We have this solder ball & beads around chip components since day one of manufacturing using non-clean process. We managed to eliminate the solder beads around ceramic type chip componenets when we have a slow ramp up of pre heat at solder reflow. However we are still seeing the solder bead around crystals which has a metal body. We suspect the crystal having a metal body heats up faster than the ceramic body, therefore stll causing a faster out gassing of flux at the area. If anybody had an experience solving this problem, do enlighten. Thanks.
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
You might want to check your paste application. We eliminated this problem with reduced aperatures, and homeplate designs on our stencil. The paste process is very important in eliminating solder balls. See Solder aperature guidlines in the forum.