Correlation between dispersion process of aluminum-silver paste and castability
Dispersion technology is the "key to molding" of the castability of silver paste: dispersion determines whether the aluminum-silver paste is free from agglomeration, fully wetted and evenly distributed, and directly determines whether the castability is stable and evenly dispersed. Even the best formula will lead to immobile flow, uneven paving and defects.
Let's make it clear with the logic that can be directly corresponding to the scene: good dispersion → good casting; Poor dispersion → casting collapse.
First, good dispersion → good castability (ideal state)
1. The aluminum-silver paste is completely depolymerized without agglomeration.
The particles are dispersed independently, the flow resistance is the smallest, the shear thinning behavior is standard, and it spreads smoothly and fills in place during dip coating.
2. Uniform fineness
No large particles, smooth wet film, no orange peel, no pockmarks, no wire drawing, and high consistency of end thickness.
3. Rheological stability
The viscosity and thixotropy index TI fluctuate within batches/between batches, and the casting behavior is controllable.
4. The powder is completely wetted with the organic phase.
Good interface bonding, no shrinkage, no shrinkage, no leakage, and even wetting of ceramic substrate.
5. The system has no bubbles/few bubbles.
There is no pinhole and no material interruption during flowing.
Second, poor dispersion → direct casting scrap (typical problem)
1. Agglomeration of silver powder → insufficient casting, unable to spread.
Agglomerated particles are like "lumps", which greatly increase the flow resistance and are difficult to flow under high shear.
Manifestations: Leaked immersion, exposed porcelain, poor filling and incomplete end face, especially for small-sized components.
2. Uneven dispersion → poor casting thickness, orange peel and pitting
Local thick, local thin, wet film uneven, corrugated, orange peel-like.
Manifestations: large thickness fluctuation, uneven edges and poor consistency.
3. Excessive dispersion → uncontrolled casting and easy sagging.
Manifestations: TI decreased, shape-keeping became worse, vertical flow, pile edge and wall climbing.
4. Dispersing and wrapping a large amount of air → pinhole, cut-off and wire drawing.
Bubbles do not come out, and bubbles burst during coating, resulting in pinholes, material breakage and wire drawing.
Performance: The electrode is porous, loose, with poor conductivity and low adhesion.
5. The powder is not fully wetted → shrinkage and non-wetting.
The surface of silver powder is not wrapped by organic phase, and the interfacial tension is high.
Performance: shrinkage, shrinkage, local non-paste, porcelain exposure.
Third, the key dispersion process parameters → directly determine the castability
1. Feeding sequence (most likely to be wrong)
Correct: dissolve the resin first → add additives → slowly add silver powder in batches (while stirring)
→ Full wetting, no agglomeration and stable casting.
Error: powder and carrier are poured together and fed quickly.
→ The package is reunion, difficult to open, and the casting becomes worse.
2. Dispersion speed/time
Low-speed premixing (300–500 rpm): wet the powder without foaming.
Medium-high speed dispersion (800-1500rpm): Turn on reunion.
Insufficient time: poor dispersion and many reunions;
Too long time: temperature rise, thixotropy and sagging.
3. Dispersion temperature
Control ≤45℃
4. Equipment combination (must be matched)
High-speed dispersion: preliminary dispersion.
Three-roller grinding: ultimate dispersion+fineness reaching the standard (≤3μm).
Both are indispensable, only dispersion without grinding → casting must be poor.
5. Vacuum defoaming
After dispersion, it must be defoamed at -0.09~-0.1MPa for 20–30 min.
→ No bubbles → Good leveling and no pinholes.