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2026-07-17
Evaporation boats are key consumables in resistance-heated evaporation coating, and their condition directly affects the purity and uniformity of the film. Evaporation boats are repeatedly heated and cooled at high temperatures, which causes thermal stress and material migration, limiting their lifespan.
The materials for evaporation boats are usually high-melting-point metals or ceramics, like molybdenum, tantalum, or boron nitride. Molybdenum boats are good for evaporating low-melting-point metals like aluminum and silver, offering good thermal conductivity and resistance to thermal shock. Tantalum boats handle even higher temperatures, making them suitable for evaporating precious metals like gold and platinum, though they’re quite expensive. Boron nitride boats are composite ceramics that don’t contaminate the coating material and are ideal for evaporating dielectric materials in optical coatings. Operators need to choose the right type of evaporation boat based on the coating material, because picking the wrong one can lead to unstable evaporation rates or early boat breakage.
Before using the evaporation boat for the first time, it needs to go through a pre-melting process. Place the film material in the boat and slowly heat it in a vacuum environment so that the material melts and any absorbed gases or impurities are released. During the pre-melting process, operators should watch how the material melts and pay attention to any splashing. Splashes usually indicate that the material contains moisture or low-boiling-point impurities, so you might need to extend the pre-melting time or replace the material.
During use, the heating current of the evaporation boat should be increased gradually, not set directly to the working current. Raising the current too quickly can cause thermal shock and crack the boat. Operators should follow the principle of slow heating, especially during the first few coatings after replacing with a new boat, to give it a chance to adjust. Once the material is fully melted into a uniform melt pool and starts to evaporate steadily, keep the current constant.
The lifespan of an evaporation boat is affected by the amount of coating material. If too much material is loaded, the molten pool can overflow the edges of the boat and drip to the bottom, causing a short circuit. If there's too little, the boat itself is directly exposed to high temperatures, accelerating oxidation. Each time you load material, it should be moderate, so that the molten pool reaches about two-thirds of the boat's depth. When the material inside drops below one-third, it needs to be refilled or replaced.
To tell if an evaporation boat needs to be replaced, you can check its appearance and electrical properties. A new boat has a smooth surface with a metallic shine. After using it a few times, the surface may show oxidation colors or tiny cracks, which is normal aging. When the boat develops through-cracks, warping, or local thinning, it should be replaced. Also, if the coating rate drops noticeably while the current stays the same, the heating efficiency of the boat has likely decreased, which could also be a sign that it needs replacing.
When changing the evaporation boat, the operator needs to first cut off the heating power and wait for the boat to cool down to room temperature. Removing it while it's hot can increase oxidation and might also burn the operator. Before replacing it, clean the electrode chuck, remove any oxides and leftover film material to ensure good contact between the new boat and the electrode. After installing the new boat, do a dry run first (without putting in any film material) to check if the current is stable, and only start using it once everything looks normal.
Keep a record of the number of uses and total time for each evaporation boat. This helps predict when it needs replacing and prevents process interruptions due to boat breakage during coating of important products.
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