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Surface Tension vs Part Mass

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bwet

#43869

Surface Tension vs Part Mass | 12 September, 2006

I have a thought question for you SMT process engineers out there.

We are attempting to have a part selectively drop off in a reflow oven on a 2-sided l-f SMT assembly. In order for this to happen we will have to make sure during the reflow cycle that the part in question falls off while the others remain.

The question is how does one calculate the force due to surface tension holding a part to the PCB while the solder is in the liquidus phase? If we know this then we believe we have a method to attach the right amount of weight to the device to make it fall off.

Thanks in advance for any surface scientists/experimentalists answers out there.

BWET

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C.K.

#43871

Surface Tension vs Part Mass | 12 September, 2006

There is a formula for that out there somewhere.

It has to do with the part mass and the area of solderable surface. If the part mass is too great in relation to solderable surface area, the part falls off in a d/s reflow cycle. Anyone remember that formula?

There really is no way that I know to measure this, but it'd be a great thing to invent. I'm envisioning a load cell tacked on underneath the part, and this load cell would be hooked up to a MOLE/SlimKIC device that has this type of measurement capability.

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#43872

Surface Tension vs Part Mass | 12 September, 2006

I found this searching the archives. Unfortunately Bob doesn't seem to have an active site anymore, just references by people he works for. It's a shame, because there was a load of good stuff there. Haven't looked at Phil's site for a while but it's worth checking out.

_________________________________________________________ Date: June 07, 2004 09:31 PM Author: davef Subject: Component size on double-sided PCB

That's thin board.

The test that you ran is the best method for determining the specific answer.

Bob Willis says, "Use pad mating to lead wetting area. Some have found the ratio to be as much as 44 grams/sq in."

Bob Willis and Phil Zarrow have posted papers on the topic on their sites. [You can find links their sites in the SMTnet Archives.] ____________________________________________________________ Also found this:

http://www.smtnet.com//forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=4184&#Message16487

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#43879

Surface Tension vs Part Mass | 12 September, 2006

I hope you don't mind if I comment Rob....one needs to read that article closely in that some of those numbers may apply specifically to SDSR. I can't read that formatting, myself. :(

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#43883

Surface Tension vs Part Mass | 12 September, 2006

Make sure you desicate the boards if they have MSDs on them.

I'm guessing you have a part on the board you want removed, but can't do it normally.

At one time I thought it might be slick to make a spring loaded Kapton tape "tent". The idea being that before reflow the sping load is pushing the Kapton tape up but it is stuck to the part. Then during reflow the spring makes the tape go up with the part.

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RDR

#43884

Surface Tension vs Part Mass | 12 September, 2006

what size part? I will almost guarantee that a 1/4- 20 nut will remove part during reflow if it will fit.

Russ

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#43888

Surface Tension vs Part Mass | 12 September, 2006

Russ: You say, "I will almost guarantee that a 1/4- 20 nut will remove part during reflow if it will fit." What are you talking about?

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#43890

Surface Tension vs Part Mass | 12 September, 2006

I assumed he intended to glue the nut to the part, turn the board upside down and run it through. Thing is, you might seriously have to crank up the heat to get it to reflow with that big sink on there.

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#43892

Surface Tension vs Part Mass | 12 September, 2006

Really Rocky? What's the meaning of Russ' "if it will fit" in your scheme?

As an alternative to your scheme, he could glue the nut on its side, tie a string through the hole in the nut, attach the other end of the string to the open door, and then slam the door shut just as the board hits the peak temperature, removing the component from the board. This could be called the 'not as tough a pulling teeth method.'

We thought that he pushed the nut under the part, so that as the nut expanded with the heat, it would lift [push] the part from the board surface.

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#43893

Surface Tension vs Part Mass | 12 September, 2006

Bob: Did you know that soldering equipment suppliers make these things that they call 'rework equipment'? This equipment employ these wacky pieces of metal, sized to each particular component, called 'nozzles'. Manufacturing companies use this rework equipment and nozzles to remove specific components from boards and leave the other components on the board. Wild, eh?

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RDR

#43902

Surface Tension vs Part Mass | 13 September, 2006

yep, steve is right, it was kind of a joke but I have attached nuts and bolts to QFPs when it was deemed that a lot of components needed to be removed from some assemblies in a quick amount of time. Fit just means that it will fit onto part. thought this may be helpful since the parts could probably be removed by nuts (pun intended hahaha)in the amount of time it took to research proper weight by calculation

Russ

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Rob

#43905

Surface Tension vs Part Mass | 13 September, 2006

Hi Steve, you're right, I pulled the wrong SMT NET article from my archive. :(

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#43908

Surface Tension vs Part Mass | 13 September, 2006

One can only speculate on the response one would receive from a maintenance tech upon discovering the hardware scattered across the blower baffles in the reflow zone. ;)

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#43909

Surface Tension vs Part Mass | 13 September, 2006

1/4-20 would be unlikely to fit on an 0603 is all.

I have to admit that when he first brought up the hardware theme I kept visualizing a jack screw assembly but the results I envisioned were less than ideal.

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RDR

#43916

Surface Tension vs Part Mass | 13 September, 2006

hahahaahaha no kidding huh?

Russ

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