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CC2650 QFN package issues on thermal vias

Other Parts Discussed in Thread: CC2650, TPS62740
Hi,
I have two general questions on this topic:
1) How much flexibility do we have on thermal vias placement?
CC2650 datasheet recommended using a 3*3 array of thermal vias (8mils diamter and 40 mils spacing)
However, in the reference design (EM-7ID, 5XD, 4XS, and 4XS-Ext_Reg), it seems packages of all sizes use a 4*4 array of thermal vias (20mils diameter and 36 mils spacing).
For the DC-DC converter TPS62740 in the design EM-4XS-Ext_reg, thermal vias is not even included, while in the datasheet, a line of 3 vias (12mils diameter and 28 mils spacing) is recommended.
I checked the application notes of both slua271a and sloa122, and they both agree that vias with 15 mils diameter and 40 mils spacing is a good starting point for QFN package, and vias size could be reduced accordingly.
I guess the reason that thermal vias placement is not strictly followed is because both CC2650 and TPS62740 are dedicated for low-power application so thermal vias or not won't hurt the thermal dissipation much?
2) Is it preferred to tent the vias or not?
I think so. But it seems we get contradicting opinions from both application notes: slua271a says yes and sloa122 says no.
Looking forward to your replies.
Thanks a lot!! Much appreciated!
  • Hi Xiaoxiao,

    You have quite a bit of flexibility on number and placement of ground vias. For the CC26xx the vias are important for a low impedance RF ground return path (as you say, the CC26xx is a low power device and does not generate any significant amount of heat). The more vias, the better, I would recommend 9 as minimum. We typically use 0.2 mm drill holes to minimize the leakage of solder paste. With this dimension we do not have any problems with soldering even without tenting. We also open up the solder mask on the bottom side to avoid air traps in the holes. I would recommend that you ask your manufacturer on what they prefer.

    The TPS62740 should probably have the 3 vias as recommended in the datasheet, and it is a mistake on our part that we did not add it for the CC26xx design. It is probably not critical as the device is connected to a big ground plane on the top layer with multiple ground vias close to the part.

    Cheers,
    Fredrik
  • Thank you very much!!!