Hot air solder leveling

Type of finish and quality requirements (hot air solder leveling [HASL] versus organic sol-derability preservative [OSP], solder mask). 

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. 

Bi forms a binary eutectic with Sn in the proportions 58Bi 42Sn (m.p., 138 C). There are numerous bismuth alloys in use, many composed of two or more metals in addition to bismuth. The Sn-Bi eutectic, if contaminated with Pb, can be problematic as it is known to form a ternary alloy with a melting point of 96°C adversely affecting solder-joint fatigne characteristics. In some applications, solder joints will fall apart if the service temperatnre is high and the low melting point Sn-Bi-Pb alloy is formed. This becomes all the more critical with low-volume Sn-Bi solder joints. The Pb can come from solder predeposited on component leads, Sn-Pb hot-air solder leveled pads, or both. 

Hot air solder leveling (HASL) or reflowed tin on copper bond pads possess a thin intermetallic layer between the applied surface finish and the pads. During surface-mount technology (SMT) reflow, the addition of molten solder on top of this preexisting intermetallic layer will further its growth. Therefore, the TAL should be limited to minimize IMC but sufficient for good solder wetting. 

For processes utilizing potentially corrosive rosin based fluxes, simple test options to determine cleanliness such as resistance of solvent extract (ROSE) tests can be used. Other more significant tests may be required to determine the residue properties of organic residue fluxes, such as those commonly used during hot air solder leveling (HASL) of boards.The most common of these are ion chromatography tests that can characterize the residuals and their potential hazards to the product. 

Ready et al. showed that the use of certain water-soluble fluxes or fusing (hot air solder level [HAST]) fluids could increase CAF formation. In examining a catastrophic field failure (see Fig. 56.9), Ready et demonstrated the presence of a copper-bromide containing salt between an innerlayer power plane and ground pin, separated by 0.005 in. Using a test coupon manufactured in the same lot as the failed product, they extracted the flux residues from an innerlayer of the multilayer board and used ion chromatography to match the residues from the board s innerlayer with the constituents of the flux. 

Thermal shocks and thermal cycles during PCB manufacturing. Thermal shocks are usually defined as temperature ramps faster than 30°C/s, but include any ramp fast enough that temperature differentials play an important role. Examples include solder mask cure and hot-air solder leveling. 

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. 

Surface Finishes. The search for alternatives to hot air solder leveling (HASL) has been ongoing for several years, primarily because of the inherent inconsistency in the quality of the HASL finish. For example, the thickness (and therefore, solderability) of HASL is difficult to control. In areas with a very thin layer of HASL, consumption of Sn by the formation of tin-copper intermetallics will render the areas non-wet-table. The HASL finish is typically non-flat (with a dome shape), making it difficult to deposit a consistent amount of solder paste during solder paste printing and difficult to place fine pitch (<25 mil) devices. The HASL process itself is not as clean and easy to control as some plating processes. The current move towards lead-free solder has provided the additional impetus towards alternative surface finishes. 

This chapter reviews Uteratme on the microstructure of solder joints and the interactions of solders with substrates and related solder joint reliability issues. The substrates here are limited to Cu, Ni-coated Cu, electroless nickel/ immersion gold (ENIG), and hot air solder leveled (HASL) Sn-Pb. The solders are mainly Sn, eutectic or near eutectic Sn-Ag, and Sn-Ag-Cu alloys. Specific reliability issues discussed here include black pads of ENIG, gold embrittlement, compatibility of Pb-free solders with Pb-containing surface finish, and Kirkendall voids. 

BASE hot air solder leveling S-N stress versus cycles to failure (plot)... 

Molten coatings such as hot air solder leveled coatings (HAST) on PCBs are applied via specialized equipment that coats the substrate with molten metal selectively in the areas where the finish is desired. The thickness distribution is not as uniform as with plated finishes. As the molten coating method involves application of a molten metal at very high temperatures, the... 

TABLE 4 Variations Typical of Eutectic Sn-Pb Hot Air Solder Leveled (HASL) Finishes... 

FIG. 21 Utilizing the hot air solder leveling (HASL) process, (a) A conventional pad coated with Sn-Pb solder, (b) A via-in-pad is filled and coated with sn-Pb solder. (Courtesy of Universal Instruments... 

A number of surface finishes have been utilized over the years to protect against oxidation of solderable features. Hot air solder leveling (HASL) and rosin (lacquer) coatings, once popular, are now disappearing due to thick and irregular deposition. With the advent of fine pitch, HAL surface finishes have become a liability. These finishes have been moving more toward thin-film deposits, which are thin, uniform, and planar. Planarity is an important characteristic for fine-line applications. This realization has directed the search for viable surface finishes that fulfill this condition. 

HASL Sn-Pb (hot air solder leveled) used only with eutectic Sn-Pb solder as a baseline... 

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