how to use barcode scanner in asp.net c# THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS in Software

Printer QR Code JIS X 0510 in Software THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS

THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS
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10.1 INTRODUCTION
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The European Union s Restriction of Hazardous Substances (RoHS) directive has had a profound impact on all levels of the electronics industry supply chain, including the base materials used in printed circuit boards (PCBs).While debate over the net environmental benefits of this initiative continues, it is clear that all levels of the supply chain must deal with the implications. The July 2006 deadline for the first group of products that must comply has come and gone, and deadlines for other product groups are established. Indeed, many of the product groups with exemptions that allow for more time to comply are being forced to transition ahead of schedule due to component availability and constraints within the assembly services industry. The product groups granted initial exemptions are generally those with the most stringent reliability requirements, and many involve complex PCB designs that even further complicate the issues with respect to base materials. This chapter will present the key issues related to base materials, important properties with respect to lead-free assembly compatibility, and examples of material evaluation. Further discussion of material selection is presented in Chap.11.
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10.2 ROHS BASICS
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RoHS restricts the use of:
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Lead (Pb) Cadmium (Cd) Mercury (Hg) Hexavalent chromium (CrVI) Polybrominated biphenyl (PBB) Polybrominated diphenyl ethers (PBDE)
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The first four substances are metals used for a variety of applications, whereas the last two are generally used as flame retardants in plastic materials. It is important to note that these halogenated flame retardants are not generally used in laminate materials for printed circuits. In addition, bromine (Br), the halogen used in these flame retardants, is itself not restricted.
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7 discussed flame retardants in more detail. The key point to be made here is that the most common flame retardant used in base materials, tetrabromo-bisphenol-A (TBBPA), is not restricted by RoHS. Furthermore, in FR-4 materials, TBBPA is generally reacted into one of the epoxy resins, and thus does not exist as a free-standing molecule, but is incorporated into the molecular backbone of the resin system. The metals that are restricted are not used in base materials either. So from the standpoint of compliance, most laminate materials are acceptable. The key issue is the restriction on the use of lead, and the implications that this has for assembly of components onto PCBs. Eutectic tin/lead (Sn/Pb) has been the primary solder alloy used in assembly of printed circuits, and has a melting point of 183 C. Peak assembly temperatures using eutectic tin/lead commonly reach 230 235 C. With the elimination of lead from electronics assembly, alternative solder alloys have been developed. These lead-free alloys have higher melting points. Tin/silver/copper (Sn/Ag/Cu) alloys, also called SAC alloys, are the most common lead-free solder alternatives, with the most common of these alloys having melting points around 217 C and peak assembly temperatures of up to 260 C. Assembly rework temperatures can be even higher. It is the increase in temperature to which PCBs are exposed to that is the primary issue with respect to base materials. In other words, the issue for base materials is not compliance with the restriction on the banned substances, but compatibility with the manufacturing processes and temperatures used.
BASE MATERIAL COMPATIBILITY ISSUES
The question of base material compatibility is very complex.This is the result of several factors. First, PCB design and construction have a significant impact on the base material properties required. Thin, low-layer-count PCBs may have different requirements than very thick, highlayer-count PCBs. Copper weights, aspect ratios, and other design features also have an impact. End-use application and the associated requirement for long-term reliability and electrical performance also impacts the decision-making process. The requirements for a cell phone, video game, or even a computer motherboard are very different than those for high-end servers, telecommunications gear, avionics, and critical medical and automotive electronics. Finally, not all lead-free assembly processes are the same. Some designs experience peak temperatures of around 245 C, whereas others experience peak temperatures of up to 260 C, or even higher in some cases. Some PCBs may experience two to three thermal cycles, others up to five, six, or even more depending on how many reworks are allowed.All of this makes it impossible to recommend one base material type for all applications without either underspecifying the laminate material and risking defects during assembly or later on in the field, or overspecifying the material and paying too much for the material or limiting availability. Beyond the complexity introduced by the PCB design and construction, there are fundamentally two issues:
Surviving lead-free assembly processes without defects Maintaining the required level of long-term reliability for the given application
The critical base material properties required for each are summarized here, and discussed in more detail later in this chapter. The impact on the base material components is discussed in Section 10.4. 10.3.1 Surviving Lead-Free Assembly without Defects The primary defects related to base materials that can occur during lead-free assembly include measling, blistering, or delamination. These defects are related to the thermomechanical properties of the base material as well as the interaction of these properties with any
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