Photoimageable ink is not easy to process; issues over through hole filling result in extra processing time and low yield rates. The biggest problem that has been troubling flexible printed circuit board (FPCB) makers for many years concerns through hole filling. The technological issue has hindered Taiwan FPCB makers' advancement to the high-end precision FPCB market segment.

If Taiwan-based FPCB makers want to enter this market segment - which is dominated by Japan-based FPCB makers and covers applications ranging from hard disk drives (HDDs), printers, precision medical instruments and aerospace control devices - they must overcome the technological challenge of through hole filling.

There are several keys to achieving success in the process. First, FPCB ink must be high-quality and easy to coat over the FPCB, which will enhance its softness and flexibility. The mask also needs to seep into the through holes to form a protection layer there. Second, FPCB makers must have the expertise to employ either the screen printing or roller coating coverlay process; otherwise there will be a high chance of seeing products of poor quality. Third, production efficiency must be high; without an efficient production, the output scale and production schedule will both be stranded.

Advanced material R&D companies in the US and Japan, which are pioneers in the FPCB industry and have a large portion of the global FPCB market, now use lamination to solve problems faced with the through-hole filling process. Using this type of solution can shorten the time for manufacturing FPCBs, reduce material cost by one-third and significantly increase yield rates and production efficiency.

Germany-based TMT GMBH specializes in making high-precision FPCBs used in high-end medical devices and equipment. As traditional photoimageable ink's manufacturing processes may cause solvent pollution and face low production yield rates, TMT, which had been eager to find an eco-friendly replacement, has already started using Taiwan-based TeamChem Material Company's photoimageable coverlay (PIC) to replace photoimageable ink.

FPC coverlays are meant to overlay and protect the fine circuits, giving them flexibility and sheltering them from damage by temperatures, humidity, pollutants and erosive substances. Coverlay development has so far resulted in three types: the dry-film coverlay, screen-printing coverlay and photoimageable coverlay (PIC).

Dry-film and screen-printing coverlays generally involve low yield rates, high costs, imprecision and poor flexibility, and more importantly, these solutions do not thoroughly address the quality issue of through hole filling. PICs are technologically more advanced than the other types and can more precisely locate the position for soldering on FPCBs, making it a superior solution for precise soldering for the creation of fine-pitch circuits and an even coating thickness. PIC is used to fill through holes by lamination instead of a coating with ductile solder mask ink. Taiwan-based TeamChem Materials Company started investing in PIC production and R&D in 2012. Now the company has started mass production of PIC and sold them to Europe.

PICs are important because they can change the FPCB production model as well as the FPCB industry ecosystem by giving Taiwan-based FPCB makers the opportunity to compete against Japan- and US-based makers in the high-end precision FPCB market. TeamChem Materials Company, headquartered in Taoyuan, northern Taiwan, in early 2012 decided to step into development and production of PICs with a goal of resolving issues associated with Japan- and US-produced PICs, such as implementation difficulties, high equipment costs, time-consuming manufacturing processes, and high unit prices. TeamChem Materials chairman Todd Yeh points out that the company wants to provide a type of PIC that is based on a low-temperature manufacturing process utilizing FPCD makers' existing equipment, with a low level below NT$500 (US17.1) per square meter.

TeamChem Materials Company is now able to mass produce and supply its PICs. R&D personnel at some FPCB makers in Shenzhen and Xiamen, China are already testing TeamChem's coverlays on their FPCBS for communication antennae and light bars used in tablets and smartphones. Commenting on PICs' future development, some R&D engineers at FPCB makers have pointed out that traditional FPCB production requires 11 steps - micro-etching the circuit, pre-tacking, lamination, baking, hole punching, sandblasting, applying photoimageable solder masks, pre-baking, exposure, developing and baking again. With TeamChem Materials' new PICs, the process can be reduced to only five steps - micro-etching the circuit, lamination, exposure, developing and baking – and can be quickly laminated through vacuum laminators to save time and personnel costs and greatly increase their competitiveness in obtaining OEM/ODM orders.

TeamChem Materials is the first Taiwan-based maker to have successfully developed ductile solder masks specifically for use in FPCBs and has been known for supplying the most flexible mask to FPCB makers. It has been working with various handset FPCB makers to develop specialty ink, and has also identified key problems FPCB makers face using PICs. PICs currently used by FPCB makers are usually expensive, and there are problems such as the coverlay's excessively high lamination temperatures, which could create excessive stress and deform the FPCB, or create bubbles during lamination when the material is not ductile enough. These problems form an obstacle to improving yield rates and reducing production costs.

There are other problems, such as: poor image development, which could fail to leave a clear land pattern on the FPC; and material absorption during the back-end electroless nickel immersion gold (ENIG) and gold plating processes due to weak chemical resistance. These problems have prevented PIC use from becoming popular among Taiwan-based FPCB makers who are inexperienced in handling such processes. TeamChem Materials chairman Todd Yeh points out that the company has many years of experience working with major FPCB makers and has for many times developed special inks exclusively for certain FPCB clients. TeamChem believes that its initiative to start PIC R&D and production will create a win-win model for itself and FPCB makers. FPCB makers can provide specification requirements and product samples, and the FPCB solder mask expert TeamChem Materials will be happy to work with them.