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How important NSMD pads are for the QFN and similar packages?

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#72481

How important NSMD pads are for the QFN and similar packages? | 30 July, 2014

Hello gurus,

I am a hardware designer with a small company. We are about to send a new PCB for fabrication. This board uses dozens of tiny QFN, DFN, and similar packages. As a final check I have been going through the datasheets of all these parts to verify the land patterns and found out that many IC manufacturers recommend using NSMD pads. Unfortunately, this is not what we have now in the design. Our solder mask opening is equal to the copper pad size. Some of these copper pads are only 0.2 mm wide and the pitch of some of the devices is only 0.4 mm. This makes me nervous.

Could you please confirm or deny the importance of using NSMD pads for QFN packages? How does it affect quality of the assembly?

Also, the pads in the datasheets are frequently drawn with the short side looking inwards rounded. Does this shape affect assembly in any way? Our pads are mostly rectangular.

Thanks.

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#72484

How important NSMD pads are for the QFN and similar packages? | 30 July, 2014

About the pads shape - you can do rectangular or oblong. For stencil design oblong is better for small pads(0.2mm) because it will give better release, so having this thought in mind you should probably design oblong shape and then print make paste apertures 1:1 or even overprint a little bit.

About the NSMD pads - you don't have to do that all the way across all the leads(having no mask between the leads might be causing shorts), but I would consider to have mask in between the leads and have it open a little bit more than the copper(especially for the part with (0.2mm wide). My last suggestion could also be considered NSMD design I guess.

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#72485

How important NSMD pads are for the QFN and similar packages? | 30 July, 2014

example

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#72486

How important NSMD pads are for the QFN and similar packages? | 30 July, 2014

> I would consider to have mask in between the leads and have it open a little bit more than the copper

Thank you. That is exactly what the NSMD pad is!

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#72487

How important NSMD pads are for the QFN and similar packages? | 30 July, 2014

Yes, but I have seen designs where the whole row of leads has no mask(which is also a NSMD)

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#72489

How important NSMD pads are for the QFN and similar packages? | 30 July, 2014

> Yes, but I have seen designs where the whole row > of leads has no mask(which is also a NSMD)

Apparently I might have to do just that for some parts. We have a few parts with 0.2 mm pads at 0.35 mm pitch. Our PCB shop says that they need at least 7 mil between the pads to guarantee the soldermask web. Since the 0.15 mm distance we have is less than 7 mil, they are saying that we either have to reduce the pad size to 0.1722 mm max or remove the web. I don't know what is a lesser evil...

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#72492

How important NSMD pads are for the QFN and similar packages? | 31 July, 2014

I know that some board houses have better capability than others. Some can do down to 4mils mask in between the leads I think. I know that it also depends on the type of the mask they are using - for some types you can't go down to the 4mil. Regards, Emil

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mm

#72493

How important NSMD pads are for the QFN and similar packages? | 31 July, 2014

> I know that some board houses have better > capability than others. Some can do down to 4mils > mask in between the leads I think. I know that it > also depends on the type of the mask they are > using - for some types you can't go down to the > 4mil. Regards, Emil

7 mil is required between the pads to guarantee the soldermask web of 3 mil. They can do better with a laser imaged mask but it is a more expensive process.

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DM

#72634

How important NSMD pads are for the QFN and similar packages? | 27 August, 2014

If at all possible, you definately want solder mask defined pads. Without it, you will struggle with solder bridging. This is especially important if you have a larger size board.

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