Solder protective finishes

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. 

Soldered joints should be moisture proofed and the joint area of an electrical bond should, after bonding, be provided with a protective finish (organic coating, sealant, paint system). All contaminants should be removed from conductor surfaces. 

The next phase will address eliminating lead from board finishes—the protective coatings applied to termination pads on printed wiring boards to protect metal conductors from degradation (e.g., oxidation, corrosion) and remain solder-wettable. Finishes are applied in a number of ways, including dipping into a molten metal bath (e.g., tin, solder), electroless plating, etc. Alternative finishes must, of course, be compatible with the lead-free alloy selected in Step 1. 

The PCB industry produces a number of lead-free finishes among them are nickel/immersion gold (Ni/Imm Au), Imm Ag, Sn, Ni/Pd, and organic solderability protectants (OSPs). 

One approach uses a single coating—the protective finish. As the terminology implies, the protective finish prevents the loss of base metal solderability due to rapid reoxidation prior to soldering. The role of the protective finish in a soldering process is illustrated in Fig. 20 [5]. The protective finish is initially wetted therefore, it must be solderable. Then, the coating is dissolved into the molten solder. The solder proceeds to wet and spread over the underlying (pristine) base... 

In those cases in which it is impractical to sustain base metal solderability, a metalhc coating is applied to the base metal. The molten solder wets and spreads over the coating and, ultimately, makes a bond to it. This coating is referred to as the solderable layer. Typically, a protective finish... 

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. 

Several solderable and protective finishes along with their thickness as specified by MIL-STD 1276D are listed in Table 5 [5,40]. One of the most commonly used solderable-plus-protective layer combinations is Ni and Au. The likelihood of Au embrittling Pb n solder joints is reduced by a hot-solder dipping operation that replaces the Au with a Pb-Sn finish [41,42]. Copper is a frequently used solderable coating it is often coated with a Pb n or Sn protective finish (electroplated, plated and fused, or hot-solder dipped). Other protective layers include... 

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

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. 

A ground system is only as good as the methods used to interconnect the component parts [9]. Do not use soldered-only connections outside the equipment building. Crimped/brazed and exothermic Cadwelded) connections are preferred. (Cadweld is a registered trademark of Erico Corp.) To make a proper bond, all metal surfaces must be cleaned, any finish removed to bare metal, and surface preparation compound applied. Protect all connections from moisture by appropriate means, usually sealing compound and heat-shrink tubing. 

Before the widespread use of solder mask, a primary method for protecting copper circuitry was to electroplate a coating of tin or tin-lead on the copper. After component insertion, the tin plating could be liquefied with oven or vapor-phase reflow to form solderjoints.The use of solder mask over bare copper to take advantage of surface mount, wave soldering, and mixed assembly restricted the use of tin plate finishing. For simple technology product, tin or tin-lead plate and reflow remains a viable fabrication and assembly method. 

To ensure good soldering, it is necessary for the assembler to ensure that aU copper is protected and no solder mask residue remains on solderable featnres. By inspecting the bare board, the engineer can reject parts that show bare copper due to skipped surface finishing or mask residue. A special case is OSP, where inspection of the protected copper is impossible because OSP-protected areas have the same visual appearance of uncoated copper with or without thin solder mask contamination. 

When solder masks were first used, their purpose was to protect the circuitry of the PCB from solder during the assembly operation. Eliminating short circuits was the main objective. As the use of solder mask became more prevalent and the material properties improved, solder mask was used for other functions, such as to provide environmental protection to the assembled board, and to serve as a plating resist for final finishes, dielectric protection, and other functions that take advantage of the properties of the cured mask. 

If the hole is simply coated with solder mask and the solder mask is not developed from the hole, the air trapped in these small holes may expand and create a bubble or blister, or may erupt, exposing the copper in the hole to potentially corrosive chemistries. These holes should be developed cleanly so that they receive the final finish, should be protected before solder mask application with an inert final finish or be completely plugged. 

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