Authored By:
Norman J. Armendariz, Ph.D.
Raytheon Company
Andover, MA, USA
Summary
Recent CCAs-circuit card assemblies manufactured using SMT-surface mount technology processes were exhibiting cleaning residues remaining on the surface and within electronic component / package housings with discoloration (degradation) of Cu-wire insulation and (oxidation) of metal contacts observed, which may compromise the surface conformal coating properties, and possibly degrade the component's electrical performance and long-term reliability.
Certain components or package configurations exhibited more entrainment and/or entrapment of cleaning residues. It appears that the component housing design is a major factor that in certain configurations, that either readily entrain cleaning solutions, and/or limit the ability of aqueous rinsing to effectively remove any remaining cleaning agents and residue from the electronic materials housed within. In other words, certain components have more "rinse-ability" than others.
This paper will discuss the "physics" of liquid entrainment / entrapment / cleaning / rinsing and their effects on
component materials validated with experiments performed to determine the minimum amount of component housing vent area required to adequately rinse components of their cleaning residues with the objective of providing electronic component designers a DfR or "Design for Rinse-ability" specification or guidelines to deploy.
Conclusions
There are two (2) major chemical interactions reacting with SMT assembled electronic component materials in cleaning bath solutions that enter component housings via capillary forces, through seams / holes and / or insufficient vent openings for adequate rinsing, are listed as follows:
1). Cleaning agents; that primarily degrade polymers (wire insulation)
2). Ionics or halides from cleaning bath; primarily chlorine (catalyst) that primarily oxidize metals (contact pads). Cleaning chemistries were reviewed with the current cleaning solution A considered more aggressive than cleaning solution B, in particular on materials, such as polyurethane used in Cu wire insulations.
The component housing construction / assembly design in terms of vent areas or openings is a major factor that determines if a component will readily entrain cleaning solutions and / or limit the ability of water rinsing to effectively remove cleaning residues.
The effect of component cover vent areas was clearly demonstrated from SMT RF / cleaning experiments that strongly correlated the effect of component vent areas on the occurrence of cleaning residue entrainment / entrapment with a minimum vent area required.
Thermal processing effects may compromise the bonding interface seals, creating conditions for capillary forces to come into play to entrain and entrap cleaning solutions within components, and at the same time, water is prevented from entering and rinsing-out contamination. In summary, certain components have more "rinse-ability" than others. If the component is designed in such a manner that water can easily flow in and out of the component, then the component should be rinsed of any residual contamination, thus reducing the effect of any latent materials degradation on the components electrical performance and reliability.
Initially Published in the SMTA Proceedings
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