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I was wondering why I cannot edit footprint of LED 1206 to take care of LED 0805 as well so that I can mount one of them depending on the availability. To do this, I have to increase the pad size of 1206 footprint on the inner side to give a clearance of around 1.2 mm between two pads. I want to know the impact of increased pad length in the PCB.

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  • \$\begingroup\$ impact on what? \$\endgroup\$ – Ale..chenski Jan 11 '18 at 7:48
  • \$\begingroup\$ Impact on overall performance if any such as capacitance. I am new to PCB design and not sure of other side effects. \$\endgroup\$ – Rajesh Jan 11 '18 at 8:04
  • \$\begingroup\$ This is called double layout. Do it and don't worry \$\endgroup\$ – Gregory Kornblum Jan 11 '18 at 8:14
  • \$\begingroup\$ Sometimes it may be a little messy in assembly, but again, don't worry \$\endgroup\$ – Gregory Kornblum Jan 11 '18 at 8:15
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    \$\begingroup\$ Will you solder by hand or in a fab? \$\endgroup\$ – pipe Jan 11 '18 at 8:23
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If you increase the pad, you will probably increase the amount of paste too. Too much paste will increase the possibility of the component tombstoning as the solder melts.

enter image description here

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    \$\begingroup\$ Tombstoning does not seem very likely to me for 0805 diodes... \$\endgroup\$ – Sclrx Jan 11 '18 at 9:03
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As long as you follow the board design rules that will not cause any problems. You will also slightly enhance the heat dissipation for the 0805 part. You need to watch out for misalignment of the 0805 part. During smd assembly the parts self align using the solder surface tension the larger pads will allow the smaller part to move around more.

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You don't have to worry about not been able to mount it. If you use 1206 footprint you will be able to mount 0805 component. It will be in it's internal size limit but you will be able to mount it without problems.

SMD_SIZES

In the link you can see different sizes.

Also maybe you can't edit the component as it's protected against writing. Try to create new copy.

My recommendation however is not to do this as this increses the posibility of creating a connection using only the paste(short circuit) if you are going to solder by hand.

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    \$\begingroup\$ If you compare soldering pads, you may not have any issues. But if you refer package dimension of the component, you will see that maximum width of 0805 LED component is 2 mm. \$\endgroup\$ – Rajesh Jan 11 '18 at 9:11
  • \$\begingroup\$ What I mean is that if you have a 1206 footprint but your component is 0805 you will be able to solder it, although it won't be a pretty and clean. archive.fabacademy.org/2017/fablabnervecentre/students/401/… \$\endgroup\$ – Dimitri Jan 11 '18 at 10:49

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