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BGA rework: Coplanarity of Xilinx before and after reflow.

#34266

BGA rework: Coplanarity of Xilinx before and after reflow. | 16 May, 2005

We have a customer that has been trying to place the Xilinx part number XCV2000E-6FG1156I which is an 1156 ball plastic BGA. When we do the X-ray and inspection we find that the package is warping up on all four corners of the package, Like so: Top ) PCB. This is taking place in a 7 zone oven with profiles that give perfect results with similar packages. The removed and reballed BGA as well as a new BGA do not sit flat on a granite block. Xilinx says a coplanarity tolerance of 8mils is acceptable. Has anyone ever had similar issues or experience with this part?

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jdumont

#34271

BGA rework: Coplanarity of Xilinx before and after reflow. | 16 May, 2005

We have the exact same issue with out 456 ball Xilinx parts. The corner balls either do not touch or are so stretched out that they are intermittant. I believe this to be a moisture ingress problem. Ive been setting up our little batch oven this afternoon to bake a tray of these parts according to J-Std-033 by tomorrow. Hopefully this will prove my theory. Ill let you know how I make out...

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#34274

BGA rework: Coplanarity of Xilinx before and after reflow. | 16 May, 2005

This has been a problem with new components coming directly from vacuum sealed desiccant packed moisture barrier bags with moisture level cards. We have also baked all boards prior to the BGA rework to ensure no moisture issues were addressed.

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#34285

BGA rework: Coplanarity of Xilinx before and after reflow. | 16 May, 2005

We believe your problem is caused by the mismatch in CTE of the material ised in your BGA. In fact, this "potato chipping" [where the corners curl-up] is present in all BGA. The issue is the matter of degree. [Mismatching CTE is how old thermostats work. Two metal strips with different CTE are bound together. As the temperature increases, the metal with the higher CTE will lengthen more than the other metal. Since the two metals are bound, the longer metal must make up the extra length by taking a longer path, or in other words, the longer metal will bend the strip.]

To prove this is correct, watch your PBGA in your rework machine. You will see it "wiggle" as the corners move from curving up to curving down to curving up etc. [Admittedly, a rework station does a more radical job of heating BGA than a reflow over, but you get the idea.] Again, this is due to the CTE mismatches before reaching Tg temperatures warping the BGA in one direction. Once either the plastic or laminate reaches its Tg, it will either exacerbate the problem or make the BGA warp the other direction. And finally, once the both the plastic and laminate reach their Tg the BGA will warp again in the direction of whichever CTE dominates.

You should find a new supplier that does a better job of selecting and controling the materials used in their product. Haaaa pretty funny, eh? Well, since that's not one of the choices, you need to minimize the problem by changing your reflow profile.

That being said, CTE is temperature dependent and not temperature change rate dependent, so you are pretty much hosed by the physics. However, we do like trying like hell to get the top and bottom of the BGA the same temperature before entering reflow. Moving into reflow slowly so the paste has time to become liquid before the BGA begins to dance. This means a long soak before reflow. Although, we admit this is probably wishful thinking since the physics of CTE doesn't care about the rate of temperature change.

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Bob R.

#34288

BGA rework: Coplanarity of Xilinx before and after reflow. | 17 May, 2005

We went through this with one PBGA supplier recently and they were able to get the problem under control by fixing some of their molding processes that were inducing residual stresses. You can really see it in a thermal moire where you measure warpage while simulating reflow. We were seeing the corners lift up as much as 0.007" at peak temperature leaving us with "ball in cup" failures and shorted thermal balls. The problem completely disappeared when the supplier found and fixed where they were inducing most of the stresses. PBGAs will warp just due to the CTE mismatches involved, but there is a lot the supplier can do to minimize it to an acceptable level.

There is a move afoot to create a standard to address this. I can't find the e-mail right now, but I believe it is through IPC and is being spearheaded by Amkor. I'll post details if I can find the e-mail.

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RDR

#34292

BGA rework: Coplanarity of Xilinx before and after reflow. | 17 May, 2005

I have had the same problem with this supplier, we found that one mfg. place or process used a .015" piece of FR4 as the base while others had .030" Fr4, We discontinued use of the .015" and never had a problem again.

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