What is "Creep Corrosion?"
Creep corrosion is a condition in which shorts are formed across copper component mounting pads or circuit traces by copper salt migration. The copper circuitry itself is the source of copper-for-copper sulfide salt formation. The copper salt can migrate through either a porous metallic or organic over-coating, or from the edges of solder mask. Creep corrosion has been observed with all surface finishes, but electroless nickel-immersion gold (ENIG), and immersion silver final finishes have a higher rate of failure.
What causes the "creep?"
Free copper, available from solder mask-defined areas or from migration through the silver pores or grain boundaries, reacts with sulfur in the presence of silver. Copper sulfide and silver sulfide are formed. Copper sulfide is soluble in water and will spread in the presence of renewed moisture. In a condensing environment, the salts creep outward from the copper source. The silver remains intact, except for areas of copper migration.
What is "Flaking Corrosion?"
A leading OEM has identified "flaking corrosion" as a secondary, yet important, failure mode in determining corrosion defects. Flaking corrosion negatively impacts unsoldered features left exposed to the environment. Even if creep corrosion is prevented through proper design and assembly materials, there still exists the possibility of corrosion of the copper beneath the PWB surface finish. If this corrosion is severe enough, it can result in lifting or flaking of the corrosion products, known as "flaking corrosion." Once this corrosion by-product separates from the feature, the conductive flakes can lead to shorts in the assembly.
Enthone's Solution: AlphaSTAR 420R
AlphaSTAR 420R is a specially formulated corrosion inhibitor for the AlphaSTAR immersion silver process. During immersion in the AlphaSTAR 420R working solution, any exposed copper at the solder mask boundary areas or from porosity is selectively coated. The working solution consists of a highly active oxidation inhibitor, dissolved in a water-based system. AlphaSTAR 420R is non-poisonous, non-irritating and may be safely applied to immersion silver-coated PWBs. Because the inhibitor is selective to exposed copper, it does not influence electrical contact resistance. A treatment with AlphaSTAR 420R is suitable for electrical contacts and other electronic components. Solderability is unaffected.
How does AlphaSTAR 420R work?
AlphaSTAR 420R is a copper preservative. The molecule will selectively attach itself to any available copper. Free copper can be found at edges of solder mask or in immersion silver coating porosity. The effectiveness of AlphaSTAR 420R is apparent, as shown in the two photos to the right. The top right photo shows an immersion silver board without post-treatment subjected to a harsh environment. In the bottom right photo the same PWB placed in the same environment, but treated with AlphaSTAR 420R, results in a tarnish-free and corrosion-free surface.
Unlike solvent-based systems, AlphaSTAR 420R is water soluble chemistry that enables ease of application, handling and rework. No additional, costly equipment is required. AlphaSTAR 420R readily penetrates micropores (where corrosion can propagate). If corrosion should form, AlphaSTAR 420R ensures that corrosion is suppressed, thus reducing the risk of shorts from flaking conductive material.
The coating also prevents flaking corrosion. Independent testing by a leading OEM has shown AlphaSTAR 420R treated parts survive 15 days of mixed flowing gas testing.
Is the AlphaSTAR 420R coating thermally stable?
Will it remain there after assembly?
Testing has shown the active ingredient in AlphaSTAR 420R is thermally stable at temperatures considerably higher then lead-free conditions generate. The coating will remain after assembly.
Does the AlphaSTAR® 420R coating affect solderability?
No. Internal and external testing on real assemblies has shown no degradation of solderability. Extensive builds by key OEMs have shown no assembly issues related to the AlphaSTAR 420R inhibitor. Wetting balance tests confirm good solderability. (See graphs to the right.)
Does the AlphaSTAR 420R coating affect long-term reliability?
Long-term reliability is dramatically improved by suppressing creep corrosion and flaking corrosion in harsh environments. There is no impact on surface insulation resistance or electrochemical migration. In fact, testing to Bellcore GR-78 standards for electrochemical migration has shown AlphaSTAR with 420R inhibitor exceeds requirements for reliability.
Ionic contamination: After one reflow cycle or a short bake cycle, AlphaSTAR 420R will pass the most stringent industry requirements.