Something that seems counter-intuitive to many customers, yet seems obvious to a rheologist is why the percent metal loading of solder paste varies so much. The most crucial control variables are:
- Alloy type
- Powder size
- Application / usage
I'll tackle the two last points in a later blog entry, but the first point is fairly simple to explain. Assuming that solder powder is formed of identically-sized, unreactive, perfect spheres, the viscosity of the solder paste will depend only on the VOLUME of solder present, and is therefore related to the alloy density. This finding goes straight back to one of Einstein's first papers* where he showed that, for dilute dispersions of identical spheres, the relative viscosity (the measured viscosity of the suspension divided by the viscosity of the carrier fluid), is 2.5 (the so-called 'k' factor or Einstein constant) times the volume fraction. The picture (right) shows the definition of the volume fraction.
As can be readily seen: the metal density of solders can vary from 6.5 to 14.5g/cm3, and changing the metal density therefore necessitates adjusting the metal weight percent accordingly. This also varies with the usage:
1/ For printable paste: a 6%w/w spread is possible
2/ For package-on-package paste: over 10%w/w spread in metal loading may be needed
For both 1/ and 2/, of course, the goal is to maintain the SAME volume fraction, so the rheology of the solder paste remains the same.
Simple algebra will allow you to derive an equation so you can plug in any density of alloy for a solder paste and calculate the required weight percent of that metal. I can email you the solution if you're stuck, just click on the "Contact Me" button (left).
Also note that changing the flux will not only change the density (flux densities can range from 0.85 - 1.05g/cm3), but will also change the rheological properties of the paste significantly. Quick plug: Ron Lasky was good enough to give me a chance to discuss solder paste rheology a few weeks back, and there will be more about this topic in the coming months.
Cheers! Andy
* A. Einstein, "Concerning the motion of particles in quiescent liquids as required by the molecular- kinetic theory of heat," Ann. Phys., i_7549-560 (1905