PCB007 Magazine

PCB007-July2018

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68 PCB007 MAGAZINE I JULY 2018 distance ΔE* can be applied. The two values ΔL* and Δb* shall be given attention in case of purely white surfaces. While ΔL* describes the change of brightness, Δb* defines the blue- yellow shift (i.e., the yellowing effect in this case [Figure 4]). The Δ value, as low as possi- ble, stands for the quality of the colour stabil- ity of the solder resist, with differences of less than 0.5 being hardly perceivable for the un- trained eye. Besides the fact that very low levels of disco- louration are desired and reflectivity is antici- pated to be in the range of 95% for the most advanced white LPiSM, also thermo-mechani- cal properties are considered and the coating is expected to withstand thermal cycling with- out displaying fissures or cracks (e.g., IMT/ HASL finish, 3X reflow conditioning, 500 cy- cles of 40°C to +125°C). Figures 11 and 12 show an example of occurring/non-occurring fissures as a consequence of thermo-mechani- cal fatigue, which are of course unacceptable. Summary Presently, the long-term thermal resistance for specialised LPiSM is 175°C. Suitable LPiSMs have a Tg of around 170°C. Adhesion and in- sulation performance are on a similar level compared to standard 150°C long-term resis- tance coatings. Only a suitable substrate treat- ment prior to LPiSM coating will allow one to achieve the desired results. Of course, suitable high-Tg substrates must be used to avoid ther- mal mismatches and resulting cracks. 200°C long-term thermal resistance is presently not possible. While the adhesion level is good, in- sulation properties largely suffer after long- term exposure. The mechanism is not yet fully understood. Thanks to the colormetric approach as used in many coating fields it is also possible in the printed circuit board/electronics industry to exactly define, compare and monitor the level of white and its stability under different condi- tions. When it comes to ultra-white and non- yellowing LPiSM, the latest available genera- tion of ultra-white LPiSM typically shows a re- flectivity in the range of 0.9 at dry thicknesses of only 20 µm, which means that due to the in- creased covering power considerable material savings compared to many existing materials can be made. PCB007 The original paper was presented at IPC APEX EXPO 2018 and published in the proceedings. Sven Kramer is manager, Application Technology, with Lackwerke Peters GmbH & Co. Figure 11: Fissures and cracks after 500 cycles of -40°C to +125°C. Figure 12: Coating with no fissures or cracks after 500 cycles of -40°C to +125°C.

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