type 3 vs type 4 paste

IPC-7525 "Stencil Design Guidelines" is a good reference document to have. Here are notes from a Tessera, (a major mBGA mfg.), study- "Application Note Solder Stencil Requirement for mBGA" The stencil aperture is to be square, equal in size to the land pattern. Although the fine-pitch BGA land pattern is generally a circular shape matching the ball contact size, stencil openings that have a square geometry furnish better solder printing quality. The square stencil pattern serves two purposes. It can increase the solder volume slightly and when adapting the smaller pitch mBGA the square pattern promotes a more uniform release of the solder paste from the stencil surface. A technique that has improved paste transfer on the smaller land geometry�s is the tapered or trapezoidal land pattern opening. The opening closest to the board surface is one mil wider than the opening at the top surface. Stencil Thickness: .005� thick stainless steel Aperture Size: .012� square (laser cut or chem etched) w/ electro polished wall Aperture Shape: Square w/ .006� R (max.) in corners Trapezoidal: .001� (.013� square) overall taper from solder sd. to board sd. The square trapezoidal pattern releases paste from the stencil surface more uniformly. End. As far as paste partical, or sphere sizes, the ideal size is to have 4-5 spheres fit the length/width, whichever is smaller, of the aperture for optimum release, according to MPM (a major printing machine mfg.). Contact your paste supplier's applications people to get their recommendation for the type paste for your aperture size and application. Board finish is a factor to consider as well. HASL boards are not recommended for mBGA.

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I assume you�re using round apertures. They are bad news. Go square.

If you do the numbers as Pete suggests, you�ll see that those little blanking balls are tough to print: * Area ratio = (LxW)/(2x(L+W)xT) SB GT 0.66 => Yours is (12 thou x12 thou)/(2x(12 thou +12 thou)x5 thou) = 0.60 * Aspect=W/T for electroformed stencils SB GT 1.1 => Yours is 12 thou/5 thou = 2.40

So, we kind of think of what you�re trying do as maybe printing a 16 pitch QFP, which gives you similar numbers.

It also shows you what you need to do.

Find a SMTnet thread by �rpereira�. It gives excellent guidelines for printing uBGA.

"stay away from type 4 if you can"

Sounds like something I'd say, if I didn't I probably will say it some time down the road. We do not like Type 4 because: * It costs more money. * It slumps. * If we put it on the machine for a specific job, we forget that it�s out there and it ends-up being used on every job until it gets used-up.

When we see 16 pitch, we start thinking Type 4 paste.

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