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Processing Circuit Boards with BGAs On Both Sides



Processing Circuit Boards with BGAs On Both Sides
What is the best way to process a 30 mil circuit board assembly that has micro BGAs that need to be soldered to both sides?
Board Talk

Transcript


Phil
Welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers Pick and Place. Sometimes going by the identity of consultants with ITM Consulting.

We are here to talk about SMT processes, components, materials, equipment, and whatever else may be bothering and bewildering to you in our industry. Jim, what is today's questions.

Jim
It comes from S.N. What is the best way to process a 30 mil FR406 circuit board assembly that has micro BGAs that need to be soldered to both sides?

I can honestly say FR406 doesn't mean anything specific. But I am assuming that it is a variation of FR4.

30 mil board is quite thin for most standards, not by cell phones. And putting components on both sides, advanced components such as micro BGAs, is getting more and more common.

Ideally putting on our DFM hats, it would be better if they were all on one side. But we know for circuit reasons that is not always possible.

Proper way of course it to go through sequential reflow on both sides. Because the components are light and multi-leaded, they have many bumps and balls on the bottom, there will probably not be an issue of ones on the bottom-side, the ones that are reflowed first, and then reflowed a second time upside down.

Probably little chance that they will fall off, although you could check them out with a weight pad area ratio if you want to. Typically they are light components, they have a lot of surface area on the pads, so they shouldn't fall off. Being a thin board, you want to watch out for warpage.

Particularly when you are doing your second side BGAs, you want to minimize warpage. Possibilities of head and pillow or other defects, although those are less common with the micro BGAs. Putting on my old sacred cow, don't make your heating cycles any longer than you have to.

For a thin board, you should heat up quickly, if it has micro components. Assuming you don't have anything big and heavy on the board, a relatively short reflow cycle on both sides will minimize any possibility of damaging this thin circuit board, or any of the other components.

As I always say, other than promoting solder wetting, high temperatures does no good to anything in electronic assembly. That is pretty much it.

The main thing is trying to support it properly in all of your processes, keep it flat and minimize warping, particularly for the second side reflow. What about you brother?

Phil
Well, let's just elaborate on what my brother Jim has said here. Yeah, particularly in that first reflow excursion you want to maintain adequate support because, besides the ultimate warpage, remember you have to process that second side and you want as flat coplanar surface as possible when you go to printing, and of course pick and place.

So again, if your oven has a board support system that actually works you should use it. You might look into material, that FR4.

I'm not that familiar with that particular nomenclature on that one. Something with a decent T sub G that will support the temperatures going through. I am assuming this is lead-free, so that throws another wrench in the works. The higher temperatures, which as my brother was saying beware.

Anything you can do to minimize your exposure at the higher temperatures. As Jim said, the better off you are.

Because it does no good. Grammatically correct or not, but it is still thermally correct.

Jim
And not to mention the creation of intermetallics. Assuming you are soldering to a copper board and not an ENIG board every time you go through that reflow cycle you are building that copper tin inner metallic.

We don't want it to get too thick because it becomes brittle and has other bad effects on the solder joints. So again, don't get it any hotter any longer than you have to.

Phil
I think that answers the question, a decent question. Good luck. I know we threw all of those factors at you, but it is doable.

And there are a lot of people out there doing a lot more challenging stuff, if that makes you feel any better. Beyond that, please be aware that Board Talk has not been FDA approved.

Possible side effects may range from enlightenment to bewilderment, to possible derangement. But whatever you do and whatever thickness your substrates are, please don't solder like my brother.

Jim
And don't solder like my brother.

Comments

I am curious about the statement "FR406 doesn't mean anything specific". It is a specific material, made by a specific company. Yes it isn't the most relevant info (surface finish, etc, would be more relevant). But it is specific.
Philip Eng, CML
Great discussion and topic! Thanks.
Tim, BSC
Run the side with the lowest component weight to pad area first. Assuming that you don't have ceramic BGAs or other components with a high weight to lead area ratio on both side, you should be okay. As always with BGAs, make sure you have a robust profile so as not to introduce warpage or head in pillow.

Robb Spoerri, USA
All of the above suggestions are great. However, if yields are low due to deflection (warping) or bottom-side parts detaching, you might consider engaging a rework partner for localized rework of only the problematic BGA(s). This will of course add cost and reduce efficiency, and it sounds like an admission of defeat. But depending on the number of boards involved, your time constraints, and the severity of the problem, it still might make sense. (Full disclosure: I work for a rework provider.
Alan Couchman, Process Sciences, Inc., USA

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