bga decoupling
ifarmer said:
Tantalum can only work around or below 1MHz. For high frequency, you'd better make power and ground plane as closer as possible. And place at least one ceramic caps under the chip, NP0 dielectrics is recommended.
In my opinion, the first cap is important.
Some observations, no pun intended...
It is normaly necessary to ensure proper decoupling to ground of
every BGA supply pin. At the high frequencies encountered now,
this means that the lowest valued cap (= the highest self- resonant freq cap)
should be as close to the BGA pin as possible which is usually under the
pin on the other side of the board.
The choice of NP0 is discutable. Although NPO is surely the most temp
stable and the less influenced by the voltage across it, its volumic
capacity is small and the capacitor would certainly need a larger
package than a X7R one. Hence, the placement of multiple NP0 caps
would become problematic in dense BGA's and the parasitic inductance
of the longer routing leads would rise because of this burden.
While Y5U or Y5V are certainly not to be considered for stability
reasons, X7R is usually the best choice for close pin decoupling.
Larger values of X7R caps are also easily found for the more distant
staggered cap values (100nF or even 4.7uF).