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28 pin QFN Pads.

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PCB

#62693

28 pin QFN Pads. | 14 September, 2010

Hi All,

I have designed 4 Layer PCB with one 28 pin QFN PKG.

I have got complaint from Production that the pads which connected to thermal pads (gnd) thru trace is bridging with solder and because of this bridge, pins are not getting soldered properly, they ask me to show green mask between those pins and thermal pads which is exactly what my gerber file represent. but in production it looks like attached pic.

i can reduce the pad size of thermal or those pins connected to thermal but it will not be as per IPC LP Calculator, Any Suggestions ? one more note i am specifing solderpaste/soldermask same size as pad size for thermal is that a problem ??

thanks for your help.

regards,

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#62701

28 pin QFN Pads. | 15 September, 2010

We had a similar problem so we shrunk the thermal and extended the pads. This allowed us to ensure solder resist was applied to the track in between.

It fixed our problem.

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PCB

#62703

28 pin QFN Pads. | 15 September, 2010

Thanks for the valueable feedback, I have two questions

1. why do you have to extend the pad size ? is it okay if i keep pad size

2. I am currently using thermal pad size recommended in datasheet max value 4.2mm x 4.2mm, how much should i shrink thermal pad without causing any other issues ?

again Many thank for your feedback

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#62711

28 pin QFN Pads. | 16 September, 2010

I extended the pads to help ensure the component locates correctly and to allow for easier re-work should problems occur. I reduced the thermal by 5% but you may want to speak to your fabricator and ask what they require to ensure that the resist is correct.

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PCB

#62718

28 pin QFN Pads. | 16 September, 2010

Ok Thanks, i have reduced thermal pad size by 4.5 mils on top and bottom part of this footprint where the problem was occuring, so now the clearance between thermal pad and lead pads are 12 mils instead of 7.5 mils on two sides. (i kept the signal pad size same as it was just reduced exposed area by 4.5 mils) Let's see if our production guys feedback, i will post soon.

thanks regards

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