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TOMBSTONE DEFECTS

DO YOU KNOW? WHAT KIND OF SOLDER PASTE IS BETTER IN ORDER TO... - Mar 11, 2003 by

MGARCIA

#23755

TOMBSTONE DEFECTS | 11 March, 2003

DO YOU KNOW? WHAT KIND OF SOLDER PASTE IS BETTER IN ORDER TO AVOID THE TOMBSTONE DEFECT IN SMT PROCESS.

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genny

#23756

TOMBSTONE DEFECTS | 11 March, 2003

Some people have said that for smaller components like 0402, a 2% silver paste (62/36/2)has helped reduce the tombstones. The reasoning is that this paste has a longer plastic zone compared to a true eutectic solder, and if there is any uneven heating, it allows both pads to start getting liquidy before the reflow actually takes place, so the component doesn't get pulled toward one land as much.

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#23766

TOMBSTONE DEFECTS | 11 March, 2003

Good Afternoon,

We have found that the occurance of tombstones is usually related to other parameters. Board design, placement accuracy, paste tack time and thermal profile are the most common. We do use a 2% silver paste to process 0201 components for the increased plastic range but most of the time we find it can be controlled through profiling.

Hope this Helps

Chris

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mr

#23789

TOMBSTONE DEFECTS | 13 March, 2003

Ditto on the profile/reflow oven...a couple years ago, we had a huge increase in the number of tombstones found post-reflow. Profiling the oven showed a decent curve with some innocuous minor deviations, and further investigation of the oven found 2 totally clogged air amps above the plenum (old oven). They were clogged with flux, of course, and were somewhere in the rear zones (critical). fyi, our tombstone defect rate (front end and backend combined) is 3 PPM or less.

If you have older ovens, check the airflow paths, or on newer ovens, check the fan speeds to ensure enough impingement (heat transfer) on the assembly.

Marc

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#23792

TOMBSTONE DEFECTS | 14 March, 2003

Hi, About Thombstone defects: 1.Placement accuracy-main reason 2.Soldering-should be right on pads.The print should have regular shape.The quontity of the solder on both pads on one component have to be equal. 3.Reflow: the profile have to be made very precise(otherwise thomstoning could appear)

About the paste:recommended with silver 1.Sn62Pb36Ag2 2.Lead free SnAg or other variations Good luck!

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MA/NY DDave

#23793

TOMBSTONE DEFECTS | 14 March, 2003

Hi

I got thinking about this and did some reading.

First as a few have already written you should look at much more than just the paste. The paste composition is probably the last of the items tackled. It looks to me like that is close to the last item to be addressed. Even paste height might be a higher effect.

So depending on the size of your problem and the lost money you could turn into revenue I would:

1>Hire a Consultant to look at everything (Again lots to be looked at and worked... (Starting right at Board Design 2>Buy or borrow some of the Books on SMT and read about all the potential contributors. Yes Uneven or Goofed Up Oven Reflow as noted already is a potential big problem. With a Bad Board design it is even worse. 3>Buy or Borrow an IPC Design Guide and see if your designers, designed the PCB up to best practices. 4>Other ideas yet that's enough.

It seems without knowing the size of your problem that: a>Board Design b>Placement Accuracy c>Reflow (if goofed up) might be the major contributors. d>Lots and Lots more reduction ideas

If you still want to work only the paste, talk to the application person at your paste supplier. Several of them are excellent. In addition to the paste Genny listed there are other eutectic pastes that provide the same advantage as 2%Ag does, by slowing the flowing or wetting so that one side doesn't grip and yank the one terminal too much.

YiEng, MA/NY DDave

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Grant

#23803

TOMBSTONE DEFECTS | 15 March, 2003

Hi,

We had tomb-stoning on a product, and it was quite severe. We eliminated it by reducing the amount of solder paste on the small passible component pads. We used a 5 thou stencil, with 20% reduction, and it eliminated the problem.

Regards,

Grant

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MA/NY DDave

#23806

TOMBSTONE DEFECTS | 16 March, 2003

Hi

Good, that is one of the things I see some have done.

With paste too thick these little components float too much off, rather than centering.

YiEng, MA/NY DDave

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#23819

TOMBSTONE DEFECTS | 18 March, 2003

I like this answer!

Part geometry, part mass, land pattern, print accuracy, reflow profile, placment accuracy, and aperture shape all play a role.

If you use a homeplate to reduce solder balling, have high profile/low mass parts AND place more towards one pad than the other (increased downward forces on one termination while on the other end the forces are decreasing), you're asking for trouble.

In other words, once you've eliminated all of these as the root cause of your tombstones, you might wanna look at one of those spiffy Ag2 pastes....or, use it as a band-aid to cover up all the other problems....hmmmmmmmmmmmm. Nah.

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RDR

#23822

TOMBSTONE DEFECTS | 18 March, 2003

Here is what I have been finding lately. We switched to a 22mil square pad on 40 mil centers (from the IPC "75/25"). ALL OF OUR TOMBSTONES HAVE GONE AWAY! (at least for now) we did not change profiles, paste, or any other parameter. Obviously placement is still important but; it appears that at least in our shop the new pad geometry has opened the process widow tremendously in regards to placement and reflow profile.

Russ

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Ren� Sandoval

#23849

TOMBSTONE DEFECTS | 20 March, 2003

Hello Garcia:

For my experience in Tombstone problems I recommend you to check the next points:

-Use Solder paste with 2% silver -Use 5 mils Th Stencil -In some machines , for 0402 or smaller componentes , you require an special holder for the feeder to avoid the movement at the time the Nozzle pick the component from the Tape ( In Siemens MAchines you have to Use Spacers in each feeder ). If not , your process will be very inconsistent in placement( sometimes ok, sometimes bad).

It helps me a lot . Saludos

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MA/NY DDave

#23855

TOMBSTONE DEFECTS | 20 March, 2003

Hi

I understand that more circular or rounded pads also reduce the effect.

I love it when many independent variables can alter the outcome.

YiEng, MA/NY DDave

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MA/NY DDave

#23856

TOMBSTONE DEFECTS | 20 March, 2003

Hi Rene

Great, I am glad to see that you are recommending by experience placement control as affected by the pick operation. Neat to see you are also noting a thinner paste deposition.

YiEng, MA/NY DDave

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kenlchin

#23942

TOMBSTONE DEFECTS | 27 March, 2003

additonally, minimizing the rising rate aroud 183'c and decrease the droping rate(near 0.8'c/s) is also a good way .

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MA/NY DDave

#23945

TOMBSTONE DEFECTS | 27 March, 2003

Hi kenlchin,

Yes,

This all helps with these little components that can be affected by the forces created during soldering more so, than their own stabilizing weight.

On this tread which started with a composition only proposition, we have seen that a lot of other process or product tunings can improve this defect/enhance yield.

YiEng, MA/NY DDave

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paul

#23947

oven profile | 27 March, 2003

can any one tell me if you should run the same heat on the same boards no matter what deffrent parts are on the board Should you do a oven profile for every deffrent board or is it the thickness of the board that matters.

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Jon Fox

#23948

oven profile | 27 March, 2003

Just to be sure, you really should. PCB thickness and component density are the key players.

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MA/NY DDave

#23950

oven profile | 27 March, 2003

Hi

I hope I am replying in the correct tread down link.

J.Fox is right. Know the Assembly / PWA that you are processing.

YiEng, MA/NY DDave

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#23952

oven profile | 28 March, 2003

Hi, Look there are some important factors.The height is first of them.Component density too.One very important thing-component type.There are some packages(might be with big pads,too heavy components,sensors,BGA's .....or some not so common,not standart parts)wich define the right profile. About density:if high -more parts ->more heat.But that's not correct!If convection there are air flaws and bare board has lower temperature than a board with high density.When you have elements on board you hava a vortex and the air flaw invreases the temperature.

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MA/NY DDave

#23962

oven profile | 28 March, 2003

Hi

Yes I agree, yet this subject is about Tombstoning and oven profile or even oven working correctly across the entire product is a sub-set of how one gets those little chip babies to sit and stay where they belong. Flat and Heeling like their masters wish.

So BGA is not quite a Tombstoning concern.

YiEng, MA/NY DDave

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Alex Yalung

#24020

TOMBSTONE DEFECTS | 4 April, 2003

Hi All,

The best solution we made on our tombstone defects is component Z dimension set at pick and place machine. Usually for 0402 components Z height is pre-set by the machine at 0.5 mm, by reducing it to 0.35mm we have solve our tombstone problem tremendously. Basically, the mounting is push down a bit more by the nozzle.

Alex

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Thomas

#24127

TOMBSTONE DEFECTS | 13 April, 2003

I saw some specifications on the Tamura RMA-012 FP stating that it counters tombstoning...hope it helps you problem...

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