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.