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How do you build a RoHS compliant membrane switch?

How do you build a RoHS compliant membrane switch? Typically, there are no banned substances in the basic building blocks of membrane switches if screen printed conductive inks are used. Pressure sensitive adhesives and the polyester film used for the substrate and the inks used for graphics are generally free of restricted substances as well. One component that can be a violation is the connector if it contains lead. Another violating component, though not commonly used, is any flame retardant: screen printed or otherwise included in the product.

This paper does not address issues that are present when printed circuit boards, copper flexible circuitry or elastomeric keypads are used for a membrane switch.

Definition of RoHS Compliance

The RoHS criteria was designed to reduce or eliminate specified substances commonly used in electronic assembly, device or equipment that have been recognized as hazardous. An RoHS compliant component is tested for the presence of:

1) Lead (Pb),

2) Cadmium (Cd),

3) Mercury (Hg),

4) Hexavalent chromium (Hex-Cr),

5) Polybrominated biphenyls (PBB), and

6) Polybrominated diphenyl ethers (PBDE).

For Cadmium and Hexavalent chromium, there must be less than 0.01% of the substance by weight. For Lead, PBB, and PBDE, there cannot be more than 0.1% of the material, when measured by weight. Any RoHS compliant component must have 100 ppm or less of mercury which cannot be added to the component.

 

The directive went into effect in The United Kingdom and Europe in 2006. The intent was to reduce production employee, consumer and recycler employee exposure to the recognized health dangers and reduce waste pollution in land, water and air. A companion directive, WEEE (Waste from Electrical and Electronic Equipment) controls how electric and electronic devices are handled and recycled. If you need to be RoHS compliant, you should also ensure WEEE compliance. For reference: www.weeeregistration.com/weee-directive.html.

Connectors

Generally speaking, connectors which are lead free will not have any banned substances in their composition. Lead and cadmium are commonly used in finishes for metal connectors, solder tabs and stakes. However, if any lead solder is used with these connectors to connect with a printed circuit board or copper circuitry, they will not meet the criteria. RoHS compliant connectors are commercially available. For those who provide value added assembly with their membrane switches, this can become a problem. Pay attention to the complete assembly when determining your compliance.

How can membrane switch suppliers verify their compliance?

A membrane switch manufacturer commonly purchases materials that have MSDS (material safety data sheets) available from their producers. These documents are normally accepted by OEM’s as proof of content. In some cases, material providers may be protecting their “recipe”, most evident with inks and pressure sensitive adhesives. In this case, the material company can provide a separate statement stating that their manufacturing process does not include the restricted substances without compromising their proprietary knowledge.

The easiest and quickest way to acquire the necessary documents is to search the web for the part number in question. If the material manufacturer does not provide an MSDS on their web site, you should be able to get one from the distributor of the product. If not, you may need to consider using a material that does have an MSDS document readily available.

Another requirement generated by the customer may be a certificate of compliance. This is the responsibility of the membrane switch manufacturer to confirm that it meets the requirements of the customer and that it has done its due diligence to ensure that no restricted materials are present in the supplied product.

The best approach is to be proactive with your choice of materials. Know ahead of time the materials that could cause a violation. When using polymer circuitry, connectors and flame retardant materials are the likely sources of banned substances.

Prepared by GDSI, 709 S. Keller Ave., Amery, WI 54001 715-268-6495

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