I generally try to have a bend that results in about 0.1" additional space on each end. So, for your PR01, set the mounting holes as L2 + 0.2". A slightly longer distance is okay, but don't get too carried away. If you are clever and base your layout on a grid (typically 0.05" or 0.10"), choose a distance, d, that makes L2 + d equal to an integer multiple of the grid spacing.
     
  3. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    From what I remember from the IPC standard, it should not shorter than 2 x the lead diameter. But I can check this out if you want, we have them at work.
    Adam
     
  4. argh

    argh

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    0
    Oct 9, 2014
    Thank you hevans1944, I will keep that in mind for my designs.

    Arouse1973, I am actually in the process of designing a product. I haven't heard of this 'IPC' standards. Could you give me a brief overview of these standards, and yes, it would be appreciated if you could find out and tell me what the standard is for bending points.
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Since you apparently do have Internet access, and are designing a product, I suggest you Google "IPC standards" for the information you need. Most of us here like to help beginners, and I am definitely in that group because I still remember what it was like back in the day, but we can help better if you have done your homework.
     
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    IPC-A-610 Rev D 2000
    Section 7.1.2 Component mounting and lead forming. States:
    Leads for through hole mounting must extend at least one lead diameter or thickness but no less than 0.8mm (0.031in) from the body, solder bead or lead weld.
    Thanks
    Adam
     
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