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JULY 2018 I PCB007 MAGAZINE 63 and a (more favourable) lower CTE. The mech- anism is presently not understood. However, it can be summarised that the tested LPiSM shows a high stability of Tg and CTE over a wide temperature range, which obviously can be advantageous when thermal shocks/tem- perature cycling and long-term thermal storage are carried out. Defining Ultra-White and Largely Non-Yellowing LPiSM Even though the following group of coatings discussed principally also belongs to the LPiSM family, these are very different in certain as- pects from the previously discussed ones. The most important and obvious difference is the fact that the colour impression is not created by using dye-stuff—which leads to a transpar- ent coating film—but is achieved by using an insoluble white pigment (usually TiO 2 ) which is there largely to hide the substrate. To describe a colour or a difference in colour from a quantitative point of view, one can il- luminate the surface to be measured by means of a defined light source and measure the light that has been remitted. In mathematical cal- culations both the type of the light source and the colour impression of the eye is considered for measuring colour. To characterise a colour through values, the so-called CIE Lab System (to be precise, the L*, a*, b* system of the CIE–Commission Internationale de l'Eclairage, International Commission on Lumination, as shown in Figure 8) is commonly referred to. This lab system is based on the transformation of the primarily calculated colour values X, Y, Z to the coordinates L* (brightness), a* and b* (yellow-blue-value). The major benefit of the CIE lab system is its suitability for determining colour distances. The value defining a colour distance, i.e. the similarity or identity of colours, is ΔE*, which Table 3: Dependency of Tg on curing temperature (+ 3X reflow). Table 4: Dependency of Tg on curing temperature + 3x reflow + thermal aging. Figure 8: Scheme of the CIE L*a*b* system and its colour coordinates.