[SI-LIST] Re: BGA vias outside the package

"For 180 MHz, the part should work."
This is not engineering.  This is speculation, at best.  The only way to 
know is to perform the engineering.  Just because "there does not seem 
to be any other option" does not make it good engineering.  That's sales 
and marketing talking.  The carcass of many dead systems and companies 
have been buried by those words.

There are no easy answers here.  I ask my engineers, would you bet your 
job on your design?  In my opinion, engineering is complete when you're 
willing to do this and defend your decisions.  Otherwise, keep doing the 
analysis, until you reach this point.  If management wants you to take a 
different approach, and ignore the necessary engineering steps, give 
them your best engineering judgment, disagree with their opinion, and 
make it crystal clear that this is a management not an engineering decision.

When I was in school, Civil engineers were generally at the bottom of 
the class ladder. However, can you imagine a Civil Engineer giving 
similar advise on the design of a structure? I'm sorry, "there does not 
seem to be any other option" does not fly in a court of law when a 
bridge or a building fails catestrophically.

regards,

Scott



Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



V S wrote:
> Under your circumstances, there does not seem to be any other option. For 180 
> MHz, the part should work. Few things that come to my mind
>
> 1. The larger via will not help much. It does provide some capacitance but it 
> will not help the inductance of the trace connecting the via to the BGA.
>
> 2. You may try to keep the power and ground traces close by ( either in top 
> layer or in the form of adjacent gnd layer or by routing them close by in top 
> layer). This should not slightly help in very high frequency noise.
>
> 3. Identify power pins not internally short. Route them slightly away.
>
> 4. I did some design using via in pad. The via went from top layer to layer2 
> ( ground) and top layer to layer3 (power). In my case I was able to get a 8 
> layer board for about $25 each for 100 boards. So I will say that you may 
> still like to explore it.
>
> Vikas Shukla 
>
>
> --- On Wed, 1/28/09, Ryan Sequeira <ryan.sequeira@xxxxxxxxxx> wrote:
>
>   
>> From: Ryan Sequeira <ryan.sequeira@xxxxxxxxxx>
>> Subject: [SI-LIST] BGA vias  outside the package
>> To: si-list@xxxxxxxxxxxxx
>> Date: Wednesday, January 28, 2009, 5:20 AM
>> Would like to understand the effects of supplying power and
>> ground to the
>> BGA by connecting
>> vias to the VCC/GND plains and traces outside the BGA
>> package. 
>>
>> Pros: Allows for larger vias outside the package.
>> Cons: Compromised noise performance through trace length
>> added to VCC/GND
>> trace length.
>>
>> Is there something else that needs to be taken care of....
>>
>> The BGA device is a CPLD, IO=3.3V, Core=1.8V, Fmax =
>> 180MHz, VCCO(3.3V) - 11
>> pins, VCC(1.8V) - 4 pins, GND - 15 pins
>> The device is a 132csBGA, 0.5mm pitch, 0.3mm pad. So cannot
>> afford to drop
>> the vias within the BGA area. Microvias would be too
>> expensive...
>>
>> Ryan
>>
>>
>>
>>
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