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Tombstones? Pb vs Pb Free

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#41435

Tombstones? Pb vs Pb Free | 9 May, 2006

I would like to know what you are seeing in production. I have read the papers that state fewer tombstones with SAC305 as well as the papers that state more tombstones with SAC305. I am seeing more tombstones with SAC305. I place a lot of 0402 components but am also seeing more tombstones on larger chip components also. A Google search is showing more articles stating more tombstones with SAC305.

What are you guys seeing in production?

Thanks,

Chris

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Chris (2)

#41438

Tombstones? Pb vs Pb Free | 9 May, 2006

Hi Chris,

We have only produced a few RoHS assemblies as of yet but we have not had one tombstone so far (knock on wood)

I'm hoping that will not become a problem. We have enough of them now!

Good Luck

Chris

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RDR

#41440

Tombstones? Pb vs Pb Free | 9 May, 2006

When we first started we had this problem, we eliminated by longer profile (less delta T) and then ultimately chnaged pad dimensioning for 0402 away from IPC standard.

i would highly recommend that you check your placement accuracy and your profile first before doing anything else.

Russ

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#41442

Tombstones? Pb vs Pb Free | 9 May, 2006

I can understand the theory behind fewer tombstones with SAC. If people were having tombstone problems with SnPb eutectic solder, often switching to Sn62Pb36Ag2 would improve it, because with the addition of some silver, a small plastic zone in the solder is introduced. SAC also has a plastic zone. However, I have no practical data to say absolutely that a plastic zone improves tombstone problems. I agree that component pad layout and temperature profile play major roles in causing or eliminating tombstone issues. I would look there first.

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Chris

#41451

Tombstones? Pb vs Pb Free | 10 May, 2006

Russ,

By a longer profile you mean a more even slope across the entire profile. Unfortunately I only have a 5 zone oven that is fairly short. How many heated zones are you using?

Did you shrink the 0402 pad design and leave less pad exposed out at the ends of the 0402? I used to work in the hearing aid industry and our 0402 pads were about 0.042" in length end to end. The 0402 just fit the pads. Never once had a tombstone. However, we had a much longer oven but still only 5 zones.

I could change the pad layout or stencil apertures. The pain is that I have over several thousand products. I will see what I can do with my profile. I do know that the placements are poor on one of my machines. The other machines place 0402 perfectly.

A paper from Agilent showed a huge increase in tombstones with SAC305. Kester says less tombstones. Cookson says increased tombstones.

Thanks again,

Chris

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RDR

#41458

Tombstones? Pb vs Pb Free | 10 May, 2006

Yes, we added some soak, we also use a 5 zone oven. We utilize a .020" x .020" pad with center to center at .042". We only get tombstones on misplacement or bad paste release now. Registration of component is key with pbfree, the parts just don't slide around anymore to their most balanced position as they did with pb.

We also believe that with SAC305 tombstones are more prevalent. This is based off a duplicate product running lead with zero defects and then when switched to Pb free tombstones "popped up", It was a profile issue which as stated did correct problem.

Good luck partner!

Russ

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