BGA fab: what's reasonable practise for double sided PCB?

I'm designing a board where the parts will only fit if I do one of the following:

1. Put BGAs on both sides

2. Put BGAs on top, smt TSOPs directly underneath and using blind vias to keep the real estate on the bottom side below the BGA parts.

Can you folks rate these two "solutions":

A. no problem, we do it all the time, and it doesn't cost much more B. it can be done, but it is more expensive are you sure you need to do this C. Hmm, we've considered this but don't have much long term reliability information, rework is tough.. D. Tried this, doesn't work well E. What are you thinking!!

Help me avoid getting lynched by the manufacturing dept! Thanks in advance, Gord Wait

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| I'm designing a board where the parts will only fit if I do one of the following: | | 1. Put BGAs on both sides | | 2. Put BGAs on top, smt TSOPs directly underneath and using blind vias to keep the real estate on the bottom side below the BGA parts. | | Can you folks rate these two "solutions": | | A. no problem, we do it all the time, and it doesn't cost much more | B. it can be done, but it is more expensive are you sure you need to do this | C. Hmm, we've considered this but don't have much long term reliability information, rework is tough.. | D. Tried this, doesn't work well | E. What are you thinking!! | | Help me avoid getting lynched by the manufacturing dept! | Thanks in advance, | Gord Wait | | Gord,

I prefer option F...YOUR MAD!!!!!!!!!!!

BGA's on both side's can be done, but you have to offset them, remember when your designing this baby that you may verywell have to repair it. That mean's you have to remove a BGA from what ever side it's on...and what ever is beneath it. Plus I'm assuming you aint planning on puting this thing near a wave solder machine....more hassle, you need to ensure that the via's on the underside of the BGA's are protected with masking and normally a selective soldering pallet but then your still going to et heat which mean's you have a chance to grwo your intermetallic's and well basically trouble.

If you have BGA's top n bottom the first problem is that your general board design has to be sound I'm thinking about the copper loading here, you've got to balance it or the board will warp either during assembly or at a rework stage and there's no point putting your heead in the sand and saying well we wont build defective boards..you will no one is perfect. Placing a BGA on a warped or slightly warped card really is asking for trouble so make sure the fab house you use are dam good, if you've still to decide on a dfab house real Earl Moon's articles in this month's SMT Net Express they will be a great help.

Putting TSOP's directly beneath the BGA's has been done, I've seen it maybe once, again your maybe going to run into problems with rework and so on but it's the lesser of the two evils. If real estate is the biggest problem then I guess I would go for the Tsops. Design the board for a double sided reflow avaoiding PTH stuff if you can and yes I would recon the blind vias are a must but there are better people on the forum to answer that one..EARL jump in anytime. the other thing is if your putting decoupling cap's or whatever around the BGA's rememebr that you may want to get a nozzrle on top of the thing if you need to remove it so leave some space, a couple of millimetres should be enough.

The only other peice of advice is talk to the guy's your going to get to build it, they will know better than us what they can do.

JohnW

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| I'm designing a board where the parts will only fit if I do one of the following: | | 1. Put BGAs on both sides | | 2. Put BGAs on top, smt TSOPs directly underneath and using blind vias to keep the real estate on the bottom side below the BGA parts. | | Can you folks rate these two "solutions": | | A. no problem, we do it all the time, and it doesn't cost much more | B. it can be done, but it is more expensive are you sure you need to do this | C. Hmm, we've considered this but don't have much long term reliability information, rework is tough.. | D. Tried this, doesn't work well | E. What are you thinking!! | | Help me avoid getting lynched by the manufacturing dept! | Thanks in advance, | Gord Wait | We have built boards with Multiple top side BGA's and QFP 100 opposing side with very high success (double sided reflow no glue). I think the key is factory capability!

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| I'm designing a board where the parts will only fit if I do one of the following: | | 1. Put BGAs on both sides | | 2. Put BGAs on top, smt TSOPs directly underneath and using blind vias to keep the real estate on the bottom side below the BGA parts. | | Can you folks rate these two "solutions": | | A. no problem, we do it all the time, and it doesn't cost much more | B. it can be done, but it is more expensive are you sure you need to do this | C. Hmm, we've considered this but don't have much long term reliability information, rework is tough.. | D. Tried this, doesn't work well | E. What are you thinking!! | | Help me avoid getting lynched by the manufacturing dept! | Thanks in advance, | Gord Wait | | Gord -

We have had designs similar to both of your options, but I would lean towards option 2, depending on the specifics. My "rating" would be somewhere between B and C.

Glenn Robertson Alcatel USA

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| I'm designing a board where the parts will only fit if I do one of the following: | | 1. Put BGAs on both sides | | 2. Put BGAs on top, smt TSOPs directly underneath and using blind vias to keep the real estate on the bottom side below the BGA parts. | | Can you folks rate these two "solutions": | | A. no problem, we do it all the time, and it doesn't cost much more | B. it can be done, but it is more expensive are you sure you need to do this | C. Hmm, we've considered this but don't have much long term reliability information, rework is tough.. | D. Tried this, doesn't work well | E. What are you thinking!! | | Help me avoid getting lynched by the manufacturing dept! | Thanks in advance, | Gord Wait | I would pick BGA's on one side and TSOP's on the other. Inspecting BGA's opposite each other with X-ray is rough, hard to tell them apart.

Get your mfg department involved now while they can prove (or disprove) process capability, now is better than after your design is committed.

Boca

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Thanks for all the feedback. I'm going for the tsop's on back, with approval from manufacturing, of course. This forum is just what the doctor ordered!

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| I'm designing a board where the parts will only fit if I do one of the following: | | 1. Put BGAs on both sides | | 2. Put BGAs on top, smt TSOPs directly underneath and using blind vias to keep the real estate on the bottom side below the BGA parts. | | Can you folks rate these two "solutions": | | A. no problem, we do it all the time, and it doesn't cost much more | B. it can be done, but it is more expensive are you sure you need to do this | C. Hmm, we've considered this but don't have much long term reliability information, rework is tough.. | D. Tried this, doesn't work well | E. What are you thinking!! | | Help me avoid getting lynched by the manufacturing dept! | Thanks in advance, | Gord Wait | | 3. Update your resume, you're working for a company that is run by Marketeers... no sense of reality.

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