When the EU’s RoHS legislation came into force in 2006, some product categories were exempted from specific aspects such as the demand for lead-free assembly; some had doubts about reliability and there was little if any long-term reliability data. Now, almost 10 years after RoHS’ introduction, some major exemptions are being phased out. Medical devices have been required to be fully RoHS compliant since 2014, while automotive electronics must come into line by 2016 and medical systems such as implantables and defibrillators by 2021. From an engineering point of view, these deadlines are extremely close.
At Active-PCB Solutions, we work extensively in the medical, automotive, aerospace and industrial sectors, and have seen lead-free develop become the dominant assembly technology. So do these sectors now have enough confidence to go fully lead-free? Some major component manufacturers and electronics OEMs are among companies that continue to apply for RoHS exemptions, seeking to use lead in capacitors and other components as well as equipment such as computer servers. Clearly, the industry has not found a suitable alternative to lead for every application.
RoHS lead-free legislation has raised two key concerns relating to reliability. These are the relatively brittle nature of lead-free SnAgCu solder joints, and the potential effects of tin whisker growth. Lead-free solder joints are harder and less compliant than SnPb joints, and so are seen to be more vulnerable to failure when exposed to shock or vibration. Of course, environments such as aircraft, cars and trucks can impose high levels of shock and vibration on sensitive electronic assemblies.
Tin whiskers have been studied extensively, and have been found to form spontaneously in the pure-tin surface finish of component leads. There is no proven limit on the length these whiskers can grow. They are believed to be able to bridge small gaps between component interconnects, and to conduct currents up to several millamps or considerably more in certain circumstances; enough to cause a troublesome short circuit. Fine-pitch components are believed to be particularly at risk. The whiskers may also break off and form conductive debris on the board surface. Matte tin (rather than bright tin) surface treatment has been shown to mitigate, but not eliminate, whisker formation on component terminations. Alternative surface finishes such as Nickel-Palladium-Gold are available, but are more expensive. On the other hand, conformal coatings can protect circuitry against the effects of conductive debris, but whiskers may still form and penetrate the coating. Physically tougher coatings are believed to mitigate growth but – again – cannot guarantee prevention. Also, conformal coatings can introduce thermal challenges.
Other issues raised by the switch to lead-free include the effects of higher reflow temperatures on substrates, especially thin PCBs, and controlling the movement of materials and components on the factory floor to prevent mixing of lead-free and non-lead-free leading to contamination and compatibility issues.
Is the automotive industry ready for 2016? Can the medical sector be confident post-2014? One thing we can say is that after several years successfully building lead-free assemblies for customers targeting numerous critical applications, here at Active PCB we now have a large knowledge base covering state-of-the-art materials, processes and techniques that enable us to deliver equipment capable of operating reliably throughout its entire specified lifetime.
Talk to our team if you have any questions about any of our services.