Surface finishes Silver, immersion

Surface finish immersion silver (Ag), electrolytic nickel/gold (NiAu)... 

Table 32.7 shows the fabrication process for immersion silver surface finish, including the chemicals used and dwell times. 

Table 32.8 compares the advantages and disadvantages of using immersion silver as a surface finish. 

TABLE 32.8 Advantages and Disadvantages of Immersion Silver as a Surface Finish... 

In all cases, the mask preparation steps are critical. H the copper is treated well to adhere to solder mask, the interaction from surface finish chemicals is greatly reduced. It is imperative that there is no wedge between the mask and the copper into which the surface finish chemicals can creep. In extreme cases of chemical entrapment, the chemical I eactions are accelerated in the wedge, leading to nonuniform plating. Immersion silver, tin, and OSP can lead to excessive copper corrosion at the solder mask edge and result in electrical opens. This phenomenon is also known as solder mask interface attack. 

The substrate surfaces of choice are usually one of the following electroless nickel/immersion gold (ENIG), immersion silver (ImAg), and organic solderability preservative (OSP). Two other finishes, hot air solder leveling (HASL) and immersion tin, are also in use, but HASL has declined in volume and immersion tin has not reached significant volumes due to concerns over reliability. 

ImAg Immersion silver provides a flat surface with good solderability and storage life. It is also less expensive than ENIG. However, the ImAg finish is prone to tarnish under certain atmospheric conditions in the presence of sulfur, it forms sulfides that darken the surface and reduce solderability. 

Corrosion resistance Some surface finishes are more prone to corrosion than others. Immersion silver and copper are two such finishes. 

Other surface finish-delated defects Immersion silver (Imm-Ag) sometimes exhibits hn-ear arrays of microvoids at the intermetalhc interface which detracts from solder joint strength. Electroless nickel/immersion gold (ENIG) sometimes results in brittle fracture if the plating chemistries are not maintained properly. 

There are a number of other surface finishes used in the industry, such as Electroless Nickel Electroless Palladium Immersion Gold (NiPdAu), Immersion Silver, Immersion Gold, Immersion Tin, OSP and Electrolytic Nickel Gold. There are reliability and process trade offs with each surface finish. That is why it is recommended that strain/strain rate characterization and thermal cycling be performed for each set of surface finish before it is selected for the specific end-use conditions in which it will be used. The industry test methods used to evaluate different surface finishes are outlined in detail in the next chapter. 

M. Arra, D. Shangguan, J. Sundelin, T. Lepistd, and E. Ristolainen, Aging Mechanisms of Immersion Tin and Silver PCB Surface Finishes in Lead-Free Solder Applications, Proceedings of the 3rd IPC/JEDEC Annual Conference on Lead-Free Electronic Assemblies and Components,... 

The surface finishes selected were Immersion Silver, Immersion Tin, two HASL (lead-free) and four Organic Surface Protections (OSPs). Four solder alloys were planned for testing, but two were eliminated due to cost, leaving SACX and Sn/Cu/Ni. Five fluxes were evaluated, two of which were chosen because they were commonly used in other experiments. 

An alternate to immersion silver is electrolytically deposited silver. Although electrolytic silver is not commonly applied to PWBs, it is a very common electronic component finish that is particularly suitable for wire bonding applications. A standard procedure used in the industry 0n contrast to a Ni/Pd finish) is to directly wire bond to an electrolytically deposited Ag pad and component lead surface as depicted in Fig. 13. 

Immersion silver plating was chosen as the board surface finish because of the consistency in plating thickness and surface topography. The major unknown that had to be evaluated was the suitability of the surface finish. Because there is a significant schedule delta among component... 

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. 

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