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Mid Chip Solder Balls

Dave Chapman

#7847

Mid Chip Solder Balls | 14 January, 2000

We are having mid chip solder balls on chips and resistors. Seeing the problem on 80% of the assemblies we run. Anywhere from 1 to six of the balls per board. We have slowed the oven down, increased the pressure on the screenprinter to get less paste. Sped the oven up as well. We know that by reducing the aperture by about 20% we can get it to work but this means we would have to rework a large number of stencils. Any suggestions? Paste guys say everything is peachy with the solder. Thanks, Dave CSI

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#7848

Re: Mid Chip Solder Balls | 14 January, 2000

Dave: You're probably correct about too much paste, given that your balling is congrigated near the center of the chips. Three things to consider:

1 Lower temperature profile @ front 3 zones 2 I've heard some stencil suppliers can plate additional material to decrease the size of apertures. I know nothing about it, never done it, but would love to know if it's true. Although I doubt that it is because it would be nice ... and nice thing happen rarely. 3 Place a piece of Kapton Tape over part of the apertures to reduce the area of the aperture ... sometimes it works and sometimes you get additional paste in other areas.

Good luck

Dave F

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John Thorup

#7849

Re: Mid Chip Solder Balls | 14 January, 2000

Yeah, it would be kind of expensive to replace all of your stencils but it's the only real answer. Be sure that all your new and replacement stencils have a home base or home plate shape for the passives. This reduces the paste volume near the center of the chip and dramatically eliminates "squeeze balls." Your CAD operator can do it or your stencil vendor will be happy to do it for a small fee. Search the archives for more info on this subject. Be careful with the tape if you try it. As Dave says you can get very bad results if the thickness of the tape lifts the stencil too near a fine pitch component, breaks the gasketing effect and lets paste ooze all over the place. Good luck John Thorup

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#7850

Re: Mid Chip Solder Balls | 14 January, 2000

Dave: Two more things:

1. Let's go backwards, so what changed that suddenly you've realized that "piles of stencils" are causing solder ballig on chips? Are we speeding down the wrong road? Or is this something you lived with and now have decided to something about?

2. You know another thing, even though your paste supplier told you everything is oke-doke (yeh sura watt else thea sa?), why not try another paste?

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Dean

#7851

Re: Mid Chip Solder Balls | 15 January, 2000

I have reduced solder balling by up to 60 percent by changing the placement height on chip parts. Essentially account for the 6 mills of "Z" height to reduce the "squeezing" effects of the chip impacting the solder bricks. Cheap and easy, especially if you can alter the Z calibration of your P&P machine.

Dean

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Mark Anderson

#7852

Re: Mid Chip Solder Balls | 17 January, 2000

As stated previous, the following elements all have a effect on solder beading. Solder Paste, stencil apertures(reduction of home plate), stencil thickness, printing parameters, placement height, reflow preheat and soaking slope and dwell, manufacturing environment.

However, we have found that the only true means to eliminate the beading is to utilize a 0.006" thick stencil along with removing the solder maskant between the pads on small chip components (e.g. 0603,0402).

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#7853

Re: Mid Chip Solder Balls | 17 January, 2000

Interesting point Mark. We prefer solder mask between pads to reduce bridging. How does removing solder mask between pads eliminate solder balling?

Thanks

Dave F

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Dave Chapman

#7854

Re: Mid Chip Solder Balls | 21 January, 2000

Having adjusted the profile speed slower and adjusting the temps to the recommended lower levels, amazingly the mid chips have almost disapeared (90%). It appears they have been trying to run a "1 profile fits all" on the ovens for Alpha 737 and UP78. I did put in a request for all future stencils to have the 20% aperture reduction on cap chips which would allow us to run a hotter profile and on the 2 stencils we have tried this on it worked. Thanks for the help.

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