Calculating Solder Volume

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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"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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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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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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