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February 2016 • The PCB Magazine 67 an xyz setup. The X-axis corresponds to red (+a) and green (-a), and the Y-axis corresponds to yellow (+b) and blue (-b). The Z-axis is the same where it represents light to dark. Figure 5 illus- trates a visual representation of the L*a*b* color space. One of the less desirable properties of the epoxy materials used in solder mask is that the epoxy material tends to oxidize over time when exposed to heat, turning the epoxy yellow. Typ- ically, you would not see this yellowing when using colored solder masks, but the yellowing is very pronounced when the mask is a white color. Unfortunately, there is no replacing the base resins in a solder mask formulation and not have it either adversely affect the physical properties of the material or the overall cost of the formulation. One of the tricks that can be used to counteract the yellowing is to add a little blue into the formulation. As you can see from the color space, yellow and blue are on opposite ends of the "b" color axis, so the blue counteracts the yellowing tendencies of the res- in by pulling the color more towards the center of the "b" axis (and more towards the light/dark L* axis). This is a great visual trick, but there can be issues when we look at a graph of the full color spectrum. When blue pigment is added, it enhances the blue end of the spectrum as seen from the bump around 450 nm in Figure 6, but blue pig- ments also absorb colors in the green and yel- low part of the spectrum (as seen from the dip in the middle of the graph). This dip reduces the overall "lightness" of the white material. In other words, the white material with the blue tint may appear whiter to the eye, but it is not as white overall, as the board that appears yel- low to the eye without the blue pigment. Board manufacturers should be aware of the require- ments of the OEM with regard to the type of white that they are looking for in a solder mask, as some may see this as an undesirable property. Figure 4: l*c*h color space (source: Konica Minolta). Figure 5: l*a*b* color space (source: ColorCodeHex.com ). Figure 6: White solder mask with blue tint. solder Mask for led appliCations: forMulation 101