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BGA Assembly

We are just getting started doing BGA assembly at our plant.... - May 28, 2002 by BenW  

BenW

#20095

BGA Assembly | 28 May, 2002

We are just getting started doing BGA assembly at our plant. Any helpful hints would be greatly appreciated.

My first main question is about stencils. We are considering using diamond shaped holes for the BGA pads. Does anyone have any experience with these? What are the advantages/disadvantages associated?

Also, is it always a good idea to bake PBGAs before pasting and reflow? If so, what are some good specs to follow for baking them?

Thanks

Ben

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Alex K.

#20105

BGA Assembly | 29 May, 2002

Hi Ben!

I was in your position just over 12 months ago.. We use round appetures for BGAs and don't have any problems with it, as long as thiknes of your stencil is correct.

As for baking BGAs befor placing. I haven't had any problems with devices straight out of the manufacturers package. But if they are sitting in your store for a while than you can check manuf. spec. I found it's usually OK to follow.

Hope it might help you

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BenW

#20110

BGA Assembly | 29 May, 2002

Thanks for your input, Alex. I appreciate it.

I'd still like some input on diamond-shaped apertures if anyone has had experience with them.

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#20112

BGA Assembly | 29 May, 2002

Just wondering why you'd use a diamond shaped aperture? Seems like it'll just fill in the corners and be pretty much round anyway, so why not just start out that way and keep the volume you want?

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JAX

#20113

BGA Assembly | 29 May, 2002

Are you using Diamond apertures to achieve a volumn you cannot get through round ap's?

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BenW

#20115

BGA Assembly | 29 May, 2002

Diamond apertures were just an idea proposed by my plant manager. They are not necessary, by far.

Thanks for all your input.

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JAX

#20116

BGA Assembly | 29 May, 2002

The only reason I can see for doing this is: Apply more paste than a one-to-one round can provide.

Square, diamond, etc... does not matter.

Normally this is done for PTP applications but,... If it gets the job done, who cares.

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CAL

#20117

BGA Assembly | 29 May, 2002

Stick with round aperatures they work fine. If you are successfully processing fine pitch QFP's then switching to BGA's should be no problem. Profiling is a little tricky, a thermal couple should be mounted at the under/ center of the component.

cal

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Jim Altland

#20119

BGA Assembly | 29 May, 2002

I have another question about bga's and stencils. We were told by a manufacturer to use .012 square apertures for .009 dia balls. The original pick & place and reflow goes pretty well (over 95% good), however when we have to select components on the reverse side of a ceramic substrate, the bga lets loose on one or more balls and the circuit stops functioning.

Do you have any suggestions as to how we can avoid this? (We cannot change the placement of the select components.)

Thanks,

Jim

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BenW

#20127

BGA Assembly | 29 May, 2002

As far as aperture dimensions go, I found a technical note written by Micron that recommends using "square apertures with sides equal in length to the diameter of the PCB pad. The corners of the square should also be slightly rounded, and the aperture should be slightly tapered or trapezoidal in the vertical axis." They then recommend using a foil thickness of 4 or 5 mils, depending on the BGA pitch.

I'm not sure if aperture sizing would help with the reflow problem on the 2nd side of the board, though. That could be a profile problem -- too hot, maybe? -- allowing the underside of the board to heat up too much. Not sure, though.

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RDR

#20129

BGA Assembly | 29 May, 2002

I have heard of using diamond apertures for BGAs to tell you in XRay inspection if you reflowed (diamond means that paste did not reflow) that is the inly thing I coud think of as to why you qould want to do this. Russ

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matherat

#20137

BGA Assembly | 30 May, 2002

Ben,

I am not sure about diamond shaped pads but if you are just getting into BGA you need to know about SIPAD ssd.

http://www.sipad.com

mkehoe

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

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Jim Altland

#20138

BGA Assembly | 30 May, 2002

Ben,

Thanks for the input on the apertures and thickness.

The side two problem, is not reflow. We have no trouble with reflow. The problem is that our product is high precision crystal oscillators. In order to get the precision needed, we have to select a number of components on each substrate to match the particular parameters of the crystal. It is after reflow when we are testing the osctllators that we must select the components. When we use a hot air pencil to remove the initial component and replace it that the solder on the bga on side one melts and the bga looses contact on one or more ball. Is there something that we can do to prevent this? Like an epoxy underfill?

Any suggestions would be greatly appreciated.

Thanks,

Jim

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Hussman69

#20144

BGA Assembly | 30 May, 2002

Diamond/sqaure shapes are good for when you X-Ray the BGA as stated above. The only problem is if you you over print too much, you can get solder beading. I recommend round pads to avoid the beading and use X-Ray to make sure you reflow correctly. Unless your manager is the type that would take offense to you for NOT using his idea (hey, we all play the game). Then use the diamond shapes, show him the failures and let him decide that round pads are better. Pat him on the back and say "Man, were would I be without you!?" - BONUS POINTS!

Voids in the BGA balls is another concern. It's a can of worms that 50 different engineers will have 50 different opinions on the subjet. But something you need to consider.

Rework equipment is another consideration. Get the proper equipment to do the job right. Reflowing just a BGA on a board is harder than most any other part, depending on the tye of BGA you're using. If you are placing QFPs you may have some good equipment to repair with already. Just be sure to profile your repair as well.

Baking BGAs is something you need to find out from the manufacturer. Generally you only need to bake them if the original container has been opened. Most manufacturers have a storage temp range in their specification - I find this very usefull as our BGA manufacturers says we can store them up to 140 F. Perfect over night temp for us. Make sure you have an oven to store them in too. Keeping them in an oven that is constantly being opened and closed for production isnot recommended (especially if BGAs start to fail). You may want to look into nitrogen cabinets as well.

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Alex K.

#20187

BGA Assembly | 3 June, 2002

Hi Jim!

Just an idea.... There are some reflow ovens which use cooling on the bottom side of pcb during reflow for doublesided boards. It prevents solder on the bottom side to melt.

You might try cooling your BGA while reflowing parts on opposit side.

Just a thought, don't know if it will work, but might be worth a try..

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ianchan

#20189

BGA Assembly | 4 June, 2002

Yo, Hussman69,

you are one scary dude... in fact you remind me of that catbert...

as an add-on, "metcal" has a good semi-auto BGA rework system and their engineering support is strong. then again, our engineers shd be optimizing the production line and NOT to give opportunity for rework of the product, right? RIGHT?

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Dason C

#20194

BGA Assembly | 4 June, 2002

We use the diamond shape on the CBGA/CCGA only, all the PBGA are using the regular round shape.

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Hussman69

#20196

BGA Assembly | 4 June, 2002

I'm not scary, just been doing this a looooong time. I've seen alot of things and I've seen a lot of good engineers get flushed for not playing the game right. Too bad some of us have to.

And yes, you are right. Removing opportunities for rework should result in 100% first time yields! (Now, am I playing the game with you?) Rework equipment is still a good idea, although there are companies that do this type of rework too. So if the your rework is small, then this may be an alternative.

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#20283

BGA Assembly | 7 June, 2002

Ben,

Get a copy of IPC 7525 and review it. This will help you understand the relationship of aperture geometry to paste transfer.

Good Luck, Greg

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Foshap

#20467

BGA Assembly | 24 June, 2002

We, too, are getting into BGA manufacturing. In order to start experimenting, we're looking for a place to buy test boards with Gerber so that we can experiment prior to "going live" with a customers BGA requirements. Does anyone know of a company that sells supplies for trial and error experimention. FOSHAP

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RDR

#20470

BGA Assembly | 24 June, 2002

Practical components, or Top line sell complete kits (parts boards, artwork, etc...

Russ

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Dason C

#20490

BGA Assembly | 25 June, 2002

Hussman, I know when the oven temp at around 140F and the relative humidity is below 10% and meet the J-STD requirement but I never see any recommendation from the supplier to store @ this temperature. Can U advise where I can get more information? Thank!

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