For low-volume and low-cost fabrication of printed circuit boards (PCBs) in a prototyping lab or in an MSME setting, PVA and benzophenone based low cost photoresists could be of high benefit for rapid fabrication and testing of the components.
The main components of the negative type photoresist are Benzophenone and Poly Vinyl Acetate (PVA), generally mixed in a solvent.
PVA is widely available in most of the over-the-counter chemical supplier stores and benzophenone, though not so common, can be easily produced in a high-yield synthesis method in a minimal chemistry lab or a college-level chemistry lab. Its precursors, depending on the method of synthesis, are widely available and are inexpensive, which are namely benzene, benzyl chloride, benzonitrile and aluminium chloride.
A patent here discusses and explains some methods of preparation of micro-resolution patterning negative photoresists using PVA, benzophenone and ethylidyne-tris-phenol. It claims that it can enable fabrication of patterns and features in micrometer range. Quoting from the paper [0089] “…it can be noted that a photoresist composition only including the benzophenone photosensitizer without the ethylidyne tris phenol photosensitizer has a high photosensitivity and a low resolution. In addition, it can be noted that a photoresist composition only including the ethylidyne tris phenol photosensitizer without the benzophenone photosensitizer has a high resolution and a low photosensitivity” we can infer that ethylidyne-tris-phenol is for increasing the patterning resolution to micrometer scale. The abandoned patent also discusses the method of preparation of the ethylidyne-tris-phenol, which can be useful for lab level prototyping and quick fabrication of MEMS devices and learning the chemistry of photoresist preparation.
A study by Department of Chemical Engineering, Diponegoro
University of Indonesia demonstrates that benzophenone improves the crosslinking of PVA molecules when exposed to UV light, thus creating a polymerized PVA group. Though the study was carried out for preparation of microfibers for different applications, we can speculate that the PVA and benzophenone based photoresist can be used for millimeter and sub-millimeter level patterning of electronic devices.
References: https://patentimages.storage.googleapis.com/8a/5c/7a/46463b04563dce/US20100167206A1.pdf
https://iopscience.iop.org/article/10.1088/1757-899X/367/1/012028/pdf