Solder solderable finishes

There are other products available—dispersions, paints, coatings, blends—for industrial commercial applications or R D use outside of polymer electronics printed circuit board final solderable finish antistatic coatings screen printable electrodes antistatic and anticorrosion coatings. 

Finally, the board is manufactured and tested. Then the board is ready for assembly and soldering. Finished boards are tested for system functionality and specifications. 

PTH reliability and on the reliability of the solder joints made to these pads. Common metal finishes for solder-mask-over-bare-copper (SMOBC) boards inclnde hot-air solder leveling (HAST or HAL), organic-coated copper (OCC), and electroless NiAn. Galvanically plated CuNiAu and CuNiSn made by another processing route are also available.These finishes provide a solderable finish for later printed circuit assembly. The pros and cons of the various finishes are discussed in turn. 

Fig. 8.20 Applications of electroless nickel plating Ni-P deposits for wear-resistant and corrosion-resistant applications in engineering, (a) A cooling-coil assembly. (Photography courtesy Ionic Surface Treatments plating by Dudley Division.) (b) Printed circuit boards for a wrist watch and calculators. Electrically isolated areas of deposited copper may be built up by electroless nickel plating which also provides a corrosion-resistant and solderable finish. The deposits are various NIKLAD electroless nickels. (Photograph Courtesy Lea Manufacturing.)...

IMC) layers. In some cases, the base materials have a solderable finish to which the solder has been joined. Then, there is a solder/protective finish interface that typically also generates an IMC layer. There is also an interface between the substrate material and the solderable finish. A solderable finish is covered with a protective finish before soldering. The protective finish is dissolved into the molten solder during the assembly process. The dissolved protective finish can impact the mechanical properties of the solder, the extent of which depends upon the thickness of the coating and the volume of the solder. In summary, the mechanical performance of the solder joint is a function of the mechanical properties of the solder, the base materials (including solderable finishes, if present), and the mutual interfaces. 

Tin Whisker Growth on Lead-Free Solder Finishes... 

The orientation distribution on the solder finish around the root of whisker is shown in Fig. 

Lead-tin solder joints are commonly made to nickel (Ni), the Ni being either a bulk material or a thin solderable finish. The reaction results in the formation of primarily Ni3Sn4 intermetallic compound. The Pb component of the solder has no explicit role in the interface reaction. The reactivity between the Sn component of the Pb-Sn solder and Ni is considerably slower compared to Cu for the liquid-state dissolution reaction (see Fig. 9) as well as the... 

FIG. 21 Schematic diagram depicting the processes associated with molten solder spreading over a solderable finish/protective finish combination. (From Ref. 5.)... 

The functional limitations placed on solderable finish/protective finish systems are illustrated in Fig. 21 [5]. As noted above, spreading of the solder front requires that the surface of the protective finish be solderable (free of contamination or oxidation). The protective finish is sacrificial as it is dissolved into the solder. Therefore, the protective finish must be sufficiently thick to protect the solderability of the underlying solderable layer, but not excessively thick to pose a significant contamination concern for the subsequent solder joint (e.g., Au in Pb-Sn solder). The solder then wets to the surface of the solderable layer. The requirements placed on the solderable layer are the following it must be solderable it must have adequate thickness so as not to be completely dissolved by molten solder during the initial assembly process or during subsequent process steps (including repair or rework cycles) and the layer must adhere to the base metal surface. 

TABLE 5 Commonly Used Solderable Finish/Protective Finish Combinations... 

The materials used for the leads of component packages for through-hole assembly are typically Cu, or Fe-Ni alloy having a Cu or Ni solderable finish with a Sn or Pb-Sn protective layer. 

Peripherally leaded, surface mount packages use lead materials of Cu or an Fe-Ni alloy with a Ni solderable finish and an electroplated Pb n or Sn protective layer. Leadless chip devices have terminations comprised of a fired-on Ag thick film conductor that is overplated with a Ni or Cu solderable coating, followed with an electroplated Pb-Sn protective finish. Leadless ceramic chip components (LCCC) use castellated terminations with a thick-fihn Au finish. 

FIG. 7 The effect of solder finish on an arbitrary wetting index. 

Choi et al. [42] intuitively felt that the microstructure of the whisker grain should be different from the surrounding grains. From the pole figures in Fig. 13, they determined that the surface layer of the Sn-Cu solder finish had a (321) texture, and from Fig. 15 they determined that the grain just below the whisker exhibited a (210) orientation. This (210) grain was, therefore, considered to be a discontinuity in the predominantly (321) microstructure. Choi et al. conjectured that this (210) structural discontinuity could be a location where the surface oxide could easily be broken by a whisker growth. 

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