Liquid Photoimageable (LPI) Soldermask
For many years the only color you would see on a printed circuit board was green. Maybe a military influence? The color of the printed circuit board is only a small part of the consideration for the circuit’s design today.
What It Is
Liquid Photoimageable soldermask, commonly called LPI or LPISM is a two component liquid ink that is mixed just prior to application to preserve its shelf life and is a relatively economical product that is designed for either spray, curtain coat or screen print applications. The mask is a mixture of solvents and polymers that result in a thin coating that adheres to the varying surfaces of the printed circuit board. The mask serves the original intention of soldermasks in general by coating areas of the PCB that do not require any of the various final plating finishes that are available today.
Unlike older epoxy inks that were applied using a screen that blocked off pads that required solder or other finishes, LPI inks are sensitive to UV light. The panel is completely covered with the mask and after a short “tack cure cycle”, exposed to a UV light source by using either photolithography – exposure through a film tool or laser direct imaging using a UV laser.
How It’s Applied
Prior to the application of the mask the panels must be clean and free of oxidation. This is normally accomplished using a chemical solution or scrubbing with a suspended pumice or aluminum oxide solution and then dried. The entire panel is then coated with the masking liquid and tack cured to remove the solvents and to provide a surface that can be handled easily through the expose process.
The most common method of exposing the surface to a UV light source is the use of a contact printer and film tools. Top and bottom sheets of film are printed with the emulsion blocking the areas to be exposed for soldering or relieved of the mask coating. The film and the production panels are fixed to tooling on the printer to assure good registration and then the panel is exposed to a UV light source on the top and bottom at the same time. Direct imaging using a laser eliminates the need for the film and tooling by controlling the laser and using fiducials etched in the panel’s copper surface.
Developing the mask occurs with treatment of an alkaline chemistry that removes the “un-exposed” material leaving behind exposed copper pads. Final curing of the soldermask is the result of a thermal process, either in a batch or conveyorized oven. If the production process includes an LPI legend ink, the legend may be applied prior to the final mask bake process and both cured at the same time.
Soldermasks in use today are being refined to provide more than simply defining where solderable surfaces are to left exposed. They have to pass stringent testing to meet the requirements of the IPC, Bellcore and various end users. They must be able to withstand the chemicals and processes that are required for plating ENIG, Immersion Tin, Immersion Silver, as well as leaded and lead-free solder, while still in the production phase. The materials are generally tested for flammability with the majority receiving the 94-V0 rating from UL.
Requirements for smaller feature sizes, 3 mils and less, electrical insulation capabilities, via tenting, resistance to CAF and out-gassing are being pressed by today’s PCB designers for better mask performance. Additionally, registration tolerances are decreasing from the +/- 5 mil range to 2 mils or less for some component types.
LPI masks are also available in a wide range of colors and finishes. Standard green – either matte or semi- gloss is the most requested color, but white, red, blue, black, yellow & more are also available. LED and laser applications for PCBs has influenced the mask market to develop newer, more resilient white and black materials. Lead-free market products are typically a different color for a number of customers to make it easier to distinguish between product capabilities. Solder masks have developed well beyond the original capability requirements and have migrated to become an integral part of a circuit board’s design as well as its ability to perform.