RoHS solid-state relays

Dec. 14, 2006
Solid-state relays are among the switching elements that now use materials allowed under the RoHS mandates.

Edited by Leland Teschler

RoHS-compatible SSRs such as this Opto 22 hockey-puck model look the same on the outside as older versions. Inside, there are differences in components, solder chemistry, and processing temperatures. But the epoxy that coats the SSR circuit boards, like the PCB-mount model shown here during assembly, is generally identical to older versions. Ditto for the plastic that goes into the outer housing.

The specifications of these devices typically are identical to those of non-RoHS compliant versions. Nevertheless, there are differences in construction between the two that are worth knowing about.

Most solid-state relays have power lead frames made of C110 copper. These solder directly to the output terminals on a printed-circuit board. The circuit boards on SSRs typically are single sided. The metal parts on SSRs typically do not contain hexavalent chromium, a corrosion treatment banned under RoHS, so they are typically identical in RoHS and non-RoHS versions. Soldering, of course, in RoHS versions is lead-free. It generally necessitates a change not only in solder chemistry but also in flux composition to handle the higher soldering temperatures.

Even SSRs that are not specifically RoHS-compliant typically do not contain plastic additives for flame retardance that RoHS bans (polybrominated biphenyls and polybrominated biphenylethers). The two main uses of plastics in SSRs, aside from in the plastic packaging of electrical components making up SSR circuitry, are in the SSR outer housing and the epoxy encapsulant that covers circuit-board elements.

RoHS typically has no impact on the type of epoxy used to encapsulate circuit-board elements. The epoxy typically contains a filler material that is neutral and helps retain stability at the 100°C-withstand temperatures that are typically specified for SSRs.

The plastic used for SSR housings is typically either Valox (polybutylene terephthalate) or Noryl (polyphenylene oxide, or PPO). These materials generally contain a glass filling that enhances their thermal-withstand qualities and helps them earn flame ratings typically ranging from V-0 to V-4. The filling is strictly to increase the temperature at which the SSR package still maintains its integrity. It is not a flame retardant and so does not come under RoHS.

Opto 22 ( provided information for this article.

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