Lead-free assembly reliability

In addition, the advent of the European Union s Restriction of Hazardous Substances (RoHS) directive and the lead-free assembly processes that result are redefining the requirements for base materials. RoHS has a severe impact on all aspects of base materials technology. The impact of lead-free assembly on base materials and a method of selecting materials for lead-free assembly are discussed in Chaps. 10 and 11. Requirements to support circuit densification, reliability, and electrical performance are also critical and will be discussed in Chap. 9. This chapter discusses grades and specifications of base materials, as well as the manufacturing processes used to make them. 

As a material is heated to higher temperatures, a point is reached where the resin system will begin to decompose. The chemical bonds within the resin system will begin to break down and volatile components will be driven off, reducing the mass of the sample. The decomposition temperature, Td, is a property which describes the point at which this process occnrs. The traditional definition of Td is the point where 5 percent of the original mass is lost to decomposition. However, 5 percent is a very large number when multilayer PCB reliability is considered, and temperatures where lower levels of decomposition occur are very important to understand, particularly with respect to lead-free assembly. To illustrate this, consider Fig. 6.4. 

Lead-free assembly processes are driving the need for greater thermal reliability. This will be discussed further in Chaps. 10 and 11. Other trends are also driving the need for greater performance.These include ... 

Lead-Free Assembly and Long-Term Reliability Issues... 

Section 10.5 discusses the material properties relevant to both surviving lead-free assembly as well as ensuring long-term reliability. Although the test methods used to assess longterm reliability are beyond the scope of this chapter, where relevant, a brief description will be provided to help explain performance differences. However, before discussing these properties in more detail. Section 10.4 presents some of the concerns related to the components used to make base materials. 

Resin decomposition can result in adhesion loss and delamination. A 5 percent level of decomposition is severe, and intermediate levels are important for assessing reliability since peak temperatures in lead-free assembly can reach onset points of decomposition. 

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. 

In addition to the works already cited, there have been excellent studies on the impact of lead-free assembly, specifically on PWB reliability. These works present statistical analyses showing the impact of lead-free assembly on PWB reliability and reach important conclusions regarding the base materials. Althongh there is not perfect agreement among all published works, the differences typically are the resnlt of a different focus— for example, whether the focus is on complex, thick PWBs with stringent reliability requirements versus relatively less complex PWBs with shorter intended field lifetimes or less stringent reliability requirements. Conclusions inclnde ... 

Kelley, Edward, An Assessment of the Impact of Lead-Free Assembly Processes on Base Material and PCB Reliability, IPC/Soldertec Conference, Amsterdam, June 2004. 

The mandated conversion to lead free assemblies has led to the convergence of several critical issues in the microelectronics industry. The conversion has further compounded several other factors that have been pushing the envelope in Component to Printed Wiring Board (PWB) reliability, including... 

Temperature cycling tests used to determine the thermomechanical reliability of tin-lead and lead-free assemblies... 

Effect of Lead-Free Conversion, Because of the conversion to lead-free solder, the intermetallics formed at the package-joint and PWB-joint interface would be different from those in Pb/Sn assemblies. Consequently, the response to bending of a lead-free assembly would be significantly different from that of a Pb/Sn assembly. There are relatively little experimental data in the industry comparing the mechanical reliability of lead-free soldered assemblies with Pb/Sn assemblies. One data set showing a relative comparison between Pb/Sn and SnAgCu for two different surface finishes is shown in Fig. 59.13. ... 

C. Handwerker, T. Siewert, J. Bath, E. Benedetto, E. Bradley, R. Gedney, J. Solm, and P. Snugovsky, NEMI Lead-Free Assembly Project Comparison Between PbSn and SnAgCu Reliability and Microstmctures, SMTA International Conference Proc., 2003 (CD-ROM)... 

Significant volumes of research and development work on manufacturing issues associated with lead-free assembly have been conducted and published in the past decade by the industry, national laboratories, consortia, and academia worldwide. Reliability studies of lead-free solder interconnects, however, are still emerging. 

Excessive solder voids can create reliability issues, especially in applications where the lead-free assembly is exposed to thermal... 

Higashi, O., et al. Lead-free Assembly Technology for High Reliability Server Computer, presented at the EIAJ Conference, Japan, December 1999. 

The reliability of lead-free assemblies was shown to be functionally equivalent to tin lead soldered assemblies, in terms of cycles-to-failure in thermomechanical fatigue tests, intermetallic compound growth and resistance to vibration and shock. There was no evidence to suggest that Sn-3.8Ag-0.7Cu will allow substantially increased operating temperatures. 

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). 

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