| | | | | | | | | | | | Hello, | | | If you are using no-clean and they are entrapped in the flux it is also IPC accepted but i havent found a customer yet who agrees with that. Im sure you have checked your profile. I had the same problem with chip components but after changing aperture shapes to trapezoids it disappeared. | | | | Hello Wayne, | | interesting point, I heard of it and there were some evaluations on this subject here in germany with the result that it helps prevent solderballs. | | We actually never tried it because our designfolks didn�t want to put to much work in it, we just adjusted our square pads, stencil data and the printing rules to control the amount of paste and it also did the job. | | Another cause of balls I noticed in the wavesoldering process with noclean flux and nitrogen at the site of one of our subcontractors, they had to add an extra rotating brushcleaning system right after the soldermachine but it wasn�t much help. I think changing the flux and the solderresist were more successful. | | | | Wolfgang | | We had a lot of solder balls when we first started SMT. Turned out these were "squeeze balls" caused when the placement pressure squeezed paste toward the center of chip components where it was over the mask rather than the pad. As the paste melted and the flux volitalized it blew this displaced paste out the side as balls along the side of the component.| We changed the aperture shape for chip components to a baseball "home plate" shape with the points toward the center. The reduced paste volume at the center eliminated the formation of balls. Many CAD systems and most stencil manufacturers can make this modification with little effort. | John Thorup | | My wo pennies worth.....The common theme with the home plate / trapizoid design's is the redusction of the paste available, I run a std 4 thou reduction of chip component's and 2though on fine pitch to 20 thou on rectangular appetures and have no problems. Mid ship solderball's are as John say's caused by excess paste being 'squeezed' under the component at placement and colelesting in the middle of the component, again it can be removed by better stencil design. The other thing that causes the solderballing is poor profiling / ramp rates, excess moisture in material ( including the solderpaste) so there's the dicipline eliment there too and it's so easy to fix when you find the problem...past experience talking there! a bad batch of paste can have similar effect's. Wave soldering under N2 does produce more solderball's than in air but then you got dross to worry about so it's swing's and roundabouts, as someone else pointed out flux and resist have a big influence but then if your prheat's are wrong you'll get them, or again excess moisture in the boards. IPC std's and customer standard's will give target's like 5 ball's of .25mm dia per square inch or what ever but it's got to be zero or as close to zero as we can get.
And if you think it's bad now...wiat till you try it with lead free solder!!!!!!! ooooo deep joy
JohnW
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