Pad to Pad vs Pad to Thermal Tab

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   Hi to all,
Could anyone explain me why the minimum distance between pads should be different to the distance between pad and thermal tab?

I cannot understand it both are copper and from the point of view of the pcb construction should be the same, isn't it?

Thanks for your comments.
Regards.
dramos

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dramos Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 18 Feb 2021 Status: Offline Points: 61	Post Options Post Reply Quote dramos Report Post Thanks(0) Quote Reply Topic: Pad to Pad vs Pad to Thermal Tab Posted: 02 Mar 2022 at 7:49am
	Hi to all, Could anyone explain me why the minimum distance between pads should be different to the distance between pad and thermal tab? I cannot understand it both are copper and from the point of view of the pcb construction should be the same, isn't it? Thanks for your comments. Regards. dramos



Tom H Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 05 Jan 2012 Location: San Diego, CA Status: Offline Points: 5718	Post Options Post Reply Quote Tom H Report Post Thanks(0) Quote Reply Posted: 02 Mar 2022 at 9:51am
	You can change the Options to whatever values you want. The pin to pin 0.15 minimum distance would allow a 0.0375 (1.5 mil) solder mask swell and have a 0.075 mm solder mask web in-between every SMD pad. Ask your fabrication shop if they can handle a 1.5 mil solder mask swell. Or ask your fabrication shop if they can handle a 0.05 mm solder mask web. Then your solder mask swell can be 0.05 and the solder mask sliver can be 0.05 and you will have solder mask in-between every SMD pad. The Pin to Thermal clearance was established by the component manufacturer's recommended patterns. All mfr. recommended patterns have a minimum pad to thermal space of 0.20 mm.
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dramos Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 18 Feb 2021 Status: Offline Points: 61	Post Options Post Reply Quote dramos Report Post Thanks(0) Quote Reply Posted: 03 Mar 2022 at 12:27am
	Dear Tom, Many thanks for the explanation. What I understand from it, it is that we always should try to place mask between Thermal pad and pin pad. The recommendation of the manufacturer is to reduce the thermal pad https://www.nxp.com/docs/en/package-information/SOT650-1.pdf The component has a thermal pad of 1.6mm +/-0.15mm and the thermal pad on the component is only 1.5mm and due to this the distance of pd to thermal pad is bigger than 0.2mm. I think that I've seen a reduction of the solder mask swell, placing mask over the pad but I think that it would be something special and the manufacturer will explain it on each datasheet. It is really grateful to learn from you Best regards, dramos