Lead-free adjustments

A new LTCC material was designed to be able to sinter under 1000°C because of co-firing with copper conductor. The LTCC is composed of lead-free, Si02—AI2O3—MgO—ZnO—B2O3 system glass and ceramic fillers. In order to satisfy electrical and thermal properties, we adjusted the amount of crystalline phases precipitated after sintering [8]. 

A simple example helps illustrate the practical application of this tool. This example is based on a real PCB design that was being converted to a lead-free assembly process. Some key features of the PCB as they relate to how it differed from a typical PWB as defined in Fig. 11.4, and the recommended adjustments suggested, are shown in Table 11.4. 

Rework for lead-free solders has been found to be more difficult, because the lead-free solder alloys typically do not wet or wick as easily as the Sn-Pb solder due to their difference in wettability. This can be easily seen with QFP packages. In spite of these differences, successful rework methods (both manual and semi-automatic) have been developed (Ref 74-75) with lead-free solders (Sn-Ag-Cu, or Sn-Ag), for many different types of components. Most of the rework equipment for tin-lead can still be used for lead-free solder. For area array packages, it is helpful to use a rework system with split vision and temperature profiling features. The soldering parameters must be adjusted to accommodate the higher melting temperature and reduced wettability of the lead-free solder. The other precautions for tin-lead rework (such as board baking) still apply to lead-free rework. 

The wave-solder-pot temperature for lead-free soldering should range from 265° to 270°C. The specific temperature to use depends on board layout and if pallets are used, and should be adjusted to maximize wetting. The dwell time in the wave should... 

With lead-free rework on 93-mil-thick boards, the bottom-side-heater set point needed to be elevated above the temperature used for typical SnPb rework. This adjustment was made to keep from overheating the top of the package beyond J-STD-020 limits. A higher heat was applied to thermally challenging 135-mil-thick boards. 

Process Adjustments from Eutectic to Lead-free Consequences... 

Lead-free solder has a rougher surface finish and generates a different-shaped fillet. It also is more prone to voids and tombstoning. These and other deviations can require adjustments to commonly used inspection techniques, such as automated optical inspection (AOI). While the results of a National Physical Laboratory (NPL) study confirm that AOI systems can be used to inspect lead-free surface mount assemblies, many defects created by lead-free processes are not visible. The added loss of visual and electrical access due to the growing complexity of PCBAs compounds the problem. 

A2002 NPL study independently evaluated the ability of AOI to inspect lead-free solder joints. The study evaluated 15 conventional leaded and lead-free target boards, with and without defects, using identical algorithms. Study results demonstrated that most AOI systems could be used to inspect lead-free surface mount facilities. It is also clear that AOI systems will require considerable adjustment to address varying defects and surface conditions found in lead-free manufacturing. 

The process parameters key to the stencil printing process include the squeegee type and hardness, print speed, print pressure, and print gap. An optimal stencil printer setup provides a clean sweep on the stencil surface and a repeatable solder paste deposition process. Shore A scale polyurethane squeegee hardness level between 85 to 95 is appropriate to achieve repeatable solder deposits for lead-free solder pastes. Print parameters such as the print speed, print pressure, and the print gap are adjusted to accommodate the variety of solder paste rheologies available. There are no specific equipment modifications necessary to stencil printer lead-free solder pastes. 

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