Mechanical relays are clearly better than solid state-relays…but not really. The fact is, you need to consider your application and your budget carefully before you choose a solid-state or mechanical relay solution. Each has pros and cons, so rather than pick a side, it would be better to explore common uses of each, and see where your application lands.
Mechanical relays are ideal for most applications, as cost is frequently a factor in deciding any technology solution. Mechanical relays are capable of switching any type of load from small signals to high current, high-induction, audio, and sometimes video provided the correct relay is chosen for each computer controlled switching application. Mechanical relays are the low-cost workhorse of the computer industry.
The lifespan of a mechanical relay can easily exceed 1,000,000 on/off cycles at the rated load. In many cases, we have seen mechanical relays exceed 10,000,000 cycles before failure. Lifespan is frequently a reason people choose solid-state over mechanical. However, I would argue that solid-state relays can suffer from failure for the same reasons as mechanical relays, so lifespan is less of a factor when choosing my own applications.
Mechanical relays have one major pitfall. Inductive switching wears down the contacts, and in the process of switching inductive loads, noise is inducted onto the driver circuit, causing potential problems with logic if not properly handled.
Mechanical relays are also available in a wide variety of low-cost configurations, including 20 Amp, 30 Amp, small signal, DPDT, and much more.
Solid-State relays are mostly available in a SPST configuration. Rarely are they available in SPDT or DPDT configurations. Solid-State relays are expensive, they can generate a lot of heat when switching large loads, and may even require external cooling. Solid-State relays also require a MINIMUM load before they operate correctly. Inductive loads can easily destroy a solid-state relay, and are more susceptible to damage caused by induction than mechanical relays. The maximum load rating of a solid-state relay typically comes from fairyland, and in no-way resembles the actual use case of said relay. Further, solid-state relays are typically rated for AC switching applications, though a few DC solid-state relays are available for select applications.
So why on earth would anyone want to consider using a solid-state relay given all of these limitations? Solid-state relays employ optical isolation, keeping computers safe from the most demanding high-power loads and even lighting strikes. Solid state relays are PERFECT for computer interface solutions because of their isolation from the logic circuits of the USB controller and other logic. Further, if you want to switch a high-power load, say 100 Amps at 240VAC, a high-current USB mechanical relay controller is destined to be an interface disaster without a ton of support electronics. Solid-state relays are the answer to extreme current switching applications when combined with mechanical contactors and induction suppression components. Solid-state relays can have a much higher cycle frequency over mechanical relays, so turning a high-power device on and off frequently is best handled by a solid-state solution. Solid-state relays seem to have an infinite lifespan when working with resistive loads provided the load is properly sized for the solid state-relay. Like all relays, solid-state relays should be protected from inductive loads using external components. This will reduce the damage to the silicon used in the switching process.