FreeLists / si-list / [SI-LIST] Re: Decoupling capacitors for BGA

[Joel Brown <joel@xxxxxxxxxx>]

Lee,

We are dealing with this same issue of bypass caps for BGAs.
If we place the caps outside the shape of the BGA as you suggest, how much
inductance is added by the planes over a distance of say 0.5" if the planes
are spaced close together, say 5 mils.

Thanks - Joel


Lee Ritchey wrote: If you connect capacitors to pins with traces, the added
inductance will degrade theri performance enough that they will likely be of
little use. Better to place the capacitors outside the shape of the BGA and
connect them into the planes directly with vias. No, they don't need to
directly under the BGA as long as the planes are continuous. [Original
Message] From: Dagmara Avanindra <dagmara.avanindra@xxxxxxxxx>[1] To:
<subramanian1683@xxxxxxxxx>[2] Cc: <si-list@xxxxxxxxxxxxx>[3] Date:
5/12/20068:46:23 PM Subject: [SI-LIST] Re: Decoupling capacitors for BGA
Subbu, if your board is not thick, you may want to consider using 0201 caps
directly underneeth the BGA aray, especially if GND and VDD are located
directly diagonally across from each other. If you connect caps via
individual traces to the pins, you will get alot of resistivity. Which can
bea plus if your goal is to dampen high-frequency noise. However, if you
maketraces very thin and long, you may slow your response time too. So
calculate your resisitivity per square to figure out what is the optimal cap
connection for you. Dagmara On 5/12/06, Subramanian R
<subramanian1683@xxxxxxxxx>[4] wrote: Hello All, We are using a 0.5mm Pitch
457-Pin BGA Processor. It is running at a maximum of 532MHz. It has 60 Power
pins (Core &I/O) How do i place the Decoupling capacitors? Now we have
placedall decaps (0402 pack) around the BGA (like an island) and connected
itto the VCC plane. Should i need to route it to the individual pins itself?
The BGA does not have much space between the Balls. So not more than 4-5
capscan be placed right over the BGA on the other side. The other thing i
cango for is Inter-plane capacitance. But for that VCC-GND dielectric should
be around 2-3mils, i understand. That doesn't look possible with our PCB
thickness requirements. It is a 6-layer PCB Thanks for any suggestions.
Regards Subbu
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