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Reflow Oven vs 0402 Tomb Stone



Reflow Oven vs 0402 Tomb Stone | 10 June, 2001

Everybody aware of the tomb-stone problem on 0402 package. I've done everything I could i.e. Profile, placement, 10 Deg placement, stencil design, but still this culprit exist. One things for sure is taht I have a very bad pad size for 0402 ( 25 mil X 27 mil with the gap of 16 mil ), but before I propose the change in land size, there is one concern that I have over my oven. The oven used is 3 zones oven. What does this effects Tomb-Stoning comparatively if they were run on 7 or 8 zone oven. ( Flexibility in profiling ? )

2nd Question, I used to work with oven with non-adjustable air. The machine's has their own amplifier that amplifies the required amount of air. Theoven that I'm working on now, has regulators with mercury indicators. How do I know what is the best setting for "these" air, in order to get the best out of my machine before concluding that my current machine is a piece of crap.

Inputs are welcome. Thanx.


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Reflow Oven vs 0402 Tomb Stone | 10 June, 2001

Hello Danial,

Leave it up to most to blame the relow oven for their tombstoning problems when in fact it is usually the last to cause it. The biggest culprit of tombstoning is the pad design followed by the print quality. Usually, we find that the pads may be different sizes from one side to the other where the 0402 lays, this is a no-no. Another is the pad size, if it is to big when the solder liquifies it flows over the pad from the center to the edges. This causes surface tension and will pull at the 0402 edges. Tombstoning will occur when one side of the 0402 is pulled with greater force than the other causing it to stand-up. Improper printing can also have the same effect. More paste on one pad will cause greater surface tension on that side. These are just a few things to consider before blaming the oven.

Air flow settings in your oven. Most all Air amplifier ovens have flow-meters to control each zone. BTU was the original developer of the air amplifier for use in convection reflow. Unless the oven was manufactured with automatic flow control valves or hard plumbed with fixed shims for airflow it should have flowmeters. Anyhow, The best setting for airflow is the strongest setting you can utilize without blowing parts of your product. The profile created will determine the best setting. Airflow is used for heat transfer and is designed to adjust so you can eliminate delta t's from component to component. BTU uses static pressure monitoring in setting up their ovens. Airprssure is measured for each zone in inches of water. they had a standard spec but has since through out that idea. To feel more comfortable, the setting most commonly used for ramp zones is 2-3" water, soak zones, 1-1.5", reflow, 2.5-3.5".


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Reflow Oven vs 0402 Tomb Stone | 11 June, 2001

I had the very same problem but, did not question my oven as I knew the profile was good and I should have had even heating. So I then looked at pad size and layout. When Layout and size were found to be ok. I attached a prob to each pad and ran a profile. I found that they heated at different rates and one side was just hitting liquidus. (Had to run the board at lower temps do to component constraints). Anyway found that 1 pad was tied to a very large ground plane 2oz copper which sat directly below it. Probably one of those 1 and a million cases but something to think about.

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Reflow Oven vs 0402 Tomb Stone | 11 June, 2001

Way to go CPI!

This is a classic example of design. Many people experience this when the have an 0402 close to a QFP or other large components. The pad closest to the larger component will become liquidous later than the pad farthest. Hence, a new tombstone to add to the collection..

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