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The Complexities of a Lead-Free Transition
Wednesday, February 15, 2012 | Randy Schueller, DfR Solutions

When engineers and product developers think about a transition to lead-free products their focus is primarily on assembly optimization and product testing to ensure reliability. These are, indeed, critical aspects of a successful transition; however, other areas are equally important. These include material selection, component specification/qualification, rework, failure analysis, and remediation actions when failure occurs during assembly or testing.   

Material Selection

One key material decision that impacts the rest of the lead-free assembly process is the surface finish on the PCB. Many options are available and each has benefits and drawbacks. The correct selection depends greatly on the complexity of the board, the assembly process, and the use environment. Common options include OSP, ImAg, ImSn, lead-free HASL, or ENIG (with some newer options also coming to market). In addition, the optimal PCB laminate must also be selected. Often the laminate is over-specified (and, thus, a higher cost is paid) or under-specified (and reliability compromised) due to a lack of understanding of the robustness of various laminates in the lead-free assembly process.  

Other important material choices include the solder alloys used in surface mount, wave solder, and for the BGA balls. SAC305 is the typical surface-mount alloy and it performs reasonably well for most applications. A variety of wave solder alloys are in use today including SnCu, SnAgCu, and SnAgCuNi. The best choice for your application will depend on the complexity of your wave solder requirements. SnCu is a cost effective solution if you have low-aspect holes to fill on thin boards. SnAgCuNi may be the best choice for more challenging holes, however, the choice of flux and the surface finish play an important role as well. The best choice of alloy for BGA terminations will depend primarily on the dynamic strain requirements of your application. If board flexing or shock events occur in the product you may require lower modulus alloys, such as SAC105 or a variety of others being introduced for this purpose. 

Component Specification/Qualification

The temperature and moisture sensitivity requirements for surface-mount LF components are pretty well understood and suppliers do a good job of meeting them. Temperature limits for wave solder components, on the other hand, are less understood and early adopters of lead-free found many cases of melted plastics on wave solder connectors. As a result, the use of nylon 66 has been mostly eliminated and replaced with nylon 46 or LCP.  

Heat damage to other components such as electrolytic capacitors is more difficult to detect and, therefore, less understood. The electrolyte can boil if the time and temperature limits are exceeded, but such limits are not always provided with the capacitor. A common specification might say that the component will survive six seconds with the leads dipped in 260°C solder. But what happens if the solder bath is increased to 270°C? What about preheat? The component engineer is left to guess what peak and duration of preheat is acceptable while the wave solder engineer continues to increase these preheat conditions in a quest to achieve hole fill requirements with less cooperative lead-free alloys. 

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