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100 PCB007 MAGAZINE I AUGUST 2019 Article by Nikolaus Schubkegel Ultraviolet (UV) bump, also called UV cure, is a processing step in which the solder mask pattern is irradiated with ultraviolet and infra- red light. This step is performed with special equipment that is built as a continuous flow system. The system consists of a conveyor belt and tubular UV lamps mounted above and below the belt. The UV lamps (burners) are mounted under a hood. For safety reasons, it must be avoided that UV radiation penetrates outside because it is very dangerous for the human eye. Typically, the light sources are powerful tu- bular mercury lamps with strong emissions in the UV and IR bands. It is an infrared-as- sisted UV exposure. UV radiation is desired, and infrared (IR) radiation is a needed side ef- fect. Generally, the light stays constant in the system; UV energy is adjusted by varying the transportation speed of the panels. The UV en- ergy is selected in the range between 500–4,000 mJ (milijoules), depending on the solder mask and the purpose. The IR radiation will heat the PCB up to 60–70°C or more, depending on the selected UV energy and panel thickness. Key Facts About UV Curing Systems In general, un-doped burners are used. Iron- doped burners can also be used; then, less energy is needed. Almost all equipment is equipped with un-doped burners, which is his- torical. These types of equipment are used in many industries for curing of paints and inks. The un-doped lamps fit for almost all require- ments. On request, the un-doped burners can be replaced with doped burners. In the PCB in- dustry, iron-doped lamps are generally used; in other industries, it can also be gallium-doped lamps. Depending on the solder mask (see later for further discussion), UV bump is performed either before final thermal cure or after final Solder Mask Curing: UV Bump Overview

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