Recently, a customer complained that he had a hard time correlating the wire bond sweep with material properties like spiral flow, filler content, filler size, etc. He was using mold flow analysis to predict wire sweep, but the results didn’t seem to make sense to him.
The reason for this is because these properties can be mutually exclusive, and combinations of these properties and result in vastly different properties.
Let me start with a basic example to make my point. Imagine you have a mold compound that is 80% filled by weight with spherical silica with an average particle size of 100um. If we did nothing else but swap out this filler a silica nanoparticle with an average particle size of 0.1um, what would be expect in material property differences?
Though both particle sizes may predict similar wire sweep, in reality, the nanoparticle-filled mold compound would be so thick that it wouldn’t be able to flow at all. Epoxy mold compound formulations often have multiple different sizes (from nanoparticles to several hundred microns) and shapes of filler including spherical, angular and other. These combinations are done exactly to optimize packing density, viscosity and flow.
Likewise, spiral flow itself is not a measure of viscosity. It is a measure of, well… flow. Spiral flow measures the distance a mold compound will flow when exposed to mold temperature and pressure. All other things being equal, a material with a lower viscosity will flow further than one with a higher viscosity. But all things are rarely equal. If the low viscosity material has a shorter gel time, than it may flow further while it is still liquid, but will stop flowing sooner than the material with the longer gel time.
Epoxy molding compound formulation is certainly as much of an art as a science. Add epoxy mold configuration, part geometry, layout and part-count to the complexity and the puzzle becomes even more complex. So what is an engineer to do when confronted with this information? The short answer is collect information and test.
What data should be collected?
Start by collecting the following data on the epoxy molding compound:
- Spiral Flow
- Gel Time
- Filler Type
- Filler Size
- Filler Shape
Once you have this, ask the epoxy mold compound manufacturer to explain the trade-offs made in each of the formulations and then ask for samples to test based on your requirements.
Please visit us at www.caplinq.com to learn more about our whole range of epoxy mold compounds including our Transfer Mold Epoxy Compounds and our Compression Mold Epoxy Compounds. If you have any other questions about how to process and cure epoxy molding compounds please feel free to leave a comment below or don’t hesitate to contact us.