Electromechanical Relays (EMR) and Solid State Relays (SSR) are very different parts with many differences in characteristics. Here is a comparison table for general cases to help you figure out which type of relay is right for your project.
| Characteristics | EMR | SSR |
|---|---|---|
| Sensitivity to withstand misuse or misapplication | Good | Poor |
| Sensitive to corrosion, oxidation, or contaminates | Yes | No |
| Sensitive to shock, vibration or acceleration | Yes | No |
| Sensitivity to radiation | Fair | Poor |
| Package versatility | Good | Fair |
| Cost per pole | Best | Fair |
| Input TTL & CMOS (buffer) compatibility | Fair | Best |
| Operate and release time | around 5-20 ms | around 0.25-10 ms |
| Compatibility of military/aerospace specs. | Good | Poor |
| Ease of troubleshooting | Good | Poor |
| Input to output isolation capability | around 4Kv | >4Kv |
| Normal failure mode (output) | Open | Shorted |
| Normal wearout mechanism | Contacts | LED |
To learn more about how electromechanical and solid state relays compare in output switching capability, please refer to TE application notes on Relay Technologies.