Figure 28 Structure composition of PCB dry film photoresist.
Figure 29 Structures and photoinitiation mechanism of BCIM/NPG systems.
EMK and PSS303 are used as photosensitizer in DFRs formula for many years, however, it is essential to develop more efficient photosensitizers due to the requirements of higher sensitivity and resolution in DFRs. Jiang, et al.[113-114]reported several novel species of photosensitizers that can be utilized for DFRs, respectively distyrylpyridine derivatives (DSPs) and anthracenes (ANs) derivatives, and their structures are exhibited in Figure 30 and Figure 31. Firstly, among all DSPs, the 24DMOP-DSP has the highest photosensitivity, which was reached 14 mJ.cm-2 and increased by 150% compared to contrastive commercial coumarin-based DAMC with 35 mJ.cm-2. The singlet state of DSPs is easily quenched by BCIM, which proves that it is easy to sensitize BCIM then initiate polymerization reaction. Next, for ANs, they possess strong light absorption from 350 nm to 450 nm with high molar extinction coefficients, which can be matched with multifarious light sources. BCIM can be photosensitized by ANs to initiate the polymerization reaction of Hexamethylene diacrylate (HDDA) effectively. The DFRs formula containing ANs/BCIM can obtain fine high-resolution micro-patterns by 405 nm laser direct-writing photolithography.
Figure 30 Chemical structures and synthesis methods of DSPs.[113]
Figure 31 Chemical structures of ANs.[114]
Ho, et al.[112] synthesizes a series of p-substituted NPG derivatives OMe-NPG, Cl-NPG and NO2-NPG as shown in Figure 32. The OMe-NPG, Cl-NPG and NO2-NPG have good thermal stability and red-shifted absorption compared with NPG, the photopolymerization performance of trimethylolpropane trimethacrylate (TMPTMA) initiated by BCIM/NPGs systems was tested, the measured conversions sequence is BCIM/NPG (51.7 %) > BCIM/OMe-NPG (41.8 %) > BCIM/Cl-NPG (35.5 %) > BCIM/NO2-NPG (0 %), which concluded that introduction of substituents may weaken the performance of initiating for NPG.
Figure 32 Structures of N-phenyl glycine (NPG) and NPG derivatives (OMeNPG, Cl-NPG and NO2-NPG).[112]
As we know, radicals photopolymerization system possess some defects in practical productions such as apparent volume shrinkage, poor precision and low adhesion.[115] Contrastively, the cationic photopolymerization system has lower shrinkage, stronger adhesion and hardness, making them particularly suitable for laser photolithography technology which requires high precision and strong adhesion.[116-118] Based on this, as shown in Figure 33, our group designs and synthesizes a series of cationic photocurable resins which consists of methyl methacrylate (MMA), methacrylic acid (MAA), ethyl methacrylate (EMA) and 3-ethyl-3-(methacryloyloxy) methyloxetane (EMO) by radical copolymerization.[109] Next, we used these resins to prepare DFRs formulations with radical/cationic dual-curing. The DFR based on the resin which containing 10 wt % of EMO has the highest photosensitivity (11 mJ⋅cm−2, ST = 20/41) and 20 μm (ST = 20/41) can be obtained by using this resin. In conclusion, the DFRs that contain cationic photocurable resin may have potential application in PCB field for creating high resolution patterns.