2. Root cause analysis:
By analyzing the root causes of defects, defect rates can be reduced through process improvements and parameter optimizations. The Gianni effect typically occurs where cracks form between the solder resist film and the copper surface. During silver deposition, these small cracks limit the supply of silver ions to the solution, while copper can corrode into copper ions. Silver precipitation then occurs on the exposed copper surface outside these cracks. Ion conversion drives the silver precipitation reaction, with the extent of copper surface attack under cracks directly correlating to the thickness of the silver deposit.
3. Cracks can form due to several reasons:
– Excessive lateral erosion or poor adhesion of solder film to copper surface.
– Uneven electroplated copper layer (thin copper or voids).
– Deep scratches on the substrate copper under the resist film.
4. Corrosion occurs when sulfur or oxygen reacts with a metal surface. Silver reacts with sulfur to form a silver sulfide (Ag2S) film, which turns black with high sulfur content. Silver also reacts with oxygen to form dark brown cuprous oxide, especially when the silver deposition rate is rapid, creating a low-density layer prone to oxidation.
5. Exposure of copper usually occurs due to chemical processes before silver deposition or incomplete removal of residual films obstructing silver layer deposition. Residual films, often from the resist stripping process, block silver precipitation reactions.
6. Mechanical treatment processes and the circuit board’s surface structure affect solution contact uniformity, influencing the evenness of silver deposition. Inadequate solution circulation can lead to uneven silver layers.
7. Ion contamination from ionic substances on the circuit board surface, originating from the silver immersion solution or trapped under the solder resist film, can impact electrical properties. Different silver solutions have varying ion contents, affecting pollution levels after washing. High porosity in silver layers retains more ions, complicating washing processes and increasing ion pollution.
8. Post-washing effectiveness directly impacts ion pollution levels; inadequate washing or poor water quality can exceed standards.
9. Microvoids, typically less than 1 mil in diameter, occur at the interface compound between solder and the welding surface on OSP, ENIG, and silver sediment surfaces. Their presence significantly reduces solder joint strength.
10. The root causes of microvoids are complex, involving factors such as:
– Rough copper surface structure beneath thick silver layers (>15μm).
– Types and components of organic matter co-deposited in the silver layer.
11. Despite numerous simulated welding studies by OEMs, EMS providers, PCB manufacturers, and chemical suppliers, microvoid elimination remains elusive.
2. Preventive measures
In order to avoid or eliminate defects and improve yield, preventive measures need to address the contribution of chemicals and equipment to defects during production. The prevention of the Gianni effect begins with the pre-process copper plating stage. For high aspect ratio holes and micro through holes, achieving uniform electroplating thickness helps eliminate the hidden risks associated with the Gianni effect. Excessive corrosion or side erosion during film stripping, etching, and tin-stripping processes can lead to crack formation, often retaining residual corrosive solutions within. However, the main cause of the Gianni effect remains solder film issues. Most flawed boards exhibiting the Gianni effect show signs of side erosion or solder film shedding, primarily stemming from the exposure development process. Thus, developing the solder film post-“foot-forward,” ensuring complete solidification, can effectively mitigate the Gianni effect.
To achieve optimal silver deposition, it is essential that the silver be 100% over copper, with each tank solution offering excellent perforation capabilities and effective through-hole exchange. For intricate structures such as HDI boards, integrating ultrasonics or injectors into pre-treatment and silver sinking processes proves beneficial. Managing silver precipitation processes can enhance Gianni effect prevention by controlling micro-erosion rates to achieve smooth, semi-bright surfaces. Original Equipment Manufacturers (OEMs) should avoid large copper surfaces or high aspect ratio through-holes connected to thin lines to eliminate potential Gianni effect risks. Chemical suppliers should provide silver precipitation solutions that maintain a balanced pH, controlled silver precipitation rates yielding desired crystal structures, and thin silver thickness for corrosion resistance.
Reducing corrosion involves increasing coating density and decreasing porosity, utilizing sulfur-free packaging during storage to prevent airborne sulfur contact with silver surfaces. Packed boards should be stored at 30℃ with 40% relative humidity, following a first-in, first-out principle despite the long shelf life of silver plates. Optimization of pre-settlement processes can minimize exposed copper, confirmed by water break or bright spot tests after microetching to ensure a clean copper surface retains a water film for at least 40 seconds. Regular equipment maintenance ensures uniform solution circulation, while DOE optimization of time, temperature, and agitation parameters for silver settling operations ensures desired thickness and high-quality silver layers.
Mechanical aids such as ultrasonics or injectors enhance silver sink solution wetting of micro through-holes, high aspect ratio holes, and thick plates, facilitating HDI board production. To reduce ion contamination, maintain low ion concentrations in the silver precipitation solution, and periodically test for compliance with industry standards using deionized water for cleaning. Record and retain results to distinguish major contaminants, especially microvoids, whose prevention involves identifying and minimizing causal factors. Adjusting silver deposition thickness and microerosion rates can yield smooth, uniform surfaces, while monitoring organic matter content ensures purity throughout the tank liquid’s service life, maintaining silver content above 90% atomic ratio.
3. The ideal process – AlphaSTAR
In addition to superior performance, an “ideal process” must meet electronics industry requirements for safety, environmental protection, and reliability. Introduced by Les Chemical in 1994, the AlphaLEVEL product series continually evolves, culminating in the third-generation silver sinking technology for printed circuit boards—AlphaSTAR. This process, comprising seven steps (including three washing stages), not only enhances performance but also addresses PCB obsolescence, cost escalation, environmental concerns, and regulatory compliance since July 1, 2006.
AlphaSTAR’s pre-treatment includes oil removal, water washing, micro-etching, and subsequent water washing steps. The low surface tension of the oil removal solution ensures all copper surfaces are wetted, resolving copper exposure issues and promoting silver layer deposition in high aspect ratio and micro-through holes. Its unique microetching formula yields a slightly coarse, semi-glossy surface structure conducive to forming fine, dense crystal silver layers, even at minimal thickness. This significantly boosts silver layer density and corrosion resistance.
The silver precipitation process involves pre-leaching, silver precipitation, and deionized water rinsing. Pre-leaching serves multiple purposes: sacrificial solution to prevent contamination, ensures a clean copper surface for silver precipitation, and facilitates automatic silver sink replenishment. The controlled substitution reaction between copper and silver ions, aided by AlphaSTAR’s micro-etched copper surface, yields a gradual, uniform silver layer deposition at a moderated rate. This dense, moderately thick (6-12U”) silver layer offers high corrosion resistance and excellent electrical conductivity. Moreover, AlphaSTAR minimizes downtime, reduces ion contamination, and lowers equipment costs.
4. Conclusion
AlphaSTAR combines exceptional finishing properties meeting and exceeding PCB industry standards for solderability, reliability, safety, and regulatory compliance. With a broad operational window, ease of operation, control, and maintenance, it ensures cost-effective production with rework capabilities. Addressing key silver precipitation-related issues discussed earlier, AlphaSTAR adheres to RoHS and WEEE regulations, ensuring completely lead-free silver layers.
