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How to do with tombstoning for component '0402'

speedy-tech

#2586

How to do with tombstoning for component '0402' | 31 October, 2000

Now we are face with tombstoning for component '0402',we have tried many ways,it seems no change,Below is our experient: 1,The thickness of stencil is 0.12mm,and the aperture is circle.the diameter is 0.55mm.The pads size is 0.5*0.6. 2 Change thick of stencil from 0.12mm to 0.1mm. 3,Change profile of the oven, Can you give us your opion about this problem as soon as you receive our mail. Thanks&Best regards

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DL

#2587

Re: How to do with tombstoning for component '0402' | 31 October, 2000

Have You adjusted the placement properties of your pick n place machine/software for a 0402, I'm thinking x, y and add a little z. seems to work for me :) (as a temp/perm fix) thinking also about the numerous variables involved. Your printing process has been sound till now? Hope it helps.

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#2588

Re: How to do with tombstoning for component '0402' | 1 November, 2000

Have you tried the forum�s archive ( an abundant source for tombstoning related things ) and checked for the many things you found ? Immediate help just like snipping with your finger isn�t that easy. Best thing to help is take all the info you get from the archive and check step by step until you find a solution.

It�s already there, somewhere.

Wolfgang

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MRMAINT

#2589

Re: How to do with tombstoning for component '0402' | 1 November, 2000

We had some similar issues on one of our products.The oven profile is critical. We found that with a gradual soak ramp up it took care of about 80% of the tombstoning.The other thing that we found was pad design issues. Take a look at the sept. issue of SMT magazine has some good information. We also found that using paste with 3% silver also helped reduce tombstoning.

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CHRIS MAY

#2590

Re: How to do with tombstoning for component '0402' | 2 November, 2000

Tombstoning primarily occurs due to the fact that the paste may melt and wet the metallized face of the chip earlier or better at one end than the other. This could be due to a difference in heat capacity (thermal shadowing ?) or that one end of your 0402 is more solderable than the other. It could be that there is more paste at one end, so during reflow more surface tension would be present. Check for any vias that could be robbing one end of paste.

Regards,

Chris May

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bierleinb

#2591

Re: How to do with tombstoning for component '0402' | 2 November, 2000

I would check trace-pad interactions as you may be robbing heat from one of the pads causing uneven heating during reflow. Also, measure paste deposits on trouble parts to see volume depsoted across the pads. You should easily be able to do 0402s with a 5 or 5.5 mil stencil(.197 or .217mm) stencil. Also think about using round or oval pads - not just stencil apertures.

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Erdem Bilaloglu

#2592

Re: How to do with tombstoning for component '0402' | 2 November, 2000

We are using semi-circular land pattern and 0.004" smaller appertures on screen. With stable rising slope (max:2.6degC/sec)we have not experienced any tombstonning so far.

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Dason C

#2593

Re: How to do with tombstoning for component '0402' | 2 November, 2000

For 0402 or less, it is prefer to start on the pad design but it doesn't help right now. Senju Metal say that they have develop one of the paste can reduce the tombstoning problem but I don't have chance to test it. Please try and post your finding.

Good Luck! Dason.

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Philip A. Reyes

#2594

Re: How to do with tombstoning for component '0402' | 3 November, 2000

Hi there!!!

Actually, all mentioned ideas were significantly related to the cause of tombstoning. But, based on what DL said, you have to check the placement of your 0402. Coz, based on experience, i evaluated the different variables for the rootcause of this defect and one of the main rootcause is mounting accuracy. This means that placement of 0402 must be 100% with +/-5% aligned to the pads. Unequal distribution of component body/termination on the two pads causes this defect, because the surface tension in the other side is greater than the opposite side(not equal, if the component was not placed properly on the center between two pads) that tends to pull the component to tombstone. As an advise, look first on your process capability, try to figure out first your process before changing design, materials...etc. Use 4M's and 1E during problems like this, materials will always be the last thing to consider. FYI.

Phil

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Dean Stadem

#2595

Re: How to do with tombstoning for component '0402' | 6 November, 2000

All of the replies have merit. Here is another one. Because the cause of the tombstoning is wetting forces that are imbalanced, and one end wets faster or more readily than the other, the 0402 lifts up on that end easily because of its low mass. Increasing the soak time or raising the soak temperature helps, because then there is less of a delay between wetting from one pad to the other. Here is another tip that has helped me several times in the past. I purposely place the 0402 packages at a slight angle on their pads, plus or minus 15 degrees. This leaves only a corner of the end termination in the paste. What this does is make the solder paste be more fully wetted before it exerts enough pull on either end to move the part. By the time the paste is fully wetted to start pulling the 0402 into alignment, usually both ends are fully liquid, and this tends to even out the surface tension on each end. It works for me every time when I get pads that are rectangular, are too large, or have ground planes or heavy traces connected directly to the 0402 pads that prevent simultaneous reflow on both pads. Try it, it should at least help.

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Kevin

#2596

Re: How to do with tombstoning for component '0402' | 6 November, 2000

Masking thickness can also play a factor in tombstoning (or the Manhattan Effect). If the pad is 2 mils or more below the surface of the masking this can create a problem. Depending on the thickness of the solderpaste deposition, the solder height (after reflow) may actually tend to be below the soldermask. This causes the soldermask to be a fulcrum (or pivot point).

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