Solder mask Liquid masks

The conventional HVLP spray process utilizes a low pressure stream of solder mask liquid that is atomized by a high volume stream of air. The atomized solder mask is directed to the PCB surface in a specific pattern so that as either the panel or the spray head(s) move, a uniform coating is applied to the panel. In some systems, the ink is heated just before spraying to lower its viscosity and aid in the evaporation of solvent between the spray nozzle and the panel. Typical process variables for HVLP spray include ink viscosity, ink (pot) pressure, atomization air pressure, conveyor speed, and ink and atomization air temperatures. 

Manufacture of Printed Wiring Boards. Printed wiring boards, or printed circuit boards, are usually thin flat panels than contain one or multiple layers of thin copper patterns that interconnect the various electronic components (e.g. integrated circuit chips, connectors, resistors) that are attached to the boards. These panels are present in almost every consumer electronic product and automobile sold today. The various photopolymer products used to manufacture the printed wiring boards include film resists, electroless plating resists (23), liquid resists, electrodeposited resists (24), solder masks (25), laser exposed photoresists (26), flexible photoimageable permanent coatings (27) and polyimide interlayer insulator films (28). Another new use of photopolymer chemistry is the selective formation of conductive patterns in polymers (29). 

Tenting or plngging of vias A hole plugging epoxy may be preferred over liquid photoim-ageable solder mask with a secondary via plugging process. 

Normally, embedded formed polymer capacitors are made from purchased sheet materials. It is difficult for the board fabricator to apply a liquid dielectric formulation precisely to laminate, but one photolithographic capacitor formation technique has been developed. Unneeded dielectric is removed with a developer solution similar to hquid photoimageable solder mask. 

The most suitable photosensitive dielectric was the photo-imageable solder masks, due to their proven compatibility with printed circuit assembly processes, ability to withstand exposure to service environments, and possession of the necessary via-imaging characteristics. SLC technology was originally developed and implemented with a liquid PID solder mask applied by curtain coating. In 1995, it was also qualified with a dry-film PID with virtually identical electrical properties. The dry-film version did not require the extensive surface grinding processes that the liquid PID process required. 

Photoimageable solder masks were developed in the mid-1970s and were offshoots of photoresist materials used for patterning the circuitry. Photoimageable solder masks were developed in both liquid (LPI) and dry film (DFSM) and both were used widely. Today, more than 98 percent of the solder mask used is applied in liquid form, and most is photoimageable. 

Liquid Masks. Solder masks applied in liquid form can be either photoimage-able or not. Most solder mask in use today is photoimageable, meaning that after application it will be patterned in a photolithographic process. A non-imageable product must be applied only to those areas to be coated, and areas where metal or base material must be exposed cannot be coated. 

Application—Liquid Photoimageable (LPI) Solder Mask. LPI solder masks are applied typically by screen printing, spray, or curtain coating. Although each method has pros and cons, all are used successfully in the industry. 

Solder Mask. The three major types of solder mask— liquid screen-printed, dry film, and liquid photoimageable (LPI)—come with different benefits and concerns from a reliability perspective. The solder mask material should be selected for its compatibility with the heat and solvent characteristics of the assembly process, its capability to provide good conformity over surface features on the PCB, and its ability to tent vias if required. Since many of these characteristics are product-specific, only a few general guidelines can be provided here. Where tenting of vias is required to keep solder, moisture, or flux from wicking up... 

A screen-printing process similar to that used for rigid circuit boards is available for flexible circuits as the low-cost coverlay process. However, specially conditioned liquid solder mask materials are required to produce appropriate flexibility for the circuits. Standard solder mask materials designed for the rigid boards have small cracks and de-laminations when bending. 

Basically, the same process and the same screen printer are available for applying liquid coverlay materiais as a solder mask on rigid circuit boards. The solder mask materials have to have appropriate flexibility to avoid the cracks during the bending.The only difference in the processes is the... 

In the electronics industry, the list of commonly dispensed fluids includes adhesives (anaerobic, cyanoacrylates, epoxy, and UV) conformal coatings flux (liquid and paste) RTV silicone (room-temperature vulcanization — curing without heat) solder mask, and solder paste. Given the vast differences in the properties of these fluids, in particular their viscosities and typical dispensed volume, dispensing... 

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