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Problems with .45 mil ball diameter BGA....

Good day all, we are having problems with some new BGAs that... - Apr 26, 2005 by

jdumont

#33903

Problems with .45 mil ball diameter BGA.... | 26 April, 2005

Good day all, we are having problems with some new BGAs that are slowly filtering their way into our designs. Engineering really wants to use them and I am pressured to make them work. They are 54 ball BGAs. The pitch is .8 mils and the ball diameter is .45 mils. The package is 16 x 8 mils. Now I know there are probably much worse challenges out there but we are having issues getting consistantly accurate places from the P&P. Also I am seeing issues with paste clogging and not transfering to the PCB during the stenciling process. We are using a 5 mil thick stencil with 1:1 aperture to pad size. Any recomendations?

There is a TSOP version of this part that I am trying to get engineering to switch to to save me some headaches but in the meantime i need to make the best of what ive got.

Thanks in advance JD

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RDR

#33906

Problems with .45 mil ball diameter BGA.... | 26 April, 2005

How does your machine recognize the part? Does it use vision for ball inspection or a just a body scan?

What is the aperture size on the stencil? Have you calculated the area ratio? What is the aperture shape?

What type of paste are you using? is it expired, old, type 2?

Are the paste release problems only present on the BGA location?

It doesn't seem like this package should give you any problems unless there may be something wrong in the above list.

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FD

#33909

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

Are you sure about your dimensions?

.45mil ball diameter and .8mil pitch, and a body size of 16x8mils is really, really tiny. I assume you mean millimeters and not mils? 1mil = 0.0254millimeters

If your stencil is 5mils and your ball diameter is only .45mils, then your solder paste height (thickness) will be 10x your ball diameter. That is not good.

Now a 0.45mm ball diameter and 0.8mm pitch sounds more normal. This is right below the border of microBGA and standard BGA specifications. Standard BGA are said to be at least 1.0mm pitch and 0.5mm diameter (or larger). So I would verify that you PnP equipment could handle microBGA. If it is out of spec for your mounter, then I would recognize off of the body only and do 100% X-ray inspection of each placement to verify there are no missing balls or miss alignments.

If your mounter is capable of doing microBGA I would consult the mounter's manufacturer and see if calibration, or something else is needed.

I am not a screen printing expert, so I will leave that to others.

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#33910

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

Hi,

There are a number of things you can try. You are already using 1:1 hole to pad ratio, and that's good as you need the size.

I would try an electroform stencil as that will have a smooth side of the aperture and help paste release. Don't use chem etched. We had a problem with an old manual stencil printer and went for 4 thou thick stencils, and it helped. So going thinner might help.

Use very fine solder paste, and we use Koki fine paste, but there are heaps of brands, and get something for fine pitch parts.

You should be able to work with these parts no problems, however it can be frustrating getting going with them. I had similar problems a few years ago, and people here on the SMT forum helped with ideas until I solved the problem. Going 4 thou and laser cut solved the problem for us at the time.

Regards,

Grant

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jdumont

#33913

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

Thanks a lot guys. The part dims are in mm by the way. So last night i was looking at the .pcb file and saw that the pad size and stencil are both .013" while the ball on the BGA is .018" (sorry for switching from metric). Is this ok or can i have engineering open up the pad on the board to be closer to the ball size. Also, I didnt think about using a thinner solder paste. Would type 4 be good for this size or will type 5 be necessary? Will I see any problems using a thinner paste on the rest of the board that doesnt need it?

Thanks a lot for the suggestions... JD

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HOSS

#33916

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

A 1:1 pad to ball diameter will cause the balls to collapse more giving you less post solder component standoff. Depending on the application of this product and required reliability, you may want to stay with the padstack being used. You may try changing your stencil apertures to a square vs round. Below 17 mils we have done this and acheived better paste release. You stencil supplier should have an opinion about this. If they don't, get a new supplier.

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jdumont

#33919

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

Square you say? Wouldnt this give less paste deposition than a circle and basically reduce the standoff height the same as making the pad bigger? Would using a type 4 paste for this size stencil aperture (.013") be a good idea?

Thanks JD

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#33921

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

Hi,

That was one of the recommendations made when I had the problem, but I never followed it up.

Would the outside corners of the square aperture be on the edge of the pad, so you would be putting down less paste on a smaller aperture, but it's square? I have heard good things about square apertures over round though.

Regards,

Grant

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RDR

#33923

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

You will want the square in this case to be .013" with rounded corners (slight overprint). I do not see why you would need a type 4 paste for this. the aperture as it sits should release well but obviously there is something wrong.

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jdumont

#33925

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

If square apertures are supposedly better than round why arnt we using them for all BGA locations, even the larger ones?

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RDR

#33929

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

no problems with release on the larger ones.

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KEN

#33930

Problems with .45 mil ball diameter BGA.... | 27 April, 2005

One thing that may have been over looked: Not all solder pastes are created equal. Period.

The best way to avoid these situations is to characterize you material (and your process). The fact is you could try every aperture shape under the sun....and you may still not have acceptable results (with this existing solder paste and this device).

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jdumont

#33945

Problems with .45 mil ball diameter BGA.... | 28 April, 2005

Good point about the paste. We are using Kester 256 anyone have any experiences, good or bad, with this stuff? We have had pretty good luck thus far with it.

Thanks JD

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KEN

#33971

Problems with .45 mil ball diameter BGA.... | 28 April, 2005

You just found your problem.

And yes, I have had experience with this material.

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jdumont

#33974

Problems with .45 mil ball diameter BGA.... | 29 April, 2005

Really you think the 256 is the issue eh? I have some a sample of the type 4 powder size coming in hopefully today. Hopefully that will help, if not its back to the old solder eval drawing board. I love my job...

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Woodsmt

#33987

Problems with .45 mil ball diameter BGA.... | 29 April, 2005

I have used many of these devices recently and have used them several times over the last few years. I have been successful using a type 4 paste on a 5 mil stencil. The 13 mil round aperture should provide you with an area ratio just adequate for release. If you change to a square you would want to bump up to 14mil to keep you area ratio around .7. You may also need to optimize stencil cleaning. If your printer has under stencil wiping you may want to wipe as often as after every print. You will need to monitor your process to find out how often this should be done. If you printer is equipped with Vacuum and wiping, this will really help. If you do not have these options you may need to pull and clean you stencil more often. Don�t forget to optimize all you printer parameters. Snap off speed adjustments may also help.

This message was posted Add this forum to your site! Click to learn more. the Electronics Forum @

reply »

Woodsmt

#33988

Problems with .45 mil ball diameter BGA.... | 29 April, 2005

I have used many of these devices recently and have used them several times over the last few years. I have been successful using a type 4 paste on a 5 mil stencil. The 13 mil round aperture should provide you with an area ratio just adequate for release. If you change to a square you would want to bump up to 14mil to keep you area ratio around .7. You may also need to optimize stencil cleaning. If your printer has under stencil wiping you may want to wipe as often as after every print. You will need to monitor your process to find out how often this should be done. If you printer is equipped with Vacuum and wiping, this will really help. If you do not have these options you may need to pull and clean you stencil more often. Don�t forget to optimize all you printer parameters. Snap off speed adjustments may also help.

This message was posted Add this forum to your site! Click to learn more. the Electronics Forum @

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KEN

#33998

Problems with .45 mil ball diameter BGA.... | 1 May, 2005

What is your price difference between the #3 and the #4 powder.

....see where I'm taking this?

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#34020

Problems with .45 mil ball diameter BGA.... | 3 May, 2005

Consider ssd for your BGA's. http://www.sipad.com/SIPAD_NASA_Plots.htm www.sipad.net mkehoe@sipad.net

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