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

Encoding QR Code in Software THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS

THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS
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absorbed moisture that may be contained within the PCB during assembly, which can be volatilized as the PCB is heated. Fundamentally, these defects represent either the creation of voids within the PCB due to the volatilization of moisture, retained solvents, other organic components including the by-products of resin decomposition, or the loss of adhesion between the fundamental base material components: glass cloth, resin system, and copper. The adhesion loss and subsequent blistering or delamination may be the result of pressure from volatilized components, mismatches between component thermal expansion rates, degradation of the components at the interfaces, or combinations of these factors. Figure 10.1 is an example of delaminations within a PCB after lead-free assembly processing. This PCB used a standard dicyandiamide (dicy) cured, high-Tg FR-4 base material. The left and rightside images in this figure are from the same area of the PCB, but photographed with different lighting to highlight the specific interfaces where delamination occurred.
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FIGURE 10.1 PCB delamination after lead-free assembly.
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The key base material properties that relate to survivability in lead-free assembly processes include:
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Decomposition temperature (Td) Coefficients of thermal expansion (CTEs) Moisture absorption Time to delamination
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Time to delamination, or T260 time, which is the time it takes for a sample to delaminate when held at an isotherm temperature of 260 C, is not a fundamental base material property, but is a functional test related to the other material properties. In multilayer PCBs, this property can be affected by the quality of the innerlayer copper surface preparation and multilayer lamination quality, among other things. 10.3.2 Lead-Free Assembly and Long-Term Reliability Issues The second fundamental issue for base materials and PCBs is understanding the impact to long-term reliability assuming a PCB has survived assembly. Other things being equal, the long-term reliability of a PCB assembled in a lead-free process with higher temperatures is not as good as a PCB assembled in a tin-lead assembly process with lower temperatures. The magnitude of the difference is dependent on many factors, but they include the type of base
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material used. The key properties of the base material with respect to long-term reliability include:
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CTEs Glass transition temperature (Tg) Decomposition temperature (Td) Moisture absorption
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Additionally, the specific construction of the PCB in terms of resin content, the types of fiberglass cloth used, and the adhesion and long-term thermochemical compatibility between the resin system and copper which is plated within vias can have a significant impact on longterm reliability. Some of these factors relate to the impact they have on CTEs, both in the z-axis as well as in the x- and y-axes. For example, using a very high resin content prepreg layer adjacent to a very low resin content layer increases the stress at the interface of these layers due to the differences in thermal expansion that result from the differences in resin-to-glass concentrations.The higher temperatures of lead-free assembly result in a greater absolute level of expansion or stress at these interfaces, which can lead to blistering or delamination. The distribution of the glass filaments within the resin system can have a similar effect. Within a given layer or prepreg, if there are resin-rich versus glass-rich areas, the differences in thermal expansion between these areas cause stresses that can lead to delamination, or at the interface with a plated-through hole (PTH) will result in points of stress-strain discontinuity resulting from differences in the z-axis expansion values. It is not uncommon to see cracks in the PTH copper begin at these points and ultimately lead to failure. 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.
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