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December 2017 • The PCB Magazine 23 solder mask, which is important to address. All white solder masks are not made equal. A lot of LED customers are looking for consistency in the color of their white solder mask. The mar- ketplace today has a number of different solder masks that are marketed as LED solder masks. The issue is that they visually look different when you put them side by side. Some solder masks have a bluish hue to them and others have a yellowish hue (Figure 2). Also, the colors look different with one coat versus two coats of solder mask, so this is an- other decision that will need to be made. In ad- dition, there can be an interaction between the surface finish, the solder mask and subsequent heat processing steps in the assembly process. Some solder masks tend to change colors more than others with additional heat. Boards with lead-free HASL tend to become yellower the more heat they are subjected to. It is generally best to permit only one pass through the lead- free HASL process (therefore, no re-work at this station). Figure 3 illustrates the same solder mask af- ter lead-free HASL versus a board that has been through two assembly reflow cycles. Boards with ENIG after the solder mask process may turn slightly pink with subsequent reflow. This is caused by the formation of a complex be- tween gold residues in the final rinse of the ENIG process with the titanium pigment in the solder mask which can shade the solder mask pink during the high temperature assembly process. It is thus important for the PCB suppli- THERMAL MANAGEMENT: A PCB MANUFACTURER'S PERSPECTIVE ON INSULATED METAL PCBS Table 1: Properties of various base metals. Figure 2: Different colors on two different types of solder mask. Figure 3: Solder mask "browning" with multiple reflow cycles.