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SMT electronics assembly manufacturing forum.


Solder Balls

Dave Vulcano

#8601

Solder Balls | 10 November, 1999

Hello,

Looking for some insight.

We just finished a run and experienced a lot of solder balls. We used a no clean paste, nitrogen atmosphere reflow oven, and saw the solder balls on 1206 - 0805 chip components. All of the solder balls appeared and were attached at the center of the components.

Would this be a print issue (excessive paste "oozing" under the the component then out the side during reflow) or a reflow issue (paste "exploding/popping" out the sides due to a profiling issue)or some other variable.

Any suggestions, help, or insight would be appreciated!

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#8602

Re: Solder Balls | 10 November, 1999

Hi Dave Sounds like what we call a "squeeze ball". Usually caused by excess paste squeezed over the mask between the pads of the component when placed. This sometimes escapes the pull of surface tension back to the fillet and escapes to the side, usually aided by the gasses of the flux volitalizing. This is normally cured by a stencil aperture modification. We use what is referred to as the "home base" (or home plate, after American baseball) shape where the aperture is narrowed at about a 60 degree angle toward the middle of the pads (center of the component) to reduce the volume of paste at the center. It has all but eliminated their occurance on out boards. Some say that a simple aperture reduction is more effective for components smaller than 0805. If your designer cannot do this, most stencil vendors will be happy to do it for a nominal fee. By the way, you didn't say if this was a new process or an established one that has changed. Good luck - Hope this helps John Thorup

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Charley Qin

#8603

Re: Solder Balls | 10 November, 1999

Hi, Dave

Which brand of solder paste are you using? Is it Alphametal UP78 family? If so. According to our experience, you could try other solder pastes without any change on stencil aperture. Maybe it could work.

Good luck.

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#8604

Re: Solder Balls | 11 November, 1999

For me it sounds like to much paste. If it worked in former runs with the same design and stencil look for changes in your printing process, check the volume and shape of your deposit. Redesigning pads and playing with aperture size allowing for more tolerances in printing eliminated that problem for us and IMO is a good approach to prevent this and similar problems .

DFM like Earl would say

Good luck Wolfgang

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#8605

Re: Solder Balls | 11 November, 1999

Dave: I think you�re correct. The solder balls are probably from either:

� Excessive paste � OR � Paste "exploding/popping" during reflow � OR � Some other variable.

Two things:

1 It�s virtually impossible to determine the cause of your ball generation without additional information, like: What's your paste? What are the measurements on your stencils? What are the measurements on your pads? What�s your oven temperature profile? Have you used a profiler or oven thermocouples to measure your temperatures? Etc, etc 2 Solder ball generation is not a new topic on SMTnet. Would some of the previous discussions help?

My2�

Dave F

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