Indium Blog

The Correct Formula for Solder Alloy Density

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  • Phil Zarrow: Ron, your career has taken you down some very interesting paths.
    I remember one of your more popular blogs was the result of you becoming the oldest senior longbow puller. You got a new world record for that. Now, what are some of your other posts.
    Dr. Ronald C. Lasky: By far the most popular post has been calculating the density of a solder alloy.
    Phil Zarrow: Interesting. Has it—you've raised some controversy?
    Dr. Ronald C. Lasky: Well, most people think that to calculate the density, say you've got an alloy that has two metals in it, then you take the mass fraction of metal one and multiply it by the density of metal one and add it to the mass fraction of metal two and multiply it by the density of metal two. Let's call it equation one. That gives you the formula.
    Phil Zarrow: Okay. Sounds right.
    Dr. Ronald C. Lasky: But it doesn't work because density is mass divided by volume. The only way the equation like that would work is if you multiplied it by the density fraction, which we don't have.
    So, the actual formula is a little complicated and requires a derivation. This is discussed on my website.
    Phil Zarrow: So this has really raised a lot of interest and discussion.
    Dr. Ronald C. Lasky: It's almost comical. I get probably, at least one email a month and this has been over a period of seven, eight years, asking either for the formula or, "Isn't equation one right?" I say, "No, the derived equation, equation two, is right."
    Phil Zarrow: Wow. Who would have thought? So tell us about some of the love letters that you get.
    Dr. Ronald C. Lasky: Well, there's a, probably the most comical was a PhD from a company that we won't name who said, “I'm Dr. Such-and-such, I've been working in this company for thirty years and I've always used equation one. Are you meaning to tell me equation one is not right?”
    Phil Zarrow: And your answer was?
    Dr. Ronald C. Lasky: My answer is, "Yes, equation one is not right." But one of the other that I found even a little more surprising, is a professor from a junior college sent me a note just wanted me to know that she was a believer in equation two, but that she had a textbook that she was using in a class she was teaching and the textbook used equation one. So, even in something like that, that you think would be vetted, there can be mistakes.
    Phil Zarrow: Well, it doesn't surprise me, but you're getting your story out there. We are seeing the belief and blind following of equation one dying down?
    Dr. Ronald C. Lasky: I think so. I posted another post. I've posted two explaining posts why equation one is wrong and I posted one recently, because I spent a lot of time thinking about how to explain that equation one is wrong.
    Phil Zarrow: Right.
    Dr. Ronald C. Lasky: I posted another post that I think finally has caused people to be believers in equation two. I've gotten less people complaining about equation two. I should point out that I developed an Excel spreadsheet that calculates alloy density that's available at
    Phil Zarrow: So we can find the rest of the story at your blog.
    Dr. Ronald C. Lasky: Right. I guess I should encourage any equation one believers that are still out there, contact me at and we'll have an interesting discussion.
    Phil Zarrow: Great. Thank you, you heretic. Keep up the great work. Thanks, Ron.