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Printed Circuit Board Assembly & PCB Design Forum

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BGA Pull test

Mike

#31078

BGA Pull test | 25 October, 2004

I work for a PCB Manufacture and I am looking for info on BGA Pull test or strenght test.

I have a CM that is experiencing bga's with some weak joints. they are telling me that when they do their pull test the joints are breaking at the nickel instead of the pads removing.

This product is ENIG, the immersion measured in at 4.3 micro inches and the Nickel was 140 micro inches, We had the lab look for corrosion on the nickel (Black Pad) and did not see any, This board is .028 thick and see's three heat cyles at the assembly house 210 for 90 sec's each,

Bga is placed on first cycle, balance of components on Second Cycle and last cycle board gets bonded to another PCB.

Does any one have any suggestions what else could be causing the weak joints?

Thanks for your help,

Mike

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#31083

BGA Pull test | 25 October, 2004

First, any results of pull or shear tests are unscientific at best. [We pop our BGA from boards with, appropriately enough, a beverage can opener.]

Second, we have no have problems with your ENIG specification.

Third, as with your customer, we'd expect foil tearing and board-based distruction when popping BGA [or any other part] from the board. Breaking at the nickel tin intermetallic is not necessarily a bad thing. While we have no ENIG boards, in that case, we: * Don't expect to see a flat nickel plane for nothing. * Expect a rough solder surface attached to the pad.

So, let's assume that your customer is seeing nickel pads with no [or little] solder. Failure of this type does not have to be hypercorroded nickel [black pad]. From a board fab stand-point, oxidized nickel could result in similar problems. * Could the gold be pourous or unevenly plated for any reason? * Could the nickel have sat around for a while prior to immersion gold? * What are the results of the customer's in-bound solderability test of the this lot of bare boards?

Finaly, OK. OK. Let's give the poor fab a break. [Everyone blames the fab. They're just such easy targets.] You only mentioned the BGA popping from the board. * If this is a fab problem, other components should be just as poorly attached. * What if your customer [or their BGA suppler] changed their process? Maybe the alloy of solder between the paste and the BGA solder balls has changed and it's not as strong as it used to be.

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Bob R.

#31086

BGA Pull test | 26 October, 2004

When we first got into BGAs on ENIG we were getting joint cracking at in-circuit test. The joints were breaking in the Sn-Ni intermetallic. We did a lot of pull testing while working with our board suppliers and our conclusion was that pull testing is not a good indication of board or solder joint quality. If you pull test, for example, a 324 ball PBGA on ENIG you'll see 300+ balls break in the intermetallic and most of the rest will tear the copper from the board. Repeat the test on HASL, OSP, or silver and you'll get 300+ copper pads tearing from the board and the rest breaking at the top of the ball and leaving solder on the pad. Having the board supplier mess with the ENIG process will have minor effects, but the bottom line is that Sn-Ni is brittle so it is the weak spot, especially at high strain rates. What your customer is seeing (breaking in the Sn-Ni) doesn't surprise me and isn't an indication of a process or board problem - that's just ENIG.

Are the joints breaking in the field or are you catching them at a test in your factory? If you're catching some then I'd get out the strain gages and start working on reducing the loads in your processes - especially those where the loads are applied quickly such as ICT. If you really want a test to evaluate joint strength then set up a 3 or 4 point bend test. Don't use an Instron to drive it because they can't reach the strain rates you'll need to simulate ICT. Build a fixture with an air cylinder so you can apply the loads quickly (~ 20,000 ustrain/sec).

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Mike

#31099

BGA Pull test | 26 October, 2004

Hi Guy's thank you for the Info,

This is a New Assembly house that our OEM has Moved to and they have not been very open in discussing this issue other than they are telling us this is board related.

I have ask for samples of faild product with the chips so we can see what their seeing and they have not supplied us with that as of yet. So for know all I have to go on is what I see on a bare board and what the Enig looked like, We checked things like Solder mask Reg and cure and both looked good.

So for know I'm in the dark to what they are seeing, until they can supplie us with a Sample so I can send it out to a Lab to have it check..

Thanks again for your Idea's,

Mike

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