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70 The PCB Magazine • February 2016 With the rise in flexible LED devices, there is also a need for flexible white solder masks. The flexibility of a sol- der mask is generally adjust- ed by using resins that have a lower cross-link density. However, there comes a point where the cross-link density gets too low to be able to have the solder mask withstand the temperatures in the solder- ing process (around 260 o C– 270 o C). Additionally, the more TiO 2 you load into a resin, the less flexible it becomes since there is less space in the mask for the TiO 2 particles to move past each other. Even with the current limitations of materi- als, the flexible LED market is making huge advances in the design and production meth- ods that open up new possibilities for materials that will keep the flexible market competitive. The density of titanium dioxide can also play a part in the solder mask properties. TiO 2 is a dense material (around 4.2g/cm3 for rutile TiO 2 ), so high concentrations of it in a mask can affect the rheology and bulk behavior of the liquid solder mask, otherwise known as sag and slump. At higher concentrations, the over- all mask becomes quite a bit denser. If the flow, or rheology, of the liquid solder mask is more fluid (less gel-like), then the solder mask will have a tendency to run during the tack-dry step for photoimageable masks and during the cur- ing step for thermal masks. The density of the solder mask can also affect how thick a layer of material can be laid down at a time. The critical thickness of a layer of solder mask before the bulk material moves is the slump. Since a lot of ovens in the PCB industry are vertical-drying ovens, these can both be a problem. There are some formulation additives that can be used to beef up the rheology, but they usually sacrifice the reflectance or final gloss of the cured solder mask. Switching to a horizontal-curing oven can greatly reduce this problem, but proper oven maintenance can also greatly reduce sag- ging or slumping of the liquid solder mask. If an oven has low or less than optimal air flow, then the liquid solder mask has more residual time at elevated temperatures on the board be- fore it starts to dry. A number of solder masks have slow evaporating solvents in order to im- prove material flow, workability, and shelf-life. If they are put into an oven with low air-flow, then the solvents are removed from the system more slowly, increasing the dry time. The in- creased dry time allows the resin to dwell on the surface as a liquid longer. The higher tempera- ture changes the surface energy and rheology of the liquid solder mask making it more prone to flow. So, proper air-flow maintenance of the cur- ing oven, especially if it is a vertical-curing oven, is very important for white LED solder masks. With photoimageable solder masks, straight side-walls are desirable after UV exposure and development. The property that makes TiO 2 highly reflective can be a double-edged sword in the case of straight side walls. As the mate- rial is exposed to UV light, the reflectivity of the TiO 2 can also bounce around light causing the curing to spread more at the top of the material with less energy reaching the bottom of the sol- der mask causing under-cut in the solder dam. Figure 8: The panel above demonstrates sag where the drips start at the top of the panel and slump (the thicker bottom edge of the lines). The thickness ranges from 2 mil to 12 mil from top to bottom. solder Mask for led appliCations: forMulation 101

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