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I recently downloaded a component's footprint and it comes with a peculiar solder paste pattern (see picture attached - solder paste is in gray). Until now I have just defined the solder paste to be the same size as the pad, but know I am wondering if this kind of pattern has a purpose, maybe for better thermal relief?

Should I use a full pad solder paste pattern, or there is any benefit in using something like this one?

Also, I have seen that some people put vias in the pad for thermal relief, but what kind of vias? Through hole vias to a bottom ground plane... blind vias to a inner ground plane? Is it really necessary?

enter image description here

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    \$\begingroup\$ You will need to present a better picture or at least isolate just the paste mask layer. As it stands now it is nearly indecipherable except to you who stares at your own CAD package. \$\endgroup\$ Commented May 29, 2018 at 9:59
  • \$\begingroup\$ @MichaelKaras sorry, I thought it was clear but you are right that it may be confusing for people used to other CAD packages, the solder paste is the gray color \$\endgroup\$
    – jap jap
    Commented May 29, 2018 at 10:02
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    \$\begingroup\$ "solder paste is in gray color", this is similar to drawing a snowman in a snowstorm - Right now I see Red, pink, some dashed pink area and a lot of gray area. I think that the gray color you are talking about is in the eye-attacking-red area and the pads. Your presentation makes no sense. If you look between the rows in this comment you will find a snowman in a snowstorm. \$\endgroup\$ Commented May 29, 2018 at 10:10
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    \$\begingroup\$ @HarrySvensson man, if you can't see in that image what is solder paste there is nothing I can do for making it clearer for you. Never mind, I appreciate your useful comment ; ) \$\endgroup\$
    – jap jap
    Commented May 29, 2018 at 11:23
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    \$\begingroup\$ I can’t understand why this question deserves downvotes. It makes sense to me, just looks like what you get after using a stencil. Downvotes should be reserved for bad questions, not questions that didn’t get formulated perfectly. \$\endgroup\$ Commented May 29, 2018 at 11:30

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The grey solder paste (easy to see as I am a PCB CAD Designer CID+ and not some smart butt engineer who doesn't know his a from his elbow) is normally broken like this to allow the rosin flux gasses to flow out inside of pushing the centre of the chip off the board and causing it to float to the point where it shorts out pads.

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The core reason for having the center pad separated is that large pads like that may float the ic off the outside pin pads or solder bridge to the pins due to the extra solder paste. Plus, stencil wiping on large pad aperatures tend to fill unevenly. Also, the stencil structural integrity is better with little holes.

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    \$\begingroup\$ Also, if you paste a single area you sometimes end up with a connection to only part of the thermal contact area, this matters mainly with high power devices like RF power LDMOS and such, but you do see it as a consideration in some other QFN parts (Mainly power conversion parts). Placing several paste areas and even using solder mask to create dams helps with this. \$\endgroup\$
    – Dan Mills
    Commented May 29, 2018 at 16:22
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There should be around 50-70% coverage of solder paste relative to the pad, there's a good application note here. Full coverage may lead to bridges as solder is pushed outside the pad boundary as there is too much solder on the pad. If in doubt, just pull the edges in a little from the side, rather than trying to make a complex pattern like the one you've downloaded. There may be a reason for it, but it will come from a lot of experimentation.

Thermal vias should be done according to the datasheet. Typically they are top to bottom to access an exposed area for heat to dissipate from, rather than being trapped by the package. It can cause issues by pulling solder through the vias or by raising bumps stopping the package from lying flat.

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