Liquid photoimageable (LPI) solder mask is the method of choice for protecting circuits in the manufacturer of printed wiring boards. There are at least twenty suppliers of mask products in the world today. Each vendor offers a variety of versions of their products incorporating many features desired by the end user such as optimum resistance in chemical and electroplating processes, compatibility with gold, halogen-free products, many finish characteristics, high pencil hardness, UL 94 VO, and a wide variety of colors. Solder masks have been specifically formulated to optimize the characteristics of the application systems available on the market.
Several different existing coating techniques have been adapted to the application of LPI inks, with varying degrees of success. Only one application machine—the double-sided vertical coater—has been developed specifically for LPI coating.
The three coating techniques that are most widely used are curtain coating, spraying and screen-printing. These techniques vary widely in throughput, capability, quality of coating, yield, types of inks that can be applied, capital and operating costs, and effect on the environment.
Double-sided LPI Coating
Rather than reject screen-printing as a viable high production coating technique, one equipment manufacturer, working closely with several major manufacturers of high quality multilayer circuit boards, has multiplexed screen printers into a machine that simultaneously coats both sides of a panel at one time.
Quality and reproducibility of the coatings are enhanced by the unique screen-printing technique utilized by dual-sided coating. Printing occurs at high squeegee pressure and fast print speed with high off contact with a shallow squeegee angle on a tight screen mesh. This technique ensures uniform coverage over and between traces. Leading edge printing forces the ink between and over traces. Two and three mil (50-75 micron) lines and spaces can be encapsulated without skips or bubbles. Even nine mil (225 micron) high traces are coated reliably. The coating thickness is easily controlled by the choice of screen mesh, print speed and squeegee characteristics.
Since the invention and patenting of this technique in 1990, the machine models and variations have evolved as the types of panels being coated have changed. Models were developed to coat the heavy panels found in backpanel applications.
The dual-sided coater embodies both a very new technology and a very old one. The old technology – screening – is inherently very simple to understand. However, its major shortcoming is its fussiness; many engineers perceive, with good reason, that the operation of screening equipment is fundamentally unscientific and requires a fair measure of skill or art to produce good work consistently.
However, with the development of dual-sided machines, screening has been adapted in a manner that eliminates the difficulties of screening. The squeegee design is self-adjusting, the panel handling system automatically adapts to a wide variety of panel thicknesses and rigidity levels. Most importantly, the need for a stencil, unique to each panel is eliminated. All panels of common outside dimensions can be processed without screen change or modifications.
Using a squeegee to forcefully deposit ink between the traces produces the most reliable, repeatable results. Flood screen coating is the most economical method for application of soldermask: better coverage on trace tops, consistent distribution, less waste, less solvent, and less ink per area of coated material.
Vertical dual-sided coating for flood screen coating machine incorporate innovations to allow the machines to coat flexible circuit sheets through the heaviest and thickest backpanels. These features include acute angle printing for optimal coverage and fill, QC technology that enables quick changeover of panel size or ink type; and a variety of preprogrammed operational modes and routines.
Dual-sided coaters can be charged with small quantities of ink. 1 kilo of ink is enough for the system to operate properly. This makes these machines ideal for small quantities of prototypes. In addition, in today’s market, with a wide variety of ink finishes and colors required, the system is very efficient with the ink.
SPS Smart Print System
