At 11:09 AM 2/5/2002 +1100, you wrote:
>Yes, but the power connections are made through the Vias are they not?
>There are over 100 ground and 3.3V/2.5V balls that connect to internal
>planes through Vias. At least for this particular BGA. Therefore, wouldn't
>the thermal effects still be relevant in this case?

         Actually, a related question I had that seems to tie in to this one...

         I have been told by some engineers that they often like to put a 
'via per pad' when working with BGAs, especially when doing prototypes. 
Part of the reason for this being you have all of the connection-points on 
the BGA part available as testpoints and/or solderpoints on the opposite 
side of the board for those lovely last-minute fixes on prototypes. (Also, 
several companies make direct BGA<->PGA snap-in adapters for prototyping 
that keep the EXACT BGA footprint extended into PGA, making it possible 
(from what I have been told by a sales rep) to open up your vias and simply 
drop their adapter in as a fine-pitch PGA thru-hole device. When it comes 
time to do a production run, you simply shrink the diameter of the vias and 
attach your BGA devices normally.)

         I can obviously see that putting vias into the pads directly could 
potentially cause 'robbing' to occur if too large of a diameter was used, 
as well as the 'thermal stressing' due to cooling from power planes. (Colin 
mentioned 'plugging' vias under the BGA - is there a way to do this without 
using blind/buried? I'm unfamiliar with the term...) I have also been told 
by an assembly technician that a lot of times they prefer all BGA pads to 
have a via in them for thermal conduction from the other side, as it can 
potentially help the reflow process.

         My question (for the BGA masters out there!) that may help to 
partially answer the question about thermal relief for the vias... Do you 
typically place your vias directly IN the pads, or do you create an array 
of vias offset from the actual landing pads? (I've seen the 'offset via' 
approach documented by several companies in various appnotes) In the direct 
via-in-pad approach I can see the need for the thermal-separation as was 
mentioned... But in the offset-grid approach, I suppose it wouldn't matter, 
since not much heat will theoretically get conducted across the trace from 
BGA pad to via to make an appreciable difference.

         Any comments about this from those who have worked with BGA? 
Trying to get all of the facts, since some upcoming designs we are planning 
on working on need to be based on parts only available in BGA. :/

         (P.S. THANK YOU again to all of the folks that replied to my 
question last week concerning autorouting vs manual routing! You all are an 
absolute goldmine of technical knowledge. Thanks for sharing!)

-- Matt

>At 06:10 PM 4/02/2002 -0500, you wrote:
>>At 08:34 AM 2/5/2002 +1100, Colin Weber wrote:
>>>I was talking with another designer, Phil Dutton, and he aslo 
>>>recommended the Power Connections Relieved.
>>>"Make sure that your Power connections are relieved, or they will cool 
>>>down faster than the rest of the device, inducing stresses."
>>That is "Power Connections." We've been talking about vias. Different animal.
>>Abdulrahman Lomax
>>Easthampton, Massachusetts USA
>Colin Weber

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