With OPAMP CSPs of the same size we have adopted the manufacturer's
(Maxim) 
recommendation for an asymmetrical opening in the stencil. 

http://pdfserv.maxim-ic.com/en/an/AN1891.pdf

http://pdfserv.maxim-ic.com/en/an/AN3377.pdf
also recommends a corresponding opening in the solder mask.

It looks something like this and works for us.

 ___   ___   ___
!   ! !   ! !   !
!   ! !   ! !   !
!  O! ! O ! !O  !
 ---   ---   ---
 ___   ___   ___
!   ! !   ! !   !
!  O! ! O ! !O  !
!   ! !   ! !   !
 ---   ---   ---
 ___   ___   ___
!  O! ! O ! !O  !
!   ! !   ! !   !
!   ! !   ! !   !
 ---   ---   ---


Volkmar


Volkmar Huss, C.I.D.
-----------------------------
 PRETTL  ELEKTRONIK Luebeck GmbH

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Susy
Webb
Sent: Monday, November 06, 2006 6:24 PM
To: [log in to unmask]
Subject: [DC] .5 mm pitch BGA

Hi all,

We are designing a 9 pin BGA into a board and having some board issues.
The pitch of the part is .5mm and the part is a fet (higher power
needs). The data sheet recommends a .25mm (.010 mil) pad which we have
done, but we are having problems with the part twisting a bit on
placement (requiring rework after reflow). After the initial board
build, we adapted a thinner solder paste and tied the pads on opposite
sides of the part (R & L) to vias to add some extra pull to keep the
device in place. That build was better, but still not as accurate as we
would like because of the solder mask opening size (+.004 overall) and
SM placement tolerance is so huge compared to pad size... only .010.

SO..... my question is.... Are any others using .5 mm pitch BGAs, and if
so, what are you doing to make them assemble properly.

Thanks,
Susy

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