05-2006, Tema Förbindningsteknik (7 Mbyte, pdf) - Elektroniktidningen

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RoHS Designers gear up If there was an industry prize for the subject about which the most rhetoric is currently heard, then the EU RoHS directive would probably win it. Vidar Wernöe, in his capacity as the RoHS and WEEE champion at the Swedish Association of Electronics Industries has heard all the claims. Also involved at a hands- on level as managing director of electronic design consultancy Elektronikkonsult AB in Stockholm, a company specialising in demanding analog and power electronics systems, we thought he was a good person to ask about the real issues facing manufacturers and designers. Problems can occur where components have a different thermal mass
The Print: To start at the beginning, what needs to be changed to produce RoHS compliant products? Vidar Wernöe: First of all designers need to find components that comply with the directive. Secondly they need to manufacture the product in a leadfree process. As for the banned substances, the issue is about removing lead without affecting the product`s long term reliability. The P: And? VW: Well, essentially there are several key problems that arise, at the component, PCB and layout levels and at a political one. For the first; when using the viable alternatives such as a tin/silver/copper compound (SAC), much higher soldering temperatures are required (around 40ºC higher) which can lead to serious overheating problems. Such problems may occur where products include components of greatly different thermal mass e.g in power electronics. Surface mount electrolytic capacitors, for example, are temperature-sensitive and overheating can cause irreparable damage. The P: Can they not be developed to withstand more heat? VW: Yes they can. The Japanese, for example, have been doing leadfree soldering for some time now, but mainly for consumer electronics. For industrial products we have a lack of reliability data and this is a problem. The production process window has, in leadfree, become much tighter and this requires a careful CAD version of heat reliefs design. Designers need to analyse not only the electrical data of a component but also examine the the process temperature specifications. Ideally a component should sustain a process temperature of 260 ºC. Components exposed for over-temperature may cause faults that may not be detected straightaway in the final tests. So designers need to go through each single component used, in order to check its RoHS compliance, electrical data and the soldering parameters. This can easily entail checking the data of more than 1000s of part numbers for each single new product design or design change/product update. There is also a key political issue. Here in Sweden for example we have asked the Swedish authoritative body KEMI for guidance regarding proving compliance and what kind of documentation they want us to show. They have declined to give such guidelines however and it may cause tremendous additional work if it has to be done retrospectively after a development project is closed. The P: You also mentioned PCBs and layout? VW: Yes. Again because of the effects of the higher temperatures, the board can warp in the surface mount production process if there is an imbalance in the distribution of copper used. A recommendation is to use “teardrops” in the layout design. This is where, when a wire connects to a via, it does not go straight in but the end is teardrop-formed for extra mass to diminish the chances of it disconnecting when heated up. In addition, for larger hole-mounted components that connect to a larger copper layer, so-called “heat reliefs” are required to be able to get an acceptable solder result. Another recommendation is to widen the hole slightly. During a leadfree soldering process, the PCB will be exposed to higher temperatures than in the past. As a consequence the PCB matieral chosen is an issue but the key issue is not to overspecify. Polyclad FR4 370 has better qualities than standard FR4. It has a high Tg, less z-value expansion at higher temperatures and delamination takes longer, but the higher costs must be weighed up against precise needs. The P: Inspite of the mammoth amounts of extra work, are the efforts worth it? VW: Oh yes. In about a year and a half`s time, I think the dust will have settled and even those getting to grips with the issues late will be producing good quality products with long reliability guarantees, but there is still some way to go at a political level to ensure that RoHS compliancy can be proven and audited in an appropriate and consistent manner. My advice is to ask and look for other company`s experience in RoHS design. We at Elektronikkonsult have, for example, published our best practices on our website. This information can be downloaded at www.elektronikkonsult.se The P: Finally, we have to ask. As a customer of Elprint since 2004 are you a happy one? VW: The Elprint service is reliable, quick, cost- effective and with a wide product range and choice of materials so the answer must be yes! Heat reliefs are recommended for larger hole-mounted components
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05-2006, Tema Förbindningsteknik (7 Mbyte, pdf) - Elektroniktidningen

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