0603 Solder Beads

Hello Mark, I can�t provide any hands on feedback with 0402 packages but we are using a no clean process with 0603 devices. Here are some parameters which will affect solder balling. Stencil thickness, aperture size, paste viscosity, snap off and bottom side stencil cleanliness.

The suggestions I recommend is that you leave the solder mask between pads, this will help prevent component floating, provide a solder ball dam (if you pull back the mask from the pads a few mils, it will help contain and hold the paste), and help prevent possible bridging between pads. I use a 6 mil. stainless steel stencil for 0603 packages and print with a set of metal blades. As you may already know a zero snap off on stencil height is preferable for almost all applications. Also keeping the bottom of the stencil clean will cut down on solder balls tremendously. In some cases you may need to clean between each print. Additionally solder balls can develop if your paste viscosity is too thick and tends to clog your stencil apertures, leading to paste on the bottom of the stencil.

Regarding 0402 packages, I know of other people that use a 4 mil. stainless steel stencil. Unfortunately when using a no clean process solder balls are almost impossible to eliminate 100%. Keep in mind that once they are minimized they tend to be encapsulated in flux residue. As an example I know of a case where drop tests were performed on assemblies which were heated to 80 C.. This product had excessive solder balls which could have presented a problem in the field had they become loose and shorted another components leads. The results were that the before and after pictures revealed no loosed solder balls on any of the units tested.

I hope this helps. Happy Processing.

Robert Culpepper

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Your problem is common on this solder defect. It is sometimes known as squeeze balls. The formation of these result from excess volumes of paste applied to the pads. Once the paste melts and covers the pad and component termination,the excess has no where to go and it runs under the compnent body until it pops out in the center of the part.All of Roberts suggestions are valid. You have an additional consideration at your site and that is the paste. It is desugned to print fast but has poor stencil life. An interruption in printing from any source will mean the paste properties have changed while at rest. (I know this from having visited your site on several occassions)Keep the stencil clean, remove and clean off line in a bath after use, and consider reducing the pad volume by using the baseball diamond pad shape. Regards, Ray

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