Leif Erik,

Your problem comes from the stencil thickness. 6mils is too much for such a pitch. Go down to 5, even 4 if permitted by the rest of the components.
You might also have too big pads; normally at this ball size you should have 0.25mm pads, per IPC-7095. Normally you should not be able to print these pads with 6mil stencils; the area ratio is 0.4, so your normal problem for a good pad design would be missing solder. 

You might also consider e-formed stencils for this application.

Good luck,

Ioan

-----Message d'origine-----
De : Leif Erik Laerum [mailto:[log in to unmask]] 
Envoyé : 22 septembre 2008 11:50
À : [log in to unmask]
Objet : [TN] Aperture recommendations for 0.5 mm Ball spacing

Technetters,

I am looking for recommendations for solder stencil aperture or aperture 
reduction for a CSP BGA that has 0.30 mm balls and 0.5 mm spacing 
(pitch). On same size balls, but larger pitch we have used 0% reduction 
with great success. This time this ended in disaster with shorts across 
pretty much all the balls. We are using T5 SAC305 solder (yes, we are 
still sticking to that) and a 6 mil stencil. I am thinking about a 50% 
reduction, but I am concerned that the deposit efficiency will be 
terrible and cause opens.

Thank you in advance.

-- 
Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com

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