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Long Soak Times for BGA soldering?

#29346

Long Soak Times for BGA soldering? | 1 July, 2004

I'm having a philisophical discussion with my boss regarding long reflow soak times to improve soldering under BGA devices. Is there anything to be gained by extending soak times at the 150C level from 60-90 seconds to 5-10 minutes? I've never seen times this extended and am wondering if they make sense in some situations? The boards in question are not terribly difficult, .063" thick, 14 layer, 12x16 inches with maybe a couple thousand components on top. Any comments on this practice?

Thanks in advance,

Rick Thompson

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#29349

Long Soak Times for BGA soldering? | 2 July, 2004

Never heard of this either. We'd be concerned about your flux loosing effectiveness over such a long soak and that would result in poor solder connections on all components on your board.

Just what is the problem with your BGA that you are trying to correct? What are the particulars of your processes and materials?

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#29351

Long Soak Times for BGA soldering? | 2 July, 2004

I agree, the flux will probably burn off before it gets to do its job. Def not a good idea....

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#29352

Long Soak Times for BGA soldering? | 2 July, 2004

X-RAY, X-RAY and X-RAY..

to optimize your reflow process when using BGA's you have to use your reflow profile hand to hand with your x-ray process.

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KEN

#29359

Long Soak Times for BGA soldering? | 3 July, 2004

so what did we do before X-ray? sound process shouldn't need to rely on inspection.

Extended soaks can be used to burn off high temp volatiles but I have never heard of 5 minutes! I have used 90-120 sec. to reduce solder splatter....but haven't done this in years as the modern NC fluxes are so much better than they used to be.

I had this discussion today with an Mfg. Engineer from a leading computer manufacturer....

He wanted to extend time above liquidous from 60 sec. to 90 seconds on a complex lead free assembly to reducue an open solder condition on a butt termination component with exposed copper lead frame (1 per board). My thoughts were...if it ain't wetting in 60 seconds, it won't in 90!

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#29360

Long Soak Times for BGA soldering? | 3 July, 2004

Not necessarily on inspection but in reliability. Do you want to ship product that will last only a few years or product that can last more than five years? X-Ray process will let you know if you have exposed your BGA's to an aggressive profile or a not so good reflow profile.

Thanks

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KEN

#29366

Long Soak Times for BGA soldering? | 3 July, 2004

??? Two assemblies assembled under the same conditions with identical x-ray results can have dramatically different life expectencies. Operating environments will have a direct affect on the product ie. hunidity, temperature swing, temp. duration, vibration caustic gas....

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#29375

Long Soak Times for BGA soldering? | 5 July, 2004

Hey, this is something that I've looked at and published a paper on back in 2003 at Apex, actually pert of one, the other part was lookign at how effective X-ray systems (lamanography systems) were at detecting & measuring BGA void size accuratly (conclusion... erm... no not really they're not that good !) Anyways, soak times.... Dependign on your paste you can go from the 60 to the 90 and even to 120 if need be and get good results, over that there's no difference over 150 you will have killed all the 1st level fluxes and yoru soldering will degrade. The main benefit of the longer soak is around the reduction of the flux volatiles at the 1st level i.e. those that burn off between 120 - 160 deg depending ont he particualr chemist developing the solder paste and company... mostly they will not give you a definate time / window as you can reverse engineer the paste from that. The burnign off of the flux will reduce the amount of gassing occurrign during reflow when the solder ball / paste is molten adn will infact reduce voiding int he devices. (not the selection of sn/pb or sn/pb/ag pastes vs ball material will also increase or decrease voiding) The other side is that when you take the board / device into reflow you really ahve to think about the other top end fluxes and the age of yoru devices etc and how long you want to be in reflow. If you tried a 5 to 10 min soak profile, so let's say yoru around 150 deg C, your goign to kill yoru paste, but also I would think have issues with the PCB in term's of x,y & z axis expansion not to mention a good final bow and depending on where the BGA is on the board no interconnection or a large amount of stress in the joints.

John

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