At Indium Corporation, we get a lot of questions about solder paste - its composition, its application, and best practice advice. Many of those questions often involve inquiries about the metal load of the paste. So, what exactly is metal loading?
Metal loading is the weight percentage of alloy in the flux vehicle. The metal load of a paste is very important in the case of both printing and dispensing. A different metal load is required for the two different processes. A printing metal load is higher than a dispensing metal load, the reason being you want the higher metal load for printing so that the paste does not slump after being printed on the PCB. The metal load is optimized for printing to get a balance between a low enough viscosity to be rolled across the stencil and a high enough viscosity that it doesn’t slump all over the PCB.
For dispensing, you are going to want a lower metal load so that you can get the paste through the needle tip. Again, testing is done to optimize the metal load for dispensing. A second part to dispensing is that you want to ensure that the powder size is appropriate for the needle gauge. The general rule of thumb is that seven powder particles need to be able to fit across the ID of the needle. For example, our Type 5-MC has a micron range of 15-25µm. You would take the larger particle size 25µm and multiply it by seven to get 175µm. For the needle gauge, you are going to want a larger diameter than that of those seven particles to prevent your needle from clogging during dispensing.
The metal load is an important characteristic of solder paste and can determine whether it prints or dispenses. Making sure that the paste works with your application, whether it is printing or dispensing, is important. If you have questions on this or need help finding what metal load works best for you, please feel free to reach out to askus@indium.com.