Brian,
        as Dennis stated, your assembler should have some good opinions on
your question. Other than that, my readings have led me to the opinion that
your pad size should match the devices ball pad size, i.e. 0.27 in this
case. The theory that sounds the most convincing on matching the ball pad
size is that it will give you an even ball distortion between the device
package and your board. Mismatched sizes will distort the ball shape and may
apply uneven stresses through the ball.
        Stencil size, that is also dependant on your other pad sizes and
thus would have to tie together with your other pads because it would
usually be stencilled in one pass. So your pad sizes and the desired volume
of paste would dictate the stencil openings for the ball grid and other
packages.

Sincerely,
Brad Velander.

Lead PCB Designer
Norsat International Inc.
Microwave Products
Tel   (604) 292-9089 (direct line)
Fax  (604) 292-9010
email: [EMAIL PROTECTED]
http://www.norsat.com
Norsat's Microwave Products Division has now achieved ISO 9001:2000
certification 



> -----Original Message-----
> From: Brian Guralnick [mailto:[EMAIL PROTECTED]]
> Sent: Monday, September 09, 2002 10:32 AM
> To: Protel EDA Forum
> Subject: [PEDA] BGA Footprint question.
> 
> 
> 
> Hi everyone,
> 
>     I'm making a 54-BALL VFBGA (8mm x 9mm) package.  From a 
> Micron ram chip MT48V8M16LFFF.
> 
> PDF = 
> http://download.micron.com/pdf/datasheets/dram/MobileY95W_3V_E.pdf
> 
> In the mechanical package description, it says:
> 
> 54x /O 0.35 solder ball diameter to post reflow condition.  
> The pre-reflow diameter is /O 0.33.
> 
> Elsewhere, the spec says that the Solder ball pad: /O .27mm.
> 
> This is all that we are told with regard to the pad size.
> Does this mean that the land pattern should be 0.33mm, or 0.35mm?
> What should the solder mask & paste stencil be cut out to?
> 
> ____________
> Brian Guralnick
> [EMAIL PROTECTED]
> Voice (514) 624-4003
> Fax (514) 624-3631

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