We can perform MP/CpS or Machine/Process Capability Study on the reflow
equipment.
You inoput variables would be: Converyor speed, O2 ppm level, Temperature
Output variables would be voids, skewing, etc.
If you want to gauge the reflow oven in terms of Cpk, you can gauge it with
the output variables. What would be the Cpk for voids with the optimized
reflow parameters you indentified in your MPCPS study...etc...etc.

regards
Jon

On Mon, Nov 3, 2008 at 3:45 AM, John Goulet <[log in to unmask]> wrote:

> Reflow I never read that anyone saw a need to do CPK on reflow. The CPK
> should be done on the placement machines. A good reflow oven has only a 2
> degree C variation from rail to rail. The reflow usually corrects parts
> that are
> shifted or rotated a little with a good board and stencil design.
> DOE on stencils should be tied to squeege pressure and speed as well as
> cleaning and chemistry used to clean the squeeqee, number of machine clean
> cycles (cleaning after  X number of prints), dry or cleaning fluid in
> printer.
> The DOE beside the obvious stencil thickness, laser cut versus electroform
> etc
> and aperature required for PB-free versus tin/lead are the following for
> small
> components and BGAs which are usually the ones with issues.
> In the DOE I would select Small Rnets with concave versus convex ("T"
> shaped lead) to be added to whatever components you have in mind. There is
> a new taller but narrow SMT switch with ony 2 pads, that tilt when they
> reflow. This one is a challange. Another point to test is the shifting of
> pads
> towards the heel versus the toe to prevent opens. Stencil designers have
> said
> that opposite of what you may think, adding paste to the rear or heel
> actually
> corrects opens at the toe. The paste flows toward the toe and up the lead
> during reflow. Shifting the pad or aperture toward the toe to fix an open
>  may
> not help because the leads pushes the paste away during placement, instead
> of fixing the open you may get a short or still have the open. A DOE board
> should have some pads with uneven etches to test stencil designs that may
> prevent components from shifting by changing the amount of paste via width
> and length reductions. A DOE with kings crown versus home plate pad designs
> or staggered tear drop for IC's may be selected. Some say the kings crown
> may lead to tombstoning but I haven't seen it. I know the  homeplate design
> may be good if you have an automated stencil cleaner but if the operator
> does manual cleaning a stencil with all those sharp points would be risky.
> I'd
> bet you'd have some damaged stencils if the point is bent up when the steel
> squeegee hits them.
>
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