exposed die attach pad

“exposed die attach pad”的中文意思是“裸露的芯片连接焊盘”，一般在芯片的底端。大家常常思考的一个问题是，这个焊盘要不要接地？真实的情况是：接不接地要视情况而定。比方说对于CC2420在datasheet上的管脚描述上是”Must be connected to solid ground plane“，那么这个管脚就必须接地，否则不会正常工作（前一版就是因为这个焊盘没接地，结果基准电压不工作，晶振两端没偏压）。但对于nRF系列的射频芯片有的就真的不用接地，我的一个同学就用过一款，是不用接地的，但照样会正常工作。后来，我还专门找了下相关资料验证了下：

Should we connect the die attach pad on the QFN package?

The exposed die attach pad is a pad connected to the IC substrate die ground and is intentionally not used in our layouts. It should be unconnected to avoid crosstalk from the GND pins carrying a multitude of frequencies back into the chip substrate.

In our layouts we have not covered the GND vias with solder mask, but feel free to do this in your design. All other signal lines (e.g. VDD) must be covered.

Note: This information does not apply to the nRF24LE1 QFN32, nRF24LE1 QFN48 and nRF24AP2 QFN32 package. (On these three packages there is internal down bonding to the die pad and hence the die attach pad must be connected.)

从上面的描述很明显就验证了，“exposed die attach pad”要不要接地，要视情况而定这一说法。

参考资料：

http://www.nordicsemi.com/kor/layout/set/print/Nordic-FAQ/All/Should-we-connect-the-die-attach-pad-on-the-QFN-package