>Hello all,
>
>We are now placing 56 bump uBGA's on several boards.  Occationally some of
>the parts are blown off in the reflow oven, usually in the first 2 zones.  I
>have also noticed that the parts after placement move quite easily, (appears
>that so little paste is deposited that what's left is not very sticky).
>Presently I'm using a standard .012 aperture in a .004 stepdown.  I am
>considering trying a .014 square aperture with .006 radii on the corners on
>a .006 stainless stencil  laser cut of course.  Has anyone run into this
>problem or know of any solutions?  Any help is welcomed.
>
>Russ Vertefeuille

Russ,
        We have  had quite a lot of experience with high density designs
using BGA and with PBGA using 63/37 balls . Stencils  were made by best
available method  mono level and apertures 1:1 with land pattern. Stencil
thickness was 5 or 6 mils and we also experimented with square apertures
with side length equal to diameter of land or !mil lager than diameter
These gave excellent results and placement was tolerant because  of self
centering. MicroBGA is the same as for BGA but you dont need as much paste
so a !;! 5 mil should do you fine. I am watching one of our local companies
who are getting set to put down flash ram components with a chipshooter

Edward Hale
PS. Laser stencils do not have to cost the earth!
>
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