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SELECTING BASE MATERIALS FOR LEAD-FREE ASSEMBLY APPLICATIONS
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Undercut drill bits generally provide better hole quality with these materials. Other drill process variables must also be examined, and include:
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Maximum hit counts Drill chip loads (feeds and speeds) Infeed and retract rates Drill resharpenings Stack heights Vacuum levels Entry and backer material types
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11.2.2.6 Desmear Processing. Many of the resin systems developed for lead-free assembly compatibility, including the phenolic-type materials, are more chemically resistant than the conventional dicy-cured materials. However, most can still be processed using conventional chemical desmear processes. Adjustments may be needed in the times and/or temperatures used for solvent swelling and permanganate desmear processes, and the supplier of these chemistries should be consulted when transitioning to lead-free compatible materials.The processing guidelines provided by the laminate material supplier will also typically provide recommendations for desmear processing. For some advanced products, plasma desmear may be recommended. 11.2.2.7 Surface Finishes. If a lead-free Hot Air Solder Leveling (HASL) process is used, the higher temperatures required by the lead-free alloys should be considered when selecting the base material. This will be discussed in Section 11.3 as well. The common alternatives to HASL, which are growing in use as lead-free assembly is adopted, typically do not involve a bake process or significant thermal cycle. Although this is good in terms of the reduced thermal cycling exposure to the base material, it also results in one less opportunity for moisture to be removed from the PCB prior to assembly. This should be considered when deciding whether a drying step is needed prior to assembly. 11.2.3 PCB Assembly Considerations Although a complete discussion of assembly variables is beyond the scope of this chapter, a few key points need to be outlined with respect to material selection and lead-free assembly variables. First, as has been pointed out several times, the storage conditions of the PCB prior to assembly should be examined. Moisture absorption in the PCB prior to assembly can have a significant impact, and in severe cases can lead to delamination within the PCB. Storage in a controlled environment is encouraged, and drying of PCBs prior to assembly may also need to be considered. One of the challenges in drying PCBs prior to assembly is avoiding any negative impact to the surface finish that could adversely affect solderability. Consulting with the supplier of the surface finish supplier is strongly suggested before defining these drying processes. However,Table 11.2 offers some general recommendations for drying steps prior to assembly based on the type of surface finish.
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TABLE 11.2 Drying Recommendations Prior to Assembly Based on Surface Finish Type Final Finish Tin Silver Nickel/Gold Organic Coating Temperature 125 C 150 C 150 C 105 C Time (hr) 4 4 4 2 Comments Higher temperature may reduce solderability Silver may tarnish, but solderability should not be affected No issues arise with extended bake on nickel/gold finish Extended bake cycles may negatively impact multiple heat cycle assembly processes
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Development of the assembly profile is important not just for the obvious requirements of component assembly, but also to ensure that the PCB is not damaged as a result of the thermal exposures. These objectives often compete with each other. Profiles vary with PCB thickness, copper distribution, component densities, and other factors. Defining these profiles to meet both objectives can be a very complex process. But with respect to the impact on the PCB and the base materials, care should be exercised in defining the heat ramp rate, the time the PCB is exposed to the peak temperature, and the cooldown rate. Minimizing temperature gradients across the PCB is important in reducing the stresses that arise from thermal expansion. Unequal copper distribution across the PCB and differences in component mass across the PCB can lead to hot spots that can approach decomposition temperatures or create areas of stress resulting from thermal expansion differences. These hot spots or severe temperature gradients can be exacerbated by trying to speed up the process by setting the reflow oven zone temperatures very high. You can reduce these hot spots and thermal gradients by setting up soak profiles where the PCB is allowed to stabilize at specific temperatures before heating to the peak temperature. Cooldown rates must also be controlled for similar reasons. Cooling too fast can lead to significant temperature gradients and induce thermal expansion related stresses that can result in blisters, delamination, or pad cratering, which is fracturing in the PCB base material that propagates underneath a copper pad on the surface of the PCB. Finally, rework procedures must be examined and controlled more strictly in lead-free applications. Control of the rework temperature and time that the PCB is exposed to this temperature is critical. Since rework involves local heating of a specific area of the PCB, the issue of temperature gradients can be even more severe. For all of these reasons, it is strongly recommended that manufacturers perform studies to assess compatibility among the base material, the PCB fabrication process, and the assembly process. First article qualifications when transitioning specific PCB designs to lead-free assembly are strongly recommended. Success with one PCB design does not necessarily mean that other designs can use the same material or fabrication process successfully. In short, leadfree assembly processes are significantly more demanding on the PCB and base materials used and require extensive engineering work to validate compatibility with the assembly process and the requirements for long-term reliability.
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11.3 SELECTING THE RIGHT BASE MATERIAL FOR SPECIFIC APPLICATIONS 2,3
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A very common request from PCB fabricators, electronic manufacturing services (EMS) companies, and OEMs is whether a given material is compatible with lead-free assembly. Although everyone is looking for a simple answer, the solution becomes complex because of the range of PCB designs (board thicknesses, layer counts, aspect ratios, via pitch, and so on) as well as the differences in lead-free assembly processes, such as the specific peak temperature and number of thermal cycles a PCB will experience. Previous sections also outlined how the PCB fabrication process and assembly process can influence compatibility. In addition, people usually want to use the least expensive material that is suitable for a given application. So although it is easy to specify an advanced product for lead-free compatibility, the approach taken here is to balance cost and performance. In an attempt to simplify this discussion, this section describes a material selection tool that has been developed to suggest what materials should be considered for a given application. This tool is based on data such as that presented in Chap. 10, in the references, as well as empirical results from prototype and production experience with lead-free assembly applications. The experience of several people with hundreds of years of combined experience has been leveraged in designing these tools. However, no such tool can address every specific application with 100 percent confidence. In addition, the capabilities of various PCB fabrication
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