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BGA Corner

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#64449

BGA Corner | 10 June, 2011

Any suggestions as to what can cause a PBGA's corner to warp down during installation process on a rework system? A couple solder balls bridge at the corner. It appears at the pin 1 corner where the metal indicator of a BGA is exposed. I am wondering if there is a mismatch in materials and either during longer TAL (90 seconds above 183C) or a rapid cooling that corner would compress down causing solder balls to bridge.

Would there be a case when 2 or 3 solder balls at the corner can bridge from lack of flux? PBGA is being reworked and tacky flux rather than solder paste is used.

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Ken

#64455

BGA Corner | 13 June, 2011

Hello Steve,

i think you are experiencing the potato chip effect, where temperature difference at the center and edges are huge; causing BGA edges to tilt down or up.Try narrow down the temperature difference between the center of BGA and corner. It works for me.

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#64465

BGA Corner | 13 June, 2011

It's more likely the warp is due to material issues. I've spent way more time than I'd care to admit running Thermoire with various profiles and there's pretty much nothing you can do to reduce warp via profile unless your current profile is truly awful. The warp happens when the BGA is above the Tg of the various constituent materials which are all below SnPb or SAC305 liquidous, so changing the profile within the limits you have available will have no effect. Profile changes can help reduce ball-in-cup if you have balls warping away from the board during reflow because you can optimize so the flux isn't used up when they come back into contact, but profile won't solve the root cause which is the warpage.

I did have one case where the cause of the warpage was moisture and a bake took care of it. Every other time it required a change by the BGA supplier. Sometimes fixing the warp required a material change (molding compound, adding copper to the interposer, etc) and sometimes it was a process change (fixing the overmold cure profile, once found a tooling pin in a molding machine that caused stresses to get molded into the part). In every case it was like pulling teeth to get the supplier to address it. Most times it's lot dependent, so you might get lucky and it'll just go away. Good luck.

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#64469

BGA Corner | 13 June, 2011

We agree with ScottE.

Search the fine SMTnet Archives for background discussions. For further discussion on the "being hosed by the physics": http://www.cooksonsemi.com/tech_art/pdfs/Controlling%20Warpage.pdf

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#64471

BGA Corner | 13 June, 2011

Try putting a .010" shim under each corner during rework reflow. This will keep corner balls from shorting out.

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#64496

BGA Corner | 15 June, 2011

I have seen this before with larger PBGA components. When installed using a rework system, I would advise Convective rather thn IR. In my experience having high power (not high temp) convection is the only way to deal with these problematic devices when mounting at a rework station. I have watched a number of companies struggle using IR in this capacity with these particular devices, and also have completed successful mounting of these type devices in front of those same companies using forced air convection type rework stations with low (relative) temperatures and high power capacity.

That was some time ago and I am no longer in that particular business, but it is interesting to see the same old problems still resurfacing...

If you are already using Covective heating, you will need to control the temps carefully and not be in a hurry. Get some long lasting flux paste and ramp slowly up to reflow in a straight shot at about 1.5degC/min. That will help the different substrates of the BGA to stay closer in temperature and help mitigate the TCE problems with the part. Make sure the site is perfectly clean, skip the slolder paste, use tacky flux and control the ramp rate, especially from Flux activation to reflow. That part has to be slow. I counsel patience....

These things look like they are flapping wings sometimes LOL

Best of Luck

'hege

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