Component termination finish

A number of lead-free component termination finishes have been evaluated (Ref 13, 55) and used over the years. For passive components (such as chip capacitors and resistors), matte Sn plating has been used for many years with the tin-lead solder, and can be used with lead-free solder as well. For leaded components (e.g., quad flat pack or QFP), plating of matte tin or tin alloys may be used with lead-free solders (forward compatible). The tin whisker concern will be discussed in a later section. Nickel-lead has been used with the tin-lead solder for many years, and Ni-Pd-Au is currently an alternative for leaded components for lead-free soldering Ni-Pd typically does not provide as good wett-ahifity as tin. Area array packages with SAC halls are available and work well with the SAC solder. 

Because there is no single lead-free drop-in replacement for eutectic Sn-Pb, there must be a suitable marking system to indicate the particular alloy (and its melt temperature) used for assembly. This is necessary for an operator to subsequently remove a component and select a metallurgically compatible replacement component (i.e., compatible with the rework alloy, component termination finish, and original assembly alloy). Numerous investigations indicating a variety of lead-free finishes compatible with the Sn-Ag-Cu family of alloys have been conducted [64]. 

Lead Pb 0.1 Solder PWB surface finish component terminations surface finish PVC stabilizer and colorant plastics colorant as lead chromate cables and harnesses sheet metal plating (to reduce risk of Zn whiskers) alloy agent for steel, copper, aluminum, etc. 

Electrochemically Induced Failures. The electrochemical failure mechanisms accelerated by temperature, humidity, and electrical bias that were described in Sec. 57.2.1.3 for printed circuit boards also apply to the remainder of the PCA. The solder used for the interconnects and the metal component terminations and lead frame finishes can also be involved in the reactions. The large number of dissimilar metals increases the complexity of the situation and the possibility of galvanic corrosion in a humid environment. In addition, contaminants introduced during printed circuit assembly such as flux residues can contribute to the failures. 

Reflowed Conventional Leadless SMT Assemblies with Pb Contaminant. In Fig. 8, we have plotted cycles-to-1% failures for SAC paste assemblies with various Pb contaminant, versus cycles-to-1% failures for 100% lead-free SAC assemblies under thermal cycling conditions -40 to 125 °C (-40 to 257 °F), or -55 to 125 °C (-67 to 257 °F). When data points appear above the main diagonal, life for assemblies with Pb contamination is of a longer duration than that of 100% lead-free assemblies. The data (Ref 7, 8, 13) is for conventional leadless SMT components 20 Input/Output LCCCs and resistors ( R ) of sizes 0603, 1206, and 2512. The source of Pb contamination is the Sn-Pb HASL board finish, or the Sn-Pb component termination, in the 1206 resistors. In the 20 I/O LCCC SAC boards with Sn-Pb HASL finish, Woodrow (Ref 13) measured a 0.5% Pb contamination level. The cycles to 1% failure were calculated from Weibull parameters (characteristic life and slope of failure distributions) provided... 

No-clean solder pastes can solder most popular metal finishes adequately due to improvements in the activator packages. Gold over nickel, bare copper with organic surface preservatives, silver immersion, tin plates, and hot-air leveled boards are popular, while component terminations such as tin, tin/lead, silver, silver palladium, and nickel are used. Solder pastes can be designed to solder specific surfaces and maintain the non-corrosive and electrical resistance required to qualify them as no-clean pastes. 

Generally, having a nonsolder surface finish to bond a conductive adhesive improves contact resistance [49]. For example, conductive adhesives perform well with palladium-based terminated components. Lead-free board finishes such as nickel/gold and copper/palladium have been evaluated. Failures at the interface between component terminations and conductive adhesives is a typical failure mode observed in durabiUty tests, and also the cause of an increase in electrical resistance. Oxidation and corrosion of Sn-Pb finishes take place at the interface [21]. 

Nickel/gold is another popular board finish when combined with a gold finish on a component terminal pad. Care must be exercised to not exceed the rule-of-thumb Au threshold concentration in a solder joint (approximately 3%) [55]. 

Several plated, tin-based lead-free finishes such as Sn, Sn Ag, Sn-Bi, Sn-Cu, and others have been found suitable with both Cu and alloy 42 component terminations (i.e., lead frames) [57]. 

Many component terminations are either plated or dipped into molten tin-lead solder to preserve their solderability, providing yet another source of lead. This is the normal procedure for component terminations that are difficult to solder, such as nickel-iron or nickel plating. Alternative lead-free finishes for components include palladium (Pd), and gold (Au) applied by electroless plating over nickel (Ni), and hot-dipped coatings of Sn-Ag, Sn-Cu, or Sn-Ag-Cu alloy. Some component metallizations are coated with Pd-Ag, Pd-Au, or Pd-Ni. These coating alternatives for tin-lead add new materials to the electronics waste stream, and potentially make recycling or reclamation of metals in electronic products more difficult and costly due to the increased number of separations required. 

Nickel/gold component terminations and solder pad surface finishes are the source for Au in solder joints where Sn Au intermetallics are formed. The formation of a brittle intermetallic compound (Au,Ni)Sn4 is a characteristic feature of these Pb-free alloys. The boundary between Ni3Sn4 and (Au,Ni)Sn4 is weak, and hence provides a low-energy path for crack propagation and the opportunity for the ductile-to-brittle fracture transition to occur. Subsequent aging indicates transgranular cracks at the (Au,Ni)Su4 layer [13]. Increasing the solder volume or solder joint size... 

Determining the bulk properties of various alloys is a starting point, enabling comparison among Pb-free alloys and baseline Sn Pb solders. In applications, the solder attachment process combines the alloy systems with minor elements from the component termination and from the PWB surface finish to form complex systems. The following sections describe the application of several key alloys and the reliability of the resulting complex systems they form. 

A large number of components are required for the synthesis of a polypeptide chain. These include all the amino acids that are found in the finished product, the mRNA to be translated, tRNAs, functional ribosomes, energy sources, and enzymes, as well as protein factors needed for initiation, elongation, and termination of the polypeptide chain. 

Defects are defined as those artifacts that are outside the window of acceptable attributes for the finished circuit board assembly. Thus, defects are not limited to the solder joints, specifically, but can also include damage to the circuit board material as well as degradation to component structures (e.g.,molding compound, leads or terminations, etc.). Defect types and their allowable frequencies (often expressed in parts-per-million solder joints or product units) vary with the different assembly processes and applications. Therefore, product drawings, in conjunction with industry standards (e.g., IPC-610), are used to establish accountable defect types for printed circuit boards. 

As mentioned above, the technique of minimizing the variability of isocyanate-polyol systems by using an excess of the isocyanate has limitations. An alternative approach is to employ isocyanate-terminated pre-polymers (of the type described in section 16.7.1(b)) to cross-link the polyol. Since these prepolymers are themselves film-forming materials, the use of a stoichiometric excess in a two-component system is not detrimental to the final film. This technique is applicable to the production of both flexible and hard coatings the former are suitable finishes for leather and rubber whilst the latter find use as sanding sealers. 

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