Lead-free compatibility

Phenolic lead-free compatible materials often not as good for electrical performance, especially Dt... 

Electrical Performance of Lead-Free Compatible FR-4 Materials... 

FIGURE 9.30 D, versus frequency for lead-free compatible FR-4 materials. 

FIGURE 9.32 D versus Frequency for Low DVD, Lead-Free Compatible Materials. 

Resin system CTE values above Tg are much higher than below Tg. Z-axis expansion induces stress on plated vias.The higher temperatures of lead-free assembly result in more total expansion for a given material. Several mature lead-free-compatible materials incorporate inorganic fillers that reduce CTE values. X-and y-axis CTEs are also important for reliability at component and layer interfaces. 

As you can see when these charts are presented side by side, the range of designs for which this product is deemed suitable decreases as the assembly temperature increases. This should be expected based on the earlier discussion of material properties. The other key point is that there may still be a group of products, albeit more limited, where standard 140°CTg material may be adequate and the most cost-effective option. This may help clarify some of the confusion about whether standard FR-4 materials are lead-free-compatible or not. For specific designs with specific requirements for reliability, the answer is yes. For other designs, applications, or reliability requirements, the answer is no. The value of this tool is that it attempts to define the range of PCB designs where specific materials should be considered. 

Fill Materials. Since the fill material is an additional fabrication material that becomes a part of the design construction, procurement documentation is required to specify a fill material type and thereby implement the via fill process. The selection and documentation of the fill material require the same consideration as the base laminate preference. This is especially critical when targeting a lead-free-compatible process. Currently, an industry-based material specification for via fill material does not exist.Therefore, specific fill-material brands may be named on the drawing, or some other form of user/supplier agreement must be established.The fabricator has preferences for the type of material used for via fill. Just as suppliers often have preferences for a specific solder mask brand, they also often prefer to use of a specific via fill material around which they have developed their principal processes. Supplier preferences can be driven by specific via fill material characteristics, such as accessibility, equipment compatibility, process supportability, plateability, and/or shelf/pot life. This may complicate source selection, or it might influence the use of a dedicated service center for the hole-fill process. The fabricator may not always know the reliability of its preferred material for a given via structure or end-use environment. 

D. Shangguan, Managing Lead-Free Compatibility, Circuits Assem., Nov 2003, p 30-33... 

Lead free stabiliser formulations obviously require the removal of lead stearate, a very efficient lubricant, and lubricants for Ca Zn stabilised profile extrusion have been investigated (139). As extruder running speeds increase, this puts increased pressure on the compatibility of lubricants and low plate-out lubricant systems are being developed (292). 

The recommended lead-free solder formulation is Sn-Ag-Cu for board assembly but there are other formulations such as Nickel-Palladium (NiPd), or Nickel-Palladium with Gold flash (NiPdAu). Passive components, to be compatible with a lower temperature Lead process (which is 215°C for 50/50 Tin/Lead formulations and 230°C for 40/60 formulations) and the higher lead-free process of up to 260°C, use pure matte Tin for their contacts. The use of lead in solder is partially based on several potential reliability issues. Pure Tin component leads have been shown to result in inter-metaUic migration in the termination of the electronic component and the growth of tin whiskers which could cause short circuits (which is why there is a exemption for military use (only) components). 

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. 

Performance of the package moisture sensitivity level (MSL) can be impaired by the higher temperature compatibility arising from lead-free requirements. 

The industry is also exploring benefits of an approach called backward-compatibility, where the component termination is lead-free, but the on-board solder is Pb containing solder—eutectic SnPb in many cases. Such a joint is neither Pb-free nor RoHS-compliant, but can allow for the component suppher to deliver lead-free components while letting the customer keep the assembly materials and reflow profiles mostly constant. 

The most commonly used specification for base materials is IPC-4101. This specification presents a classification scheme and specification sheets for the various materials in use. Table 6.2 summarizes the various materials by specification sheet number. Each specification sheet in IPC-4101 includes property requirements for that particular material type. As these specification sheets are updated periodically, it is recommended that the latest revision of this document be reviewed. This is particularly true in light of new requirements for materials that must be compatible with lead-free assembly.Table 6.2 is presented for reference only and is not all-inclusive. UL94 comments in Table 6.2 reference the minimum flammability requirements for that material. Materials may exceed these minimum ratings. Also note that where a non-halogen-based flame retardant is used, it is shown along with the resin system description. Definitions of the UL flammability ratings are given in Chap. 8. 

In addition, several FR-4 materials, such as those shown in sheets 99,101, and 126, show that these materials contain inorganic fillers.These fillers are often used to reduce the amount of Z-axis expansion. Some of these materials also have requirements forTa,Z-axis expansion and time-to-delamination, which historically were not properties included on the specification sheets. The addition of specification sheets with these properties was partly driven by their relevance to compatibility with lead-free assembly processes. 

Figure 8.1 provides an example of a TMA scan on a high-Tg, filled FR-4 material designed to be compatible with most lead-free assembly applications.The Tg is determined by extrapolating the linear portions of the expansion curve to the point where they intersect. In this case, a Tg of... 

However, for performance at very high frequencies, lower D /Df materials are preferred. While low Dij/Df materials have been available for many years, the advent of lead-free assembly has complicated material selection, and in these applications not only are the laminate and Dt properties critical, but their thermal properties are just as important. Critical material properties for lead-free assembly compatibility will be discussed in a subsequent chapter. Figures 9.32 and 9.33 provide Df and Djj data for three different low Dk/Df materials that are also compatible with lead-free assembly. 

Most conventional (dicy-cured) high-Tg FR-4 materials are generally not compatible with lead-free assembly, or can be used successfully only in a very limited range of applications. 

While most base materials comply with the RoHS directive, the question of compatibility with lead-free assembly processes is a more complex issue.The material properties that are important for lead-free assembly compatibility include ... 

Kelley, Ed, Bergum, Erik, Humby, David, Hornsby, Ron, Varnell, William, Lead-Free Assembly Identifying Compatible Base Materials for Your Application, IPC/Apex Technical Conference, February 2006. 

Ehrler, Sylvia, Compatibility of Epoxy-Based PCBs to Lead-Free Assembly, EIPC Winter Conference, 2005, Circuitree, June 2005. 

Drilling. Many of the materials designed for compatibility with lead-free assembly require optimization of drilling parameters. Phenolic and other high-performance resin systems can have higher modulus values and be harder than conventional dicy-cured FR-4 materials. 

SELECTING BASE MATERIALS FOR LEAD-FREE ASSEMBLY APPLICATIONS TABLE 11.7 Range of Material Types Compatible with Specific Lead-Free Applications... 

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