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66 The PCB Magazine • February 2016 Since reflectance and gloss are different properties, it is possible for something to be high reflectance and low gloss or visa-versa. Fig- ure 3 gives some examples of how the two prop- erties would work together. White Materials Typically, the reflectance is achieved in sol- der mask, and just about every other material that is white, with titanium dioxide (TiO 2 ). TiO 2 gives a very even reflectance across the entire visible light spectrum, and it is generally in- expensive due to its abundance in the earth. There are three types of titanium dioxide that are mined and used in industry: ilmenite, ru- tile, and anatase. The ilmenite accounts for about 47% of the TiO 2 produced in the world. The iron is removed from the ore and it is re- fined to the white powder used in coatings by the sulfate process or chloride process, with the chloride process being the more popular. The rutile form of TiO 2 is the second most abun- dant natural form of the material, and anatase is rarer. Anatase is also semi-stable in that it will revert to the rutile form of TiO 2 when heated around 600–800 o C. Anatase TiO 2 is reflective across a wider spectrum (400 nm–1000 nm), but has very poor hiding power. Rutile is reflective across a shorter spectrum (450 nm–625 nm), but has excellent hiding power. The lack of hiding power makes anatase a poor selection for LED products, but it can be used in photoimageable and UV products since it scatters light less than rutile TiO 2 . Thermal curable materials will gen- erally be whiter since the film doesn't rely on light or UV penetration to polymerize the film, so more TiO 2 in general can be used. We will discuss later about the formulation trade-offs that prevent a truly 100%-reflectance material. A Brief Guide to Color Theory Sometimes, white films are made to appear whiter by using some visual tricks. One such way into tricking the eye to see a whiter film is to add a small amount of blue pigment, called "bluing." To understand how this effect works, we must first understand how color is measured. There are two methods that are generally used in industry to measure color now. One method is the L*c*h* method where L* is the lightness/darkness scale of the measurement, c* is the chroma measurement that tells how pure or how grey a color is, and h* is the hue or type of color (e.g., red, blue, green, etc.). This method is used a lot in the ink and paint indus- try for color matching since it closely correlates to how the human eye distinguishes color. The hue is measured in circular coordinates while the chroma is the distance from the center of the color space (the distance from gray). The color wheel in Figure 4 illustrates this concept. The other common method for measuring color is L*a*b*, or more formally known as CIE L*a*b*. The CIE denotes that it was developed by the Commission Internationale de l'Eclairage, or the International Commission on Illumina- tion. The L*a*b* color space was developed to include all colors, even the ones that can't be seen by the human eye. While the L*c*h* color space was circular, the L*a*b* color space uses Figure 3: Examples of gloss and reflectance on various materials. solder Mask for led appliCations: forMulation 101