Fig. 2: Hot Air Solder Leveling
However, for HDI applications, the HASL process presents a highly variable topography, or inconsistent surface planarity because of the formation of solder beads/balls not conducive to SMT, especially for QFP and BGA packages. In addition, depending on the alloy used for lead-free solder, the HASL process may be aggressive on copper, reducing the shelf life. While the thermal shock may cause warping of the PCB, there can be PTH diameter issues and bridging of fine pitch traces with solder mask residue preventing HASL from flowing. In addition, contamination on the surface of the copper or resin residue on the laminate may cause poor bonding.
Organic Solderability Preservatives (OSP) is a low-cost transparent coating of organic material, which preserves the copper surface from oxidation until assembly. The process involves application in a dip tank with the PCB in a vertical position, or the use of a conveyorized chemical process, which leaves a very thin coating of the material, typically 100-4000 Angstroms thick. Although OSP is a flat, reliable planar surface, well suited to BGA and QFP packages, the shelf life is rather low, being typically 6 months or lower.
Fig. 3: Organic Solderability Preservatives
OSP is difficult to inspect, and does not stand multiple reflows very well. This raises questions of reliability of exposed copper pads after assembly. As OSP is not conductive, ICT test pads need to be soldered.
Immersion Silver (ImAg) is a metallic solderability preservative, and the process deposits 8-15 micro inches of nearly pure silver on the copper surface. Although it provides a flat, planar surface, excellent solderability, and about 6-12 months of shelf life, immersion silver is sensitive to handling, packaging, electrical tests, and suffers from creep corrosion from salt and sulfur in the environment.
Fig. 4: Immersion Silver
Immersion Tin (ImSn) forms an intermetallic joint with copper to provide a uniform, dense coating with excellent hole-wall lubricity. As it is possible to engineer immersion tin to be non-porous and with very fine grain, it is the top choice for backplane panel assemblies requiring press-fit pin insertions.
However, immersion tin has a shelf life of 6 months, and is sensitive to handling. In addition, processing of immersion tin requires using Thiourea, a carcinogen with environmental issues.
Fig. 5: Immersion Tin
Electroless Nickel Immersion Gold (ENIG) is a complicated chemical process, involving nickel plating over the copper pad and subsequent gold plating over the nickel. The gold layer prevents the nickel from oxidizing during storage, while also providing low contact resistance, good wetting for solder, and excellent shelf life of typically 12 months. The flat planar surface is well suited for fine pitch devices such as BGA and QFP. Being conductive, ENIG offers good ICT contacts.
Fig. 6: Electroless Nickel Immersion Gold
However, ENIG is an expensive process, with non-wetting issues if the process has not been executed properly. Slow intermetallic growth can result in poor joint reliability and strength.
Electroless Nickel Electroless Palladium immersion Gold (ENEPIG) is another complicated chemical process, involving depositing electroless nickel on the copper surface, followed by a coating of electroless palladium layer, topped with a layer of immersion gold. The triple layer helps to form a superior solder joint with lead-free solder. As the process allows a thinner layer of gold, the process is less expensive when compared to ENIG, although the extra process step offsets this. The flat planar surface suits fine pitch devices such as BGA and QFPs. As the shelf life is typically 12 months, ENEPIG is the fastest growing surface finish.
Fig. 7: Electroless Nickel Electroless Palladium Immersion Gold
PCB manufacturers prefer ENIG and ENEPIG to others because of the relative advantages the two techniques offer, although between the two, their advantages vary. ENIG is suitable for SMT, especially for BGA and other fine pitch components. The technology works well for lead-free soldering, and is highly reliable, which is why the flex PCB market prefers ENIG.
On the other hand, ENEPIG has a much wider acceptance and is suitable for multiple types of packages including THT, SMT, wire bonding, press fit, and more. Apart from being suitable for fine-pitch SMD components such as BGA and QFPs, ENEPIG is applicable to PCBs with different manufacturing technologies, requiring higher densities and reliability.