The final part of an encapsulation process is curing which ensures the encapsulant is permanently settled, achieves all of its physical and chemical protective properties. The curing requirements of an encapsulant are formulation specific.
The encapsulation deposition is left to cure at room temperature for a specified period of time. The advantages of air cure is that the package does not require to be heated so it does not experience any thermal stress. The major disadvantage of the air cure is that it takes significantly longer time for the encapsulation to completely cure which can reduce the overall speed of the packaging process.
Air or room temperature curing encapsulants can either consist of one part or two parts formulation. The one part formulations often cure by evaporation of a constituent solvent over a specific time period. The two part formulations cure by a reaction between the two constituent parts. They start curing as soon as they are mixed together in specific proportions and normally have an optimum application window where the rheology of the encapsulant is consistent. Normally, beyond that window the viscosity of the encapsulant increases as a function of time till it is completely cured. In this method it takes the encapsulant time to get to a point where the deposition is robust enough to resist any damage due to contact, this allows the deposition to fill any existing voids. Air curing encapsulation depositions should be handled carefully as it takes the depositions a long time to harden enough to avoid damage from contact.
Some encapsulants require heating to a specific temperature for a specific period of time to cure. Thermal curing is a fast curing process where the time to curing typically are in the range of 2 – 10 minutes. The disadvantage is that all the contents of a package go through high temperature which may be unsuitable for certain components of a package, or may add stress to delicate components such as wire bonds.
Thermal curing can utilise temperature induced chemical change for cure. It can also be based on evaporation of solvents in a formulation. Like the air curing encapsulants, thermal curing encapsulants could also consist of two part formulations. Thermal curing requires the oven to be preheated to the specified curing temperature. If the deposition is heated at a higher temperature or for shorter time period than curing specifications it may result in bubbles and voids.
It is a process where visible and ultraviolet light combine to trigger a photochemical reaction for curing. UV curing is able to instantly cure an encapsulant formulation depending on the intensity of the UV light shown on the deposition. UV curing is the fastest curing process where a material can be cured within a minute. The contents of a package to be encapsulated is not heated. The disadvantage is there is less time for the encapsulant to fill any void if present.
UV curing requires a user to prevent any possible shadows on the deposition as it may lead to under cured or uncured areas. UV curable encapsulants may not be suitable for applications where the components are likely to create shadow areas on the deposition.
Curing is the final step in the encapsulation process and although a less complex process step it still requires careful consideration and execution.
The Advanced Packaging Facility has access to a broad range of curing equipment to suit a wide spectrum of encapsulants.
Thermal curing equipment includes standalone ovens, high temperature ovens, vacuum ovens and belt dryer furnaces. The low temperature standalone ovens can be heated to a temperature of 250°C. The high temperature oven can be heated to 1200°C and the vacuum oven can be heated to 200°C with a vacuum of upto 0.01 mbar. One belt dryer furnaces can be heated to a temperature of 400°C and the other furnace can be heated to 1100°C. The high temperature furnace allows us to heat cure materials in a gas such as nitrogen; this is particularly useful for any material that is prone to oxidation at higher temperatures.
UV curing equipment includes 400W UV point source, 400W UV cabinet and a 25kW bespoke UV belt conveyer.