Lead-free reflow processes

The solder shock test is one of several methods to assess the thermal resistance of copper-clad laminates. It is easy to perform and represents another key test during the early assessment of a material. There are a number of different methods to choose from, which will be described in detail in Section 12.5.2. During the initial assessment of the material, it is important to choose at least one of the described test methods to make certain that the material meets the minimum requirements, especially if the material is nsed in higher-temperatnre lead-free assembly processes. Aside from solder shock testing of bare laminate material, it is also recommended that the PCB engineer consider PCB-level temperatnre shock as well as repeated reflow testing with a particnlar focns on resin-reinforcement delaminations. This will ensnre that not only the raw material bnt also the completed PCB will be able to withstand the required temperatnre regime. 

Understanding the stresses to which the assembly process exposes the PCB is fundamental in defining the acceptability criteria. Process elements such as leaded or lead-free, reflow or wave solder, double-sided reflow, hand-soldering, and rework all significantly impact the requirements of the PCB. 

The vapor phase lead-free solder process operates at a maximum temperature of 230°C, whereas many convection or IR reflow processes range from 250°C to over 300°C in some zones of the reflow process, depending on the type of PCBAs being processed. Vapor phase is compatible with typical PCB finishes, including HASL, bare copper with OSP, copper/nickel/gold, immersion tin, and immersion silver. 

So the search is on for approaches that will render the cheaper and relatively more workable engineering thermoplastics still usable for mainstream assembly and connection technology processes such as lead-free reflow soldering, for example. 

A flux system must be designed to be compatible with a particular solder alloy system. For example, typically the activation and volatilization temperatures for the eutectic Sn-Pb system are lower than lead-free alloys. In lead-free reflow, the soak or stabilization temperature may be in the range of 160 to 170°C compared with 140 to 155°C for eutectic Sn-Pb solder reflow. In addition, the lead-free peak temperature may approach 240 to 250 °C compared with 220 to 230°C for eutectic Sn-Pb. At the higher lead-free process temperatures, fluxes vaporize more rapidly and completely compared to standard temperature profiles utilized for eutectic Sn-Pb solder. Often there is insufficient flux left on a board due to volatilization. The volatilization... 

The lead-free solder adopted by mannfactnrer Texas Instrnments is a nickel-palladium-gold (NiPdAu) formulation which is claimed to be backward compatible with existing reflow soldering processes. It is also claimed to be free of the whisker-artifact problems which have been experienced when nsing snch alternatives as matted tin. A Texas Instrnments spokesman stated that the move to pnre tin would necessitate the increase of the reflow solder temperature to obtain the same reliable solder contacts. He claimed that Texas Instruments had shipped more than 30 million lead-free units which confirmed the suitability of its NiPdAu solution. 

The process window for lead-free soldering requires higher peak reflow temperatures and longer times above liquidus. Soldering in a nitrogen environment improves wetting and reduces oxidation of the flux residues and the solder. Due to the higher reflow temperature. 

Parts must be suited for the temperature required by the reflow process. Lastly, ensure that component contact or surface finish is compatible with the solder being used, be it leaded or lead-free. 

As mentioned previously, to permit Pb-free reflow, reflow equipment may require some oven modifications. There is little impact to the reflow process and most of the changes are material related. Because many of the Pb-free solders are slower to wet out on leads and pads, extended wetting times may be required. Many Pb-free solder pastes are more prone to solder void formation. Extended soak (pre-heat) times and slightly higher peak temperatures are known to help with void reduction. 

Solder requirements are the same as for any other process. There are no ahoy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is apphed, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. 

Defect detection where rework is easiest, before component placement and solder reflow Process characterization during the lead-free conversion with minimal program tuning... 

Reflow Process. The key parameter for the reflow profile is the peak temperature. Adequate reflow temperature is needed for the solder to melt, flow and wet, interact with the copper on the pad and the component termination, and form sound intermetallic bond when cooled and solidified. Typically, 30 °C (55 °F) superheat (above the melting temperature) is desired. Eor lead-free soldering, because of concerns about the thermal stability of the components, efforts are needed to minimize the soldering temperature. For SAC alloy with the eutectic temperature at 217 °C (422 °F), the minimum reflow... 

Studies have shown that reliable lead-free solder joints, with proper grain structures and in-termetallics formation, can be produced using appropriate rework processes. Care must be taken to minimize any potential negative impact of the rework process on the reliability of the components and the PWB. Surface insulation resistance (SIR) tests must be performed to ensure the compatibility between the reflow/wave solder flux and the rework flux, i.e., to ensure that the rework flux and any products of reaction between the reflow/wave solder flux and the rework flux do not pose any unacceptable risk for electromigration and dendritic growth for noclean applications. 

Managing the compatibility issues is critical to lead-free transition. These include materials compatibility (solder, components and PWB), process compatibility (reflow, wave soldering, rework, equipment, and yield), design compatibility, reliability compatibility, and business compatibility (cost, supply chain, and operations). 

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