Solder Beads

Hi Pavel,

what you see is most probably caused by to much solder. There�re plenty of hints to avoid this in the archives, just use the "search" function and look for "solder beads" and "solder balls". That will bring you on the right track.

Good luck Wolfgang

reply »

Just as there are a number of causes of solder balling there are also several factors which can lead to or encourage the formation of solder beads.

- During the placement of the component solder paste may be squeezed out from underneath the component and coalesce on the free surface of the substrate pad with the rest of the paste forming the solder joint. During reflow this squeezed out solder will take the form of a solder sphere in an attempt to reduce its surface tension. This is why the solder bead forms near the component. The volume of the solder printed and the extent to which it becomes squeezed out by the component will determine its size. Too much pressure or too much paste can also cause squeeze out.

- Misregistration between the stencil aperture and the pad can lead to solder paste printing onto the board mask resulting in beading.

- Operators can inadvertently transfer solder paste to the mask in an attempt to straighten out a misplaced component. Usually it is advisable NOT to attempt to straighten a component prior to reflow. Tweaking the placement, usually done with tweezers, can also lead to insufficient, shorts, and voiding. Allow the paste and the reflow oven to float the component to the lands and straighten the component. - Worn equipment, stencils, and squeegees as well as warped boards or insufficient stencil wiping can also contribute to beading and micro balls as well.

reply »

Hi Pavel,

I'll bet you're talking after reflow - so in a nutshell - it is too much solder. Paste goes under the R or C and rests on the mask after placement. During reflow, this excess paste balls up along your part (wow, look Ma, no rocket science!). The easiest way to get rid of this is to reduce your stencil apertures by 10% and use the homeplate design. The tips or points of the home plates point at each other, unless you're doing 0201's, then turn them 180 degrees. Ta-da, less volume of paste without sacrificing stencil thickness! It works and a heck of a lot easier to do than search the archives! ;^)

-Kevin

reply »