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66 PCB007 MAGAZINE I JULY 2018 the assembler should undergo the least pos- sible colour change in subsequent solder pro- cesses. As for the underlying mechanism, there are different causes for yellowing: • Heat • Sunlight • Environmental influences (e.g., humidity, chemicals) In this context, it is primarily yellowing by heat or sunlight, or by a sunlight-simulating time-lapsed UV radiation that takes place. Light-induced yellowing is produced whenev- er light beams fall on polymers with a wave- length of less than 380 nm (UV radiation). So- called chromophore groups producing a dis- colouration are created through radically-in- duced photochemical reactions. Heat-induced yellowing is the formation of such chromo- phore groups that are produced when poly- mers are exposed to temperatures >100°C for a longer period. The colour change can be expressed by the so-called yellowing value on the one hand, or by measuring the colour distance in the CIE lab system on the other. To define this co- lour change, the previously described colour the pre-cited ΔE*, ΔL* and Δa* values. Giv- en the widespread acceptance of the AE*, ΔL* and Δa* values for describing colour distances and/or colour changes, it is recommended to apply these data. White LPiSM—Covering Power A basic feature of coloured coating materi- als is the so-called covering power which indi- cates the performance in covering the colour, or colour difference, of the base material. As a criterion, a contrast ratio is predefined be- tween the contrasting areas of the base. The covering power is achieved by absorption and diffusion and is a function of the colour-giv- ing pigments. In the case of white pigments, it is only a consequence of diffusion. The cover- ing power depends on both the layer thickness and the respective contrast with the base and has a characteristic limit. The covering power cannot be adapted to any thinness of layer. The minimum layer thickness required for a white covering coating in a black-white contrast is approx. 40 µm. Unlike thermal-curing solder resists, UV-curing coating materials—including LPiSM—must be slightly transparent to ensure a sufficient UV cross-linking down to the sub- strate. There are certain consequences for process- ing, given the physical limits of the covering power. This way, a 10 µm layer looks less white than a layer of 20 µm, and again a 30 µm layer seems to be whiter than a 20 µm layer. Typical- ly for a layer thickness of approximately 40 µm or higher, the white colour no longer depends on the layer thickness since maximum covering power has been reached. Within colour evalu- ations and measurings of a layer thickness be- low maximum covering power (i.e., lower than approx. 40 µm), the base material must be in- cluded in the evaluation. This means that the reflectivity graph and the L* value also depend on the layer thickness (Figure 10). When processed, solder resists undergo sev- eral thermal loads, which may have a visible impact on the (white) colour. This is partic- ularly valid for the soldering process which causes a shift towards the yellow colour (i.e., yellowing). A white solder resist supplied to Figure 10: Reflectivity or L* value of a white solder mask depending on the layer thickness.