Issue link: https://iconnect007.uberflip.com/i/636678
68 The PCB Magazine • February 2016 A couple of other factors should be consid- ered when judging a white material visually. One factor is the light source. Sometimes a col- or can appear different visually when it is un- der indoor fluorescent light, for example, com- pared to outdoor sunlight in a phenomenon called metamerism. When the color spectrum is measured with a spectrophotometer, the light is normalized to mimic sunlight at a color tem- perature of 6500 o K, or 11,240 o F (also known as D65 or "daylight at 6500K"). This helps to en- sure that all color measurements are taken at a standard light temperature. Another factor to consider when evaluating a white material visually is the age of the person doing the observation. As people age, the lenses in our eyes have a tendency to yellow, caused by a hardening of the lens and exposure to UV radiation. The yellowing in our eyes can cause what is known as "blue blindness," leading to an inability to see shades of blue. So a younger observer may see a blue tint in a white solder mask while an older observer may not see blue tint at all. The overall lesson here is that visu- al observations are good for quick assessment of white products, but the color measurement from a spectrophotometer is a much better indi- cation of the true color of the solder mask. Formulation Tradeoffs Generally speaking, there are three types of curing methods that are utilized for solder mask: UV cure, thermal cure, and photoimag- ing (which is followed by a thermal cure). There are tradeoffs with each of these curing meth- ods with regards to how reflective a formulation can be made. The general rule regarding reflec- tance is that the more TiO 2 in a product, the higher the reflectance. While UV curing is the fastest production method, the material cannot be loaded with a lot of TiO 2 if you want to get good cure through the film. Photoimaging may be the best way to get good circuit resolution, but photoimageable materials are also limited in the amount of TiO 2 that can be added. Ther- mal cure materials are lower resolution (limited by the resolution and alignment of the screen being used to print the solder mask), but can be loaded with a high amount of TiO 2 giving the highest reflectance out of the different materi- als. Figure 7 gives a good representation of this tradeoff. Figure 7: Comparison of productivity speed and reflectance for different types of solder mask. solder Mask for led appliCations: forMulation 101