Calculating the exact amount of solder paste needed for a given circuit board or for a full production run can be difficult for several reasons, including:
- print deposit variations
- paste left on the stencil and squeegees after the build is complete
- bead size needed for the build (which depends on the squeegee size)
The theoretical volume of solder paste can be calculated for each board using the Greely Formula and a simple volume calculation.
The Greely Formula:
Specific Gravity of the flux vehicle is generalized to 1.
Example:
- Solder Paste: SAC305, Indium8.9HFA, Type 4.5, 87.75%
- Aperture Size: 0.012” Square
- Stencil Thickness: 0.004”
Solder Paste Specific Gravity = 4.14
Volume for this aperture can be calculated using the following formula:
Length x Width x Height
0.012” x 0.012” x 0.004” = 0.000000576 inches3
To get the theoretical weight of the solder paste for the 0.012” square aperture you must multiply the solder paste theoretical volume by the solder paste density.
0.000000576 inches3 * 4.14gm/cm3 = 0.000009439cm3 * 4.14gm/cm3 = 0.0000391gm
To calculate the theoretical amount of solder paste that will be used for each board, the weight of solder paste for each aperture on the board will need to be calculated. Once all of the weights have been calculated they can be added together which will result in the amount of solder paste per board.
Of course this is the theoretical value and not an actual value. The easiest way to determine the actual paste weight per board is to weigh a board before the paste has been printed and then again after the paste has been printed. The difference is the actual solder paste weight or consumption of solder paste per board. Of course, there will be some margin of error even in this calculation due to the weight tolerances of the board and the variations in solder paste deposits from print to print.
Let me know if I can help you calculate or estimate the amount of solder paste your project will consume.
Chris