Drying, Curing & UV
Optimising PCBA Baking, Drying & Curing Processes
I
n PCBA production, various processes necessitate Baking, Drying and Curing to meet specifi c quality standards and enhance process yields across manufacturing facilities. Each site may have distinct requirements, encompassing PCB preparation, component storage, conformal coating preparation, and curing, all with the overarching goal of ensuring consistent high-quality performance.
DRYING
Drying processes serve several purposes, including restoring moisture sensitivity level (MSL) component to the shop fl oor, long-term components storage, and conditioning bare boards under certain conditions. Drying cabinets, available in various sizes to accommodate varying capacities, are a staple solution. Key considerations include whether to use nitrogen (N2) to establish an oxygen-reduced environment, an option incurring signifi cant costs, and the potential acceleration of the drying process by increasing unit temperature. Notably, a component with a body thickness of <1.4mm stored at 1% relative humidity (RH) at 25°C requires fi ve days to meet specifi cations, whereas at 60°C, it takes just twelve hours. Understanding such dynamics is pivotal in selecting the optimal solution for one’s facility.
Factors such as the need for independent chambers to minimise unit openings’ impact, or the requirement for traceability and communication with Manufacturing Execution Systems (MES), must be considered. MES integration ensures adherence to specifi c criteria for accessing high- value components or determining component readiness based on drying time and temperature specifi cations.
BAKING FOR COATING PREPARATION Conformal coating necessitates thorough preparation of PCBs to ensure consistent, high- quality PCBA inputs. Typically, PCBAs undergo cleaning to remove fl ux residues and ensure proper surface tension before baking to guarantee complete dryness prior to entering the conformal coating process. The baking duration depends on the PCBA specifi cs, with considerations including work in progress (WIP), available fl oor space, and system capacity.
CURING FOR COATING Although some materials eliminate the need for
extraction needs when handling solvents, providing a safer operator environment. After UV curing, the PCBS can be handled and will cure in ambient temperature, facilitating easier WIP storage. For acrylic materials, sites often use infrared (IR) ovens of varying lengths with top and bottom IR zones. Horizontal IR curing ovens require substantial fl oor space, depending on the curing time, temperature range, and throughput needs. The process can take 30 seconds to several minutes based on the programmed profi le. Such units suit CEMs with diverse requirements across batch sizes, from low to high volume. After IR curing, PCBAs can be handled, but should also be stored at ambient or in a controlled environment considering WIP volume, downstream storage type including shelve or cabinet, and total post process storage time.
curing, many sites utilise UV or acrylic-based materials, necessitating fl ash curing and subsequent baking. UV materials are typically fl ash cured for handling, before undergoing full curing at room temperature or in a baking oven. Maximising the curing program reduces defects like bubbles and cracks, with buff ers like fl ash off supporting this process.
For curing itself, there are many options, including UV wand systems integrated into dispensing solutions for immediate curing within the coating unit. There is also a range of UV options from benchtop to inline, with various bulbs types (such as ARC and LED) available. These ensure the medium is exposed to the correct light bandwidth for the required time, while supporting the desired throughput. UV curing requires less energy than IR curing and reduced
Vertical curing ovens are also an excellent solution for facilities with minimal fl oor space or requiring longer cures with a smaller footprint. These systems move the PCBA slowly upwards from input, then downwards to the output over a consistent temperature, or use a rotating tower function for the same performance. While a signifi cant investment, their capacity makes them ideal for high-volume manufacturing or storing PCBAs in a consistent, controlled environment without product handling.
While electronics manufacturing has many unique processes, the above covers key processes that sites in the UK and Ireland are considering. Each site is diff erent, but understanding all options and considerations regarding technical capability and cost is crucial before selecting the right solution.
20
May 2024
www.convertermag.com
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