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Oven profiles. Linear vs. Saddle?

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#72816

Oven profiles. Linear vs. Saddle? | 27 September, 2014

I have a situation and was hoping to get some opinions on whether a saddle type profile is better or worse than a linear profile. We have an 8 heating zone 3 peak zone and 4 cooling zone single track oven that I am having a tombstone issue with. It's a leaded product and it's just the same LED 0805. We are having a nearly 60-70% fallout rate of tombstones that need to be reworked. I just thought I could get some feedback on this, I'm somewhat new to oven profiling and the theory behind a good oven profile. Thanks

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#72827

Oven profiles. Linear vs. Saddle? | 29 September, 2014

These papers are a good place to start when learning about profiling:

https://www.smtnet.com/library/files/upload/374_RSS_vs_RTS_revised_12-99_(no_pics).doc

http://www.ipcoutlook.org/pdf/best_practices_reflow_profiling.pdf

If you're having a lot of tombstoning, there are a couple of potential causes. As the RSS/RTS paper notes, the cause is uneven wetting forces...but, it doesn't consider the non-reflow part of the causes.

Some things to check for tombstones: -Registration of solder paste -Registration of parts. -Reflow profile.

If your solder paste is well registered, and your parts are well registered, then you're down to the reflow profile. As suggested, a slower ramp, and longer TAL can help. Make sure that you're in the flux activation temperature zone for the paste manufacturer recommended period of time; and make sure you're in the correct TAL period. If you're not hitting those, you'll have trouble trouble-shooting everything else.

If you don't have thermocouples to profile the machine, a good place to start is to call your paste manufacturer...tell them the type of machine, number of zones, and relative thermal density of the board, along with the paste chemistry, and they ought to be able to give you a "baseline" profile that will get you 85% of the way there.

Cheer, ..rob

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#72832

Oven profiles. Linear vs. Saddle? | 30 September, 2014

Check your pad sizes against the supplier recommendation. I have noticed that these LED packages are very light and easily move around during reflow. Human nature has us checking the profile and making changes when in fact it is usually something else. I recently had the same problem and it did turn out that we designed our pads using IPC criteria and the LED suppliers recommendation was very different.

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#72833

Oven profiles. Linear vs. Saddle? | 30 September, 2014

In 25 years of profiling, I have never seen a reflow profile make TOMBSTONES better. I have had "experts" come in an adjust our ovens to the "magic profile."

The AOI data has never shown improvement from profile changes. EVER...EVER.

As long as your parts are good, It's almost always component to land design , or thermal mismatch causing your issues. Look up the manufacturers data sheet for your LED. They will most likely offer land design recommendations

A ramp to spike profile works just fine in most situations. 4 minutes to peak temp. may be considered typical. Check your paste manufacturer guidelines for peak temp and time above liquidous.

Regards

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#72839

Oven profiles. Linear vs. Saddle? | 1 October, 2014

No one has mentioned paste, but I have tested pastes where some do much better than others at not tombstoning parts.

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#72849

Oven profiles. Linear vs. Saddle? | 3 October, 2014

1. Pad design. Most of the tombstones are result of wrong pad(land) design. One pad is significantly bigger than the other or the metal laying under one side of the part is much more than under the other side.

2. Part placement - should be in the center 3. Profile - linear 4. Solder paste choice 5. Direction of the board in the oven.

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#72871

Oven profiles. Linear vs. Saddle? | 10 October, 2014

Main issues... Pad Design Thermal Mismatch Design related to pads effected Moisture and out gassing in substrate / part terminations can move parts slightly during preheat prior to liquidous this get position shift ( smaller the parts more an issue) Placement offset Printing offset Profile is prob last on list.... Linear ramp to spike is always best for solder paste , saddle shrinks paste process window offering higher flux exhaustion prior to reflow - can cause other issues ( for open atmosphere reflow). N2 not issue as no reoxidation.

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