Electrically conductive epoxies are commonly referenced as an alternative to solder. Although both materials provide low electrical resistance and bond strength across an interface, there are usually only a few reasons to consider an epoxy adhesive over solder.
We should point out that there are a few different types of electrically conductive epoxies. Electrically conductive epoxies can be categorized by filler material and whether it is designed to be isotropic or anisotropic. We will be exclusively referring to isotropic silver filled epoxy - the most common and lowest electrical resistance epoxy adhesive.
Solder outperforms epoxy electrical resistance - Solder is typically 10-100 times more conductive. Epoxy typically provides electrical resistance of 1x10^-4 ohm ⋅cm, whereas solder will provide from 1x10^-5 to 1x10^-6 ohm ⋅cm depending on the alloy.
Solder is superior to epoxy in regards to thermal conductivity - Silver filled epoxies will usually provide <10W/mK, although there are some exceptions between 10-20 W/mK. On the other hand, solder typically provides between 40 and 400 W/mK.
Solder provides better metal to metal bond strength - This is difficult to quantify because bond strength varies greatly depending on the type of metal substrates, surface preparation, and epoxy formulation. However, as a general rule solder will outperform epoxy when bonding metals.
Epoxy allows for a much easier and less costly “cold” assembly process - This is where epoxies really shine. Epoxies can either be formulated as a two-part room temperature cure, or a one-part heat curable systems. Two-part systems require mixing but once mixed will cure at room temperature within a matter of minutes or hours. One-part systems require heat to cure, typically between 60 and 150C depending on the formulation. As a result, epoxies are best suited for assembly procedures with:
heat sensitive components
lean in house assembly procedures
In conclusion, solder generally outperforms epoxy in regards to performance factors such as electrical resistance, thermal conductivity, and bond strength. However, epoxy provides a unique assembly alternative to solder and is generally chosen over solder due to specific assembly constraints rather than performance factors.