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February 2016 • The PCB Magazine 71 Unfortunately, adding more photo-initiator to the solder mask will increase the curing at the bottom of the mask, but it will also increase the sensitivity of the upper part of the mask making the under-cut worse. The best way to counteract this problem is to reduce the amount of TiO 2 in the formulation, but this will lower the solder mask's reflectivity. The UL flammability rating of a solder mask can also be a very important property to a PCB shop or OEM. Typically, the substrate of the cir- cuit board is the largest contributing factor in de- termining its flammability rating. The substrate makes up the bulk of the circuit board with the layer of solder mask a relatively small contribut- ing factor. If the substrate does not have a good flammability rating, then the solder mask will do little to improve this rating. If an additive is needed in the solder mask to try to improve the overall flammability rating, then the reflectiv- ity of the mask is sacrificed. The additives used to achieve certain flammability ratings are not white and take up the space that would have been occupied by the TiO 2 particles reducing the overall reflectivity of the solder mask. There are additives where phosphorous is added into the polymer itself improving the flammability rating, but these tend to be expensive raising the overall price of the solder mask. Conclusion With the increasing production of LED products for everything from indoor to industrial applications, it has become increasingly important to enhance the technology that goes into white solder masks for LED appli- cations. Currently, most of the prop- erty trade-offs affect the reflectivity, physical properties, or overall cost of the solder mask. However, rapidly increasing markets can also trigger rapid improvements in technology and production methods. For exam- ple, the increase in the use of inkjet printing could be coupled with the conventional screen printing used in PCB manufacturing to improve the resolution of LED PCB boards while utilizing the increased reflectivity of a thermal cure solder mask. Using print- ed electronic additive manufacturing methods could also usher in new circuit board designs and manufacturing methods that could utilize lower temperature assembly opening up more classes of polymers outside epoxies, decreas- ing yellowing, increasing flexibility, improving speed of cure, or increasing reflectivity. Finally, new materials could be developed or applied in the near future that could improve the re- flectivity or reduce the adverse effects of other additives that are necessary for film properties. For the LED industry, the future of materials is indeed bright. Acknowledgements The author would like to thank Lisa Kenne- dy at our R&D facility for providing input and illustration for this article. PCB Josh Goldberg is a marketing specialist with Taiyo America. Figure 9: Example of undercut in a solder dam. solder Mask for led appliCations: forMulation 101

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