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In the Luxeon LED datasheet under Solder Pad Design section there is a diagram with 3 crosses around the edges of the LED. They are identified as "4x Soldermask Opening (hatched area)" but I am not entirely sure what it is exactly or why it is used. What are they, what do they achieve and how should they be drawn in Eagle or other software?

enter image description here

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3 Answers 3

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  1. It is for easy alignment with respect to the outline of the package.
  2. The Luxeon outline shows perfect alignment and easier for optical correction with a vision system on the pick N place machine.

enter image description here

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  • \$\begingroup\$ Good find Tony! \$\endgroup\$
    – DerStrom8
    Aug 29, 2016 at 19:31
  • \$\begingroup\$ I'd probably talk to the fab house whether they prefer another style of fiducial -- 3mm round soldermask cutout with an 1mm round copper dot in the middle seems to be fairly universal. \$\endgroup\$
    – Simon Richter
    Aug 29, 2016 at 20:02
  • \$\begingroup\$ @SimonRichter In my case it's just a once off, hand soldered and more for learning than anything else. \$\endgroup\$
    – DominicM
    Aug 29, 2016 at 21:10
  • \$\begingroup\$ So how would I draw it in a package? From the image it looks like the soldermask cross is surrounded by copper but what is the blue/violet outline and white space? It's a bit odd since the legend shows the violet line and blueish space as copper but then you have red soldermask in between.. \$\endgroup\$
    – DominicM
    Aug 29, 2016 at 21:12
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Soldermask openings are CAD layers usually meant to specify to the PCB fabrication house that they shouldn't put any other covering on the bare copper so that when the soldermask layer is applied, the solderpaste can be placed on the direct copper.

In the case of the cross mark you mention however, the opening is probably specified to provide an alignment mark for the pick-and-place machine that will use the cross to figure out when the part is aligned properly before placing it down. The machine probably need the bare copper to get a good image of the alignment mark

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  • \$\begingroup\$ Is it the "Mask" layer in Eagle? Looking at a fiducial footprint from adafruit library I only see Top, tStop and tKeepout layers. \$\endgroup\$
    – DominicM
    Aug 29, 2016 at 21:30
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    \$\begingroup\$ @DominicM tStop layer. \$\endgroup\$
    – Tom Carpenter
    Aug 29, 2016 at 21:35
  • \$\begingroup\$ @DominicM I'm not an Eagle expert but I would think that the "Mask" layer is the soldermask. My guess is that the adafruit footprint layer called tKeepout or tStop would negate any soldermask in the same area. I have seen this type of practice before where layers are XORed together. \$\endgroup\$
    – Daniel V
    Aug 29, 2016 at 21:42
  • \$\begingroup\$ @DominicM yes it's tStop (or bStop on the bottom). The name is "Stop" instead of "Mask", because stuff drawn on this layer indicates where there should not be solder mask (i.e. where it stops). \$\endgroup\$
    – Doktor J
    Aug 30, 2016 at 4:16
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Soldermask is used to cover the copper to prevent solder from adhering to it (hence its name, "soldermask"). Soldermask is the green stuff you see on most PCBs, though it can be just about any color (red, purple, blue, white, black, etc). By creating a soldermask cutout you will allow copper to show through the mask. This is often used for fiducials, which are used as optical markers for the position of components and used by pick-and-place machines to properly align components. I expect that's what the crosses are used for in the component layout you posted. They are either fiducials or, more likely, indicators of polarity. Having a non-symmetrical mark to indicate where the cathode and anode are can be useful for the assembler as well as anyone debugging the board, reworking it, testing it, etc. I can't quite tell, but there is also the possibility that they are being used as actual solder pads. I see the LED has matching marks, but I can't tell if those are metal tabs or just marks on the case. I'm pretty sure the crosses in the PCB layout are just there to make sure the assembler aligns the LED correctly on the board.

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