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Screen Printing for BGA

paulm

#25521

Screen Printing for BGA | 20 August, 2003

Our company if currently reviewing our BGA Screen Printing Process. The problem we are trying to eliminate has to do with BGA Ball Size Variance. Some of our vendors are allowing a +/- of 0.004" in ball size. This would allow for the possibility of a 0.008" in size variance between balls in the same package. To insure proper reflow we would need a 0.008" solder paste stencil to compensate for this variance. This becomes a problems on boards with fine pitch parts (i.e. a flash) because the 0.008" stencil leads to shorts. We are looking how others have dealt with this problem.

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MK

#25529

Screen Printing for BGA | 21 August, 2003

#25530

Screen Printing for BGA | 21 August, 2003

Woa. The more paste that you put a a BGA ball, the more likely that you'll bridge between balls.

Why do you need to compensate for the size variance in BGA balls?

How does using a 8 thou thick stencil to compensate for ball size variance?

Fine pitch balls are smaller that than those on a 1.27mm pitch ball BGA. So, why do you compensate for smaller balls as you do for larger balls? The variance in those balls should be less. Correct?

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paulm

#25538

Screen Printing for BGA | 21 August, 2003

Such balls (=/- 0.004)size variance can occur in a single part. If I have a ball in a part that could be (according to vendor specs) 0.008 larger than another ball the only way I can insure that the smaller ball makes contact with the paste is to have enough paste on the lands to compensate for the difference. We have in the past used 0.006 stencils but we were having a 5% failure rate with bga's (determined by ERSA Scope and X-ray) because of undersized balls in single packages were not making contact with solder.

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#25540

Screen Printing for BGA | 21 August, 2003

Couple of points are: * You're technically correct that ball height has the potential to vary by 8 thou, but we never see that much variation. Ball heights within a lot of components vary just about nil. * When we place BGA, we smush them into the paste that is say, 5 thou thick.

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Justin Medernach

#25543

Screen Printing for BGA | 21 August, 2003

With a eutectic ball, you'll see about a 50% collapse. This negates that .008" issue you're referring to. With a high temp ball, you don't enjoy the same freedom regarding ball collapse. Dave is correct, though. We recently got IBM to sign up to a +/-.003 tolerance on coplanarity. After speaking with their packaging guru's, they assured me that the typical deviation that you'll find across a package will be only about .001" to .002". I would wager that the loose spec is only to cover any abnormal process variations at the package supplier.

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paulm

#25550

Screen Printing for BGA | 21 August, 2003

Hi Dave and Justin,

Thanks for the advice. Dave you mentioned you "smush" your bga's. What type of machine are you placing then with it? Is the additional pressure to smush them machine or manually applied?

Also, the 0.004" variance we are being told we should expect is coming from at major part manufacturer.

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#25553

Screen Printing for BGA | 21 August, 2003

We just plant BGA in the paste when we place them with a standard flex machine. You know, just set your placement force to get the balls into the paste.

Justin is correct about collapse.

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Stephen

#25569

Screen Printing for BGA | 22 August, 2003

An asteroid the size of texas is more likely to hit the earth than to get a BGA with one ball at the absolute max and another at the absolute minimum. The manufacturer is going to give themselves some leaway. What are you going to do about the more real threat of an asteroid destroying the earth? The balls will have a range and a fairly tight standard deviation. Every possible size is not equally likely. By far most balls will be quite close to the average. The average will probably not be at the center of the spec but then there will be very few parts on the other side of the nominal value. Yes theoretically you could get one ball at one extreme and another ball at the other extreme but also you could have all the ESD protection in place and cause an air current walking by that causes enough friction to cause an ESD event that damages the BGA.

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Stephen

#25570

Screen Printing for BGA | 22 August, 2003

I hate being persnickity, I just am sometimes. You are not being told you "should expect" that variance. You are being told you can't sue them for manufacturing defects if the balls are within that range. That range is way bigger than what they will actually make. More decisions are made based on liability than on engineering. The game of electonics manufacturing is more a game of percentages than anyone will admit publicly. Even the highest quality places talk six sigma which is simply dealing with smaller percentages, but still dealing with percentages. I know I'm simplfing. And even those companies never do as well as they think they are when they are checked carefully.

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paulm

#25575

Screen Printing for BGA | 22 August, 2003

Thanks Steve for your rather colorful and sarcastic input. If I correlate your asteroid assessment with what our QA Departments data(that tells us we are experiencing this problem about 5% of the time and I am guessing higher when we are forced or required to purchased re-balled parts)then based on your theory we should expect an apocalyptic collosion with an asteroid at any time.

Interesting that our company has an employee of the same name as yours and his input in solving problems is usually about as negative and valuable (none) as yours.

Thanks anyway.

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Stephen

#25576

Screen Printing for BGA | 22 August, 2003

read the other messages. I havn't seen this problem nor have I even heard of anyone seeing this problem. If anyone else has seen this on unreflowed BGA's I would love to hear it. At first I thought you meant that your QA department was finding the problem on incoming inspection. Then I realized the probability of you having a common problem was much greater than having a problem no one else has ever had. Does the BGA in question have an encapsulated center and a "flange" around the edges? Are the "small" Unconnected balls near the edges? You probably have BGA's warping during reflow. There currently another thread about this. I have seen this and everyone I know that I have talked about it has seen it at least once. Please let us know is it on incoming inspection or post re-reflow? And I stand by my comment that most decisions on specs are liability based not engineering based.

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robd

#25590

Screen Printing for BGA | 26 August, 2003

We have seen "coplanar problems" on BGAs supplied by some suppliers. We have seen large enough spere variation to cause opens with our standard .006" paste process (paste and profile work fine "most" of the time) . Although not an extremely technical test, some of the BGAs (as supplied) will spin on a flat plate due to uneven spheres. What can be done if the parts still meet that nasty +/-.004" spec but don't work in the real world? Appreciate any constructive input.

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Stephen

#25606

Screen Printing for BGA | 27 August, 2003

Have you checked the temperature profile at the balls? Just because a recipe works "fine most of the time" does not mean it is optimum. Do you have a machine that can measure co-planarity? If I get time I plan on using the MPA to look at co-planarity or if I don't have time soon I will check with our new MFS that is going to replace it. Do any of the speres look flattened? If not can you measure some with calipers to see if you have different sizes? Have you read the thread about BGA warpage? If it seems to be supplier dependant rather than manufacturer dependant, ask for an audit of their storage practises. Maybe you are getting BGA's with moisture problems. Where I am for a while when I started we had problems with a certain BGA from open bags but not if we used ones from never opened bags. Other bga's worked fine with the profile and paste. We changed the profile and were more carefull about moisture issues. It hasn't been all quiet on the testing front but much better than it was.

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