Surface finish Termination

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. 

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 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. 

Dunford et al. investigated the metallurgical and reliability aspects of Pb-free mixed technology electronic assemblies utilizing area array packages, leadless ceramic chip passives, and small outline integrated circuits with conventional Sn-Pb terminations on a thin PWB with Au/Ni surface finish utilizing Sn-Ag-Cu-Sb solder as the interconnection alloy. The Sn-Au and Sn-Ag... 

Joint areas are also prepared during surface preparation. For instance, with bell-and-spigot-type PCCP, the inner concrete is first demohshed to expose the steel liner at both the beU and spigot ends. The extent of the demohtion depends on the thickness of the inner-core concrete and the required bond length at CFRP terminations. Adequate area is to be demolished to allow for the type of transition called for in the design. The steel surfaces at termination detail areas are also prepared to at least a near-white metal finish. 

The finished cellulose is in the form of crystalline microfibrils (Fig. 20-29), each consisting of 36 separate cellulose chains lying side by side, all with the same (parallel) orientation of nonreducing and reducing ends. It seems likely that each particle in the rosette synthesizes six separate cellulose chains simultaneously and in parallel with the chains made by the other five particles, so that 36 polymers arrive together on the outer surface of the cell, already aligned and ready to crystallize as a microfibril of the cell wall. When the 36 polymers reach some critical length, their synthesis is terminated by an unknown mechanism crystallization into a microfibril follows. 

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