Board Surface Finishes

Quaternary alloys are composed of four elements (e.g., Sn-Ag-Bi-Cu) and pentanary alloys (e.g., Sn-Ag-Cu-In-Sb) are composed of five. The latter is least understood binary systems are best understood and easiest to formulate. Some of the small differences in compositions (0.5 wt percent) as seen in ternary, quaternary, and pentanary systems are difficult for solder vendors to control accurately during formulation. Also, in wave soldering, where the solder pot contents are in constant contact with metals from component leads and circuit board surface finishes, materials will be leached into the molten charge in the solder pot and will effect solder composition over the long term. Minor constituents of a solder may be depleted more quickly during wave soldering. 

HAST coated boards have been the industry mainstay and the most common board surface finish worldwide. HASL is generally eutectic or near-eutectic Sn-Pb flowed onto exposed surface pads and traces on a finished PWB at the board shop prior to electronic assembly. It is inexpensive, eminently solderable, and easy to apply, and has excellent shelf-hfe/aging properties. There is the old adage in the industry that nothing solders like solder, which has made HASL a safe and preferred finish for many years. Previously it was thought that the HASL process could not deliver flat enough pads for fine-pitch sirrface-mount. Now it is a well-proven surface finish for component lead pitches down to at least 0.5 mm. Reputable board shops no longer consider HASL difficult to apply. 

Certain no-clean fluxes are not compatible with each other. Their chemical interaction could result in corrosion on the PCA. To avoid corrosion issues, solder paste flux, wave solder flux, and repair flux should be tested separately and in combination for SIR and ECM with the board surface finish. 

A review was performed by Clech, which examined the state-of-the-art thermal fatigue data for Pb-free solders (Ref 45). Currently available data does not indicate a strong dependence of fatigue Ufe on either Sn-Ag-Cu composition or on circuit board surface finish. (The sensitivity of solder joint fatigue performance to circuit board finish and, in particular, those finishes that include a protective layer [e.g., Au, Pd, etc.] that dissolves into the solder, will become more significant as the size of the joint and thus, the volume of the solder per joint, becomes smaller.)... 

HASLed hare-board surface finish with a soldermask o... 

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... 

TABLE 8 Printed Wiring Board Surface Finishes Used in Manufacturing Trials... 

Figure 8 shows the maximum load values for SOIC device leads pulled from assemblies made with each Pb-free solder as a function of circuit board surface finish. Considering all board surface finish conditions and accounting for the experimental variability of the data, it was concluded that the performance of all solders is nearly equivalent and, with the exception of Sn58Bi, the differences observed are not statistically significant. Compared with all other down-selected solders, the maximum load values with Sn-58Bi are the lowest for imidazole, Ni/Au, Ni/ Pd, and Pd, and second lowest for Sn. 

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