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Calculating Solder Volume

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#52384

Calculating Solder Volume | 8 November, 2007

Can someone point us in the right direction?

We are trying to find a formula to determine the "cubic mils" of solder we end up with after reflow. (No components during the reflow) Some of the discussions we've had about it produced the thoughts below.

- A formula based on the stencil thickness, aperture opening and type of solder paste should be a good place to start.

- 1/2 of the paste volume is lost from the flux and carriers being removed during the reflow.

- Someone must have a formula for this so post it in the forum and see what happens!! (my idea!)

Any help we can get is appreciated.

Matt Kehoe

770-475-4576

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#52385

Calculating Solder Volume | 8 November, 2007

Why? Just use IPC on finished solder joints as your guide. I've seen groups of people puts hundreds of hours into this task and then tell me a 3 mil stencil is all I need. After reflow, none of the solder joints meet IPC Class 1 and a lot of tombstoning. So then they go back and tell 4.5 mils is what I need. Better joints but still more tombstoning. Their third attempt was a report that Process Engineering was being resistive and skewing the tests. Yet we still use 6 mil stencils and meet IPC Class 3 standards. I don't see how anyone can mathematically design a single stencil for several different types of components that require different amounts of paste to solder properly. It really boils down to experience. Experience with your boards, your process, your machines, common sense, etc. Math can get you near the ball park, but experience gets you inside.

Now, if your goal is to save money on paste or you're just trying to get your "Green Belt", you should look else where. IPC was designed to get rid of this kind of nonsense run around engineering; which we see a lot of today. Although I've heard of guys getting Green Belts for designing a stencil aperture. All it takes is button down Oxfords, shinny shoes, and Power Point presentations. Oh and don't forget your buzzwords

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#52389

Calculating Solder Volume | 8 November, 2007

Wow, that is a lot of info so early in the morning.

What we are attempting is way less complicated than all that.

1 surface mount footprint, printed and reflowed without a component. Just a fused solder deposit on each pad.

All pads the same size and not that fine pitch. Just trying to get a calculation in cubic mils (.001 increments) based on an amount of paste printed etc etc.

Example; .005 stencil, .020 square stencil opening, normal print parameters.

How much solder on each pad after reflow?

mk

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#52391

Calculating Solder Volume | 8 November, 2007

Sorry Matt,

Just hate when people use this site as their engineering bed.

I believe you can weigh your board before and after reflow. It would have to be a sensitive scale though. From there you can calculate weight to volume ratios.

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#52397

Calculating Solder Volume | 8 November, 2007

"Their third attempt was a report that Process Engineering was being resistive and skewing the tests."

Thanks for the early a.m. laugh. I needed it.

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#52398

Calculating Solder Volume | 8 November, 2007

"1/2 of the paste volume is lost from the flux and carriers being removed during the reflow."

Don't forget the intersticial space between your solder spheres that you lose during reflow.

Not being of an engineering background I couldn't begin to address it myself, but this whole thing sounds like a big ol' turd hunt anyway. Why not just work it backwards like Hussman said?

Experiment and figure out what paste brick shape and size works for the application (do you have optical paste print inspection capability to give you an accurate volume?), then determine which aperture size gives you the best chance at achieving it.

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RDR

#52400

Calculating Solder Volume | 8 November, 2007

Just how accurate do you need to be Matt? you seem to already know the answer, calculate the volume deposit from stencil aperture and divide x 2? that will get you very close wouldn't it?

Russ

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#52403

Calculating Solder Volume | 8 November, 2007

Yes, we just wanted to make sure we weren't missing anything. Thanks for the feedback.

mk

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#52410

Calculating Solder Volume | 8 November, 2007

In the marketing description of your paste, look for "percent metal by volume." This percent metal by volume figure gives you a good approximation of the amount of metal in each solder paste brick that you print on your board. In most solder paste formulations, percent metal by volume is close to 50 percent. The remainder is flux, activators, solvents, and the magic goo that makes paste get jiggy.

Gotta go shine my shoes now, bye.

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#52419

Calculating Solder Volume | 9 November, 2007

Thanks to all. We appreciate your feedback.

mk

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