Authored By:
J. Salminen
Nokia Corporation
Salo, Finland
Summary
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 °C to +125 °C.
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.
Conclusions
This work successfully evaluated coating adhesion and solder joint reliability of WLCSP components coated with four different conformal coatings using thermal cycle testing to see differences between material combinations. Large CTE mismatch between silicone die of WLCSP component and coating materials have been a concern. Visual inspection after 1000 cycles the components on all assemblies coated with B showed that the coating had flipped. There were some cracks for all assemblies that used coating C. However there was no visual damage on coatings for assemblies coated with coatings D and E.
The solder joint electrical failure results reveal a more complex picture, as solder joint reliability not only depended on coating material, but also other variables, solder paste, solder mask and underfill. Statistically components coated with coatings B and C, give higher solder joint reliability than uncoated components, and components coated with coating 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. The type of solder paste did not have significant influence on solder joint reliability of WLCSP components.
Failure Analysis was conducted to determine the root cause for failure mechanisms of WLCSP components. Weibull analysis was conducted for failures with the main failure mechanisms. The main failure mechanism determined by cross section samples was solder joint cracking on component side. 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.
Jet dispensing technology has many advantages for selective coating of small parts, especially when coating needs to be applied in hard-to-reach spaces. It is very demanding process. The process parameters of jet dispensing must be carefully found for different types of coating materials.
Initially Published in the SMTA Proceedings
|
