Calvin, We have found several reasons for solder beading occurance. We have found that solder balling is usually directly attributed to the reflow profile while solder beading is usually (but not always) a result of either excessive solder paste volume under the component or excessive placement force/speed. The component displaces solder paste when it is placed. On a chip component, some of the paste is pushed towards the middle of the component. If the paste is pushed out far enough from the pad, the solder bead will migrate to the side of the component rather than back to the pad. There are two ways to eliminate this type of solder beading. 1) Modify the stencil to provide less paste volume under the component. The use of "baseball plate" shaped apertures that provide sufficiant solder volume but put minimal paste directly under the component will most often eliminate beading on chip components. 2) You may want to experiment with the placement force. The lower the component placement velocity, the less amount of paste displacement. I have had some sucsess approaching beading problems in this manner. I hope that this helps
M2C
Chris
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