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Printed Circuit Board Assembly & PCB Design Forum

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BGA Reballing

Views: 2060

JP

#38277

BGA Reballing | 7 December, 2005

I am attempting to reball some PBGA's with no luck. The problem is getting the solder spheres to sit in the stencil apertures and removing excess spheres. I apply light amount of flux to pads of a BGA, align a stencil over a BGA, then pour solder spheres. Some spheres get into the apertures, some apertures are empty . . . using a brush I gently try to removes excess ones but it just does not seem to work.

Any suggestions?

My mini stencil is 5 mils thick, apertures in the stencil are larger than the sphere diameter by approx. 10%-20% so that the stencil may come off after reflow. Also, the pads on the BGA are flat in some areas (after applying solder wick) and slightly spherical where the excess solder was removed but solder wick has not been applied. I have placed the stencil flat on the BGA . . . I also tried creating a standoff between the BGA and the stencil by applying Kapton tape to edges of a BGA so that the spheres would "fall in" the apertures. Nothing seems to work. The BGA is 256 ball count 1.27mm pitch with spheres 0.030" in diameter. I never tried to reball a component . . . in theory the process is simply in practice, not so (well, at least as it stands right now).

BTW, another issue that I noticed is that spheres tend to clump up with even a slight trace of flux. Trying to separate the balls is almost impossible in that case unless you have a lot of time.

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RDR

#38279

BGA Reballing | 7 December, 2005

You need a thicker mask/stencil, the openings should be .005" larger than ball diameter and the thickness should also be .005" thicker than the ball diameter. by making the mask thicker you can then just roll or scrape the extra balls off and any double stacks will show above the mask and then just look into the holes and fill any vacancies.

Or, you could buy preforms from winslow.

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JP

#38294

BGA Reballing | 8 December, 2005

Thanks Russ. As I was writing the post, it crossed my mind that stencil thickness is probably incorrect. I did not question the thickness because it was something that has been discussed with our stencil fabrication shop. When we were ordering mini stencils, an engineer there said that they make them all the time and that a recommended thickness is 5 or 6 mils. I was very specific that we will be using them for BGA reballing NOT solder paste transfer. The guy reassured me that they have experience with these. Well, so much for experience.

I have 3 stencils each being 6mils thick which I stacked. Ball diameter is .028", stencil thickness is .018" - worked well. Basically 1/2 of the ball was inside the aperture of the stencil and that made filling the holes a lot easier. Your suggestion makes a lot of sense. Now all i have to do is find a different company to fabricate the stencils.

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Gary

#38295

BGA Reballing | 8 December, 2005

Go to http://www.minimicrostencil.com and look up the rb-2000. There is a step by step discription of how to do it and as mentioned before you will need a thicker stencil for your application. there will me a list of stencil types at the web site that will help as well.

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JC

#38296

BGA Reballing | 8 December, 2005

Hey JP,

Try using I-Source for your stencils.

www.i-source.com

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