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PCB007-Aug2019

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102 PCB007 MAGAZINE I AUGUST 2019 thermal cure of the liquid photoimageable sol- der resist. Energy Levels Required for UV Cure Generally, UV bump before the final ther- mal cure is performed at a lower energy (500– 2,000 mJ) in comparison to UV bump af- ter final thermal cure (1,500–4,000 mJ). UV bumping of flexible solder mask is carried out at much lower energy than for rigid products. And exposing the solder mask pattern on a contact exposing machine with a blank glass is not UV bump, even at extremely high ex - posing energy. UV Cure Before the Final Thermal Cure As the solder mask heats to temperatures be- yond the glass transition temperature (Tg) of that solder mask (the Tg of the solder mask after exposure and developing is lower and dif- ferent to the final Tg), the solder mask par- ticles will become movable. As the particles move, cross-linking continues, the remaining photoinitiator is destroyed, and volatile com- ponents (solvents) are blown out. Also, the surface hardness increases, and the volatile emissions reduce. Furthermore, condensation in the final cure oven is reduced. The chemi- cal resistance to different final finishes (e.g., immersion tin, ENIG, and HASL) and cleaning processes is improved. Moisture absorption is reduced. In short, after direct imaging, it con- tributes to sealing of the surface. The disad- vantage of UV bump before the final thermal cure is the risk of over cross-linking and em- brittlement. This happens if the energy is far higher than required and if the system is not closely monitored. The adhesion of legend ink and the adhesion of conformal coating might be reduced. UV Cure After the Final Thermal Cure The temperature of the solder mask stays be- low the Tg. The additional cross-linking can be neglected; the molecules swing only on fixed points, and the movement of the particles is limited. The remaining photoinitiator is de- stroyed and blown out. Residues condensed on the copper surface from the final curing oven are being destroyed by photo-oxidation and oxidation with ozone. The remained vol- atiles are blown out; these volatiles might condense on copper and lead to plating defects in ENIG. Moreover, the resistance to different final finishes is improved, and the absorption of moisture is reduced. Staining after HAL and after wave soldering is also reduced. The ionic contamination and the out-gassing improves significantly as well. And the adhesion of leg - end ink and conformal coating might be re- duced. General Aspects UV bump performance, whether before ther- mal cure or after, depends on following the curing recommendations of the solder mask brand/manufacturer. Not every solder mask requires UV bumping. There are LPI solder masks that generally require no UV bump (al- though a UV bump after thermal cure for such products does reduce the ionic contamination and out-gassing). However, some products require UV bump- ing. In such cases, the technical datasheet will provide full instructions. There are also prod- ucts where UV bumping is optional either be- fore or after the final thermal cure. Some solder mask manufacturers recom- mend UV bump before final thermal curing, while other manufacturers recommend UV bumping after final thermal curing. This de- pends on the product. Again, a complete and careful reading of the technical datasheet will disclose all product-specific instructions and cautions. This refers strictly to contact exposure. If exposure is done on a direct im - aging system, UV bump is helpful and rec- ommended. In my opinion, UV bump before final ther- mal cure or afterwards is good in philosophy. In practice, sometimes it is not possible to fol- low the recommendations of the solder mask manufacturer due to the process chain and equipment alignment. Even if deviated from the recommendations of the solder mask man- ufacturer, I have no knowledge about defects that might occur.

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