Assembly process Lead-free

In view of all the unknowns and process dependencies outlined, the assembly of lead-free components with Pb/Sn paste is generally not recommended. If it has to be performed, detailed studies should be conducted to benchmark the consistency of the assembly process as well as the consequent effect on the long-term reliability (both thermomechanical and mechanical) of the assemblies. 

In assemblies utilizing lead-free solder, work must be performed to ensure that the flux, underfill, component, and PCB system is compatible. For an underfill to be effective, the underfill must bond to the die and chip carrier surfaces. Current underfill systems are designed to be compatible with flux systems for Sn-Pb solder, which may not be the case with a lead-free solder. An additional concern for underfilled systems is exposure to reflow temperatures. For example, flip-chip BGA components may experience several reflow cycles after the underfill step. The higher processing temperatures experienced during lead-free soldering can have detrimental effects such as delamination of the underfill material from the chip carrier or die surface. 

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 use of free-standing leads in the DEC process gives an advantage in the subsequent assembly steps lead attachment is not required. In addition, there are no solder joints between the leads and the substrates. This leads to a more reliable assembly. 

The need to use lead-free solder in surface mount technology applications where soldering temperatures of between 220 °C and 240 °C are likely to be encountered has necessitated the use of such high temperamre polyamides as DuPont s Zytel HTN LX resins for board assembly. It is interesting to note that some Japanese companies have been using lead-free solders in their flow soldering processes since 1997. 

TAB packages are usually naturally lead-free at the component level. Assembly to the apphcation board does require lead-free solder along with a requirement for the package to be able to withstand the high temperatures associated with lead-free processing. 

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. 

The various types of base materials can be classified by the reinforcement type, the resin system used, the glass transition temperature (Tg) of the resin system, as well as many other properties of the material. With the advent of lead-free assembly processes, properties other than Tg are becoming as important, if not more important when selecting a base material. The decomposition temperature (Ta) is one of these properties and will be defined shortly. 

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. 

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. 

Historically, the properties that received the greatest amount of attention were the glass transition temperature, Tg, and the coefficients of thermal expansion, or CTEs, particularly in the z-axis. With the advent of lead-free assembly processes, other properties have increased in importance as well.The most notable is the decomposition temperature,Ta.These properties were described in more detail in Chap. 6, and will be discussed again in Chap. 10, which focuses on the impact of lead-free assembly on base materials. However, some additional information as well as examples of the test data are included here, as well as comparisons of some common material types. 

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 ... 

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