NEPCON West - Fiberoptic Expo Conference Proceedings


Authors: Roger Jay and Alfred Kwong
Company: Solectron Corporation
Date Published: 12/3/2002   Conference: NEPCON West - Fiberoptic Expo

Abstract: Hot Air Solder Leveling (HASL) is still used widely as a surface finish for printed wiring boards because it provides a low cost, reliable finish for SMT applications in cases when land planarity is not critical. The finish is applied by fluxing the board and dipping it into molten eutectic Sn-Pb solder, then passing it through a pair of hot air knives that blow off the excess. Because there is limited control of thickness variation across the pads the allowable thickness range is between 1.5 and 5.1 microns.

This thickness variation is one of the drivers which push the use of alternative metallic coatings such as Organic Surface Preservative, Electroless Nickel Immersion Gold, Immersion Tin, and Immersion Silver. The recent occurrences of failures associated with the Electroless Nickel Immersion Gold finish has prompted many customers to revert to HASL. Unfortunately, the attempts to provide a flatter HASL coating by increasing the pressure on the hot air knives often result in thin surface coverage which may be mostly intermetallics. This wll be more prevalent with fine pitch devices i.e. 0.4 mm QFPs and 0.5 mm CSPs.

The focus of this paper will be on the effect of thin HASL coatings on SMT solderability, and its analysis using SEM/EDX and micro-sectioning techniques. Numerous examples of SMT failures related to thin HASL coatings will be shown as well as non-destructive inspection methods for bare boards. This paper can be used as a reference guide for SMT manufacturing engineers for the root cause analysis of solderability problems involving thin HASL finish boards.

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