THE SOLDER JOINT RELIABILITY OF CONFORMAL COATED WLCSP COMPONENTS
Author: J. Salminen Company: Nokia Corporation Date Published: 10/4/2009
Abstract: The compatibility of PWB coatings with assembly materials such as solder pastes, fluxes, component adhesives, underfills and conformal coatings is important issue to achieve the reliable quality in board assembly. In this work was evaluated the coating adhesion and solder joint reliability for assemblies with different conformal coatings, solder masks and underfill. The daisy chain wafer level CSP components were assembled on PWB with Pb-free solder paste. The conformal coating materials were jet dispensed on WLCSP components. The PWB specimens of three different types of solder masks, four different coating materials and one type of underfill were stressed by thermal cycling. Under thermal cycling conditions, solder joints are subject to strain and stress because of the mismatch among the coefficients of thermal expansion (CTE) in the board, component, conformal coating and solder. The samples were subjected to 1000 temperature cycles starting from -40 oC to +125 oC. The solder joint lifetime was electrically monitored by resistance measurement and the metallurgical characteristics of solder joints were analyzed by cross-section samples. Results indicate that there is clear effect on solder joint reliability of WLCSP components depending on combination of conformal coating, solder mask and underfill. Statistically components coated with coatings: B and C gave higher solder joint reliability than uncoated components, and components coated with type D have lower solder joint reliability than uncoated components. Coating E has no significant effect on solder joint reliability. Solder mask and underfill were shown to have a stronger effect than coating type. Coating performance is related to material compatibility. It is important that all materials (solder pastes, solder masks, components, underfills) are carefully screened for compatibility before applying conformal coating on PWB assembly.