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The question is related to the datasheet for LED part number MHT151WDT. It is manufactured by Lianyungang Meihua Electronics Technology Co. Ltd. The datasheet can be found here.

The pg. 2 in this datasheet gives this drawing as guide to create the PCB footprint:

enter image description here

There are two issues:

  1. What is really point of having a pad size containing 4 decimal places? This is mind boggling.
  2. Why is there a circle drawn in the drawing where the pad copper must be exlucded?

This leads to one question:

  1. How does one design a PCB footprint for this package in Altium designer? It is not clear how to cut out copper as per the circle diagram shown.

Note (on page 2):

All dimensions are in millimeters. (inches)

Tolerance is ±0.10mm(0.004") unless otherwise specified.

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    \$\begingroup\$ My guess is they just calculated the pad length from the carve-out and then didn't round whatever the result was, so you end up with a ludicrous number of decimal places. I'll just mention that the pad-terminal overlap in that dimension only needs to be 75% to satisfy IPC-A-610 inspection criteria for class 3 and 50% for class 1 and 2. You could also request a footprint from e.g. SnapEDA and make it their problem but results may vary. \$\endgroup\$
    – vir
    Apr 27, 2023 at 20:09
  • \$\begingroup\$ The SnapEDA does not appear to have the 2D model and 3D model for this part. \$\endgroup\$
    – quantum231
    Apr 27, 2023 at 20:23
  • \$\begingroup\$ What exactly is IPC-A-610? \$\endgroup\$
    – quantum231
    Apr 27, 2023 at 20:23
  • \$\begingroup\$ You can request the big online libraries to make a footprint for you from a part number, provided that the datasheet has dimensions for it. IPC-A-610 is an international standard for acceptability criteria for electronic assemblies. \$\endgroup\$
    – vir
    Apr 27, 2023 at 20:33
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    \$\begingroup\$ According to my calculations, the rectangle width is exactly 5/30 of an inch. Then 25.4 * (5 / 30) / 2 - sqrt(1.15^2 - (1.25/2)^2) = 1.1513 mm. \$\endgroup\$
    – rodrigo
    Apr 28, 2023 at 11:41

2 Answers 2

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This is called a reverse entry type. The LED is designed to project through the PCB to the bottom side to allow viewing from the bottom. I would use a rectangular pad with a small clearance from the hole. The connections remain on the top side.

The part is still technically on the top side to keep assembly on one side only.

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  • \$\begingroup\$ Are there other components that are of "reverse entry type"? I have never heard of this before. \$\endgroup\$
    – quantum231
    Apr 28, 2023 at 1:24
  • \$\begingroup\$ @quantum231: I haven't seen any, perhaps some connectors. It is a way to avoid having to put parts on two sides. It is more expensive to assemble. It is fairly common for LED indicators. \$\endgroup\$
    – RussellH
    Apr 28, 2023 at 1:28
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    \$\begingroup\$ @quantum231 Photodiodes can come as reverse. I think some buttons do too. \$\endgroup\$
    – DKNguyen
    Apr 28, 2023 at 1:35
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    \$\begingroup\$ @quantum231 I've seen some reverse entry pin headers, and a handful of sensors. It's not common, but it's not rare either. \$\endgroup\$
    – Hearth
    Apr 28, 2023 at 1:36
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    \$\begingroup\$ Some microphones do this. Good ones too, you see them in smartphones! \$\endgroup\$ Apr 28, 2023 at 15:01
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Create the outline with lines and arcs, convert outline to region, and place a pad entirely within the region.

They likely don’t want the copper right up against the hole because you could get a rogue sliver of copper that could cause shorts elsewhere on the PCB. Especially with an unplated hole.

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  • \$\begingroup\$ I will give it a try and let you know. \$\endgroup\$
    – quantum231
    Apr 27, 2023 at 22:53

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