Solder mask, solder stop mask or solder resist is a thin

lacquer Lacquer is a type of hard and usually shiny coating or finish applied to materials such as wood or metal. It is most often made from resin extracted from trees and waxes and has been in use since antiquity. Asian lacquerware, which may be c ...

-like layer of polymer that is usually applied to the

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

traces of a

printed circuit board A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...

(PCB) for protection against

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

and to prevent

solder Solder (; North American English, NA: ) is a fusible alloy, fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces aft ...

bridges from forming between closely spaced solder pads. A solder bridge is an unintended electrical connection between two conductors by means of a small blob of solder. PCBs use solder masks to prevent this from happening. Solder mask is not always used for hand soldered assemblies, but is essential for mass-produced boards that are soldered automatically using

reflow Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux (metallurgy), flux) is used to temporarily attach anywhere from one to thousands of tiny electrical components to their contact pads, after wh ...

wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...

soldering techniques. Once applied, openings must be made in the solder mask wherever components are soldered, which is accomplished using

photolithography Photolithography (also known as optical lithography) is a process used in the manufacturing of integrated circuits. It involves using light to transfer a pattern onto a substrate, typically a silicon wafer. The process begins with a photosensiti ...

. Solder mask is traditionally green, but is also available in many other colors. Solder mask comes in different media depending upon the demands of the application. The lowest-cost solder mask is

epoxy Epoxy is the family of basic components or Curing (chemistry), cured end products of epoxy Resin, resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide fun ...

liquid that is silkscreened through the pattern onto the PCB. Other types are the liquid photoimageable solder mask (LPSM or LPI) inks and dry-film photoimageable solder mask (DFSM). LPSM can be silkscreened or sprayed on the PCB, exposed to the pattern and developed to provide openings in the pattern for parts to be soldered to the copper pads. DFSM is vacuum-

laminate Simulated flight (using image stack created by μCT scanning) through the length of a knitting needle that consists of laminated wooden layers: the layers can be differentiated by the change of direction of the wood's vessels Shattered windshi ...

d on the PCB then exposed and developed. All three processes typically go through a thermal

cure A cure is a substance or procedure that resolves a medical condition. This may include a medication, a surgery, surgical operation, a lifestyle change, or even a philosophical shift that alleviates a person's suffering or achieves a state of heali ...

of some type after the pattern is defined although LPI solder masks are also available in

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

(UV) cure. The solder stop layer on a flexible board is also called ''coverlay'' or ''coverfilm''. In

electronic design automation Electronic design automation (EDA), also referred to as electronic computer-aided design (ECAD), is a category of software tools for designing Electronics, electronic systems such as integrated circuits and printed circuit boards. The tools wo ...

, the solder mask is treated as part of the

layer stack A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...

of the printed circuit board, and is described in individual

Gerber file The Gerber format is an open, ASCII, vector format for printed circuit board (PCB) designs. It is the ''de facto'' standard used by PCB industry software to describe the printed circuit board images: copper layers, solder mask, legend, drill d ...

s for the top and bottom side of the PCB like any other layer (such as the copper and

silk-screen layer Screen printing is a printing technique where a mesh is used to transfer ink (or dye) onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen in a "flood stroke" to ...

s). Typical names for these layers include tStop/bStop aka STC/STS or TSM/BSM (

EAGLE Eagle is the common name for the golden eagle, bald eagle, and other birds of prey in the family of the Accipitridae. Eagles belong to several groups of Genus, genera, some of which are closely related. True eagles comprise the genus ''Aquila ( ...

), F.Mask/B.Mask (

KiCad KiCad ( ) is a free software suite for electronic design automation (EDA). It facilitates the design and simulation of electronic hardware for PCB manufacturing. It features an integrated environment for schematic capture, Printed circuit boar ...

), StopTop/StopBot (

TARGET Target may refer to: Warfare and shooting * Shooting target, used in marksmanship training and various shooting sports ** Bullseye (target), the goal one for which one aims in many of these sports ** Aiming point, in field artille ...

), maskTop/maskBottom (

Fritzing Fritzing is an open-source software, open-source initiative to develop amateur or hobby CAD software for the Electronic design automation, design of electronics hardware, intended to allow designers and artists to build more permanent circuits fro ...

), SMT/SMB (

OrCAD OrCAD Systems Corporation was a software company that made OrCAD, a proprietary software tool suite used primarily for electronic design automation (EDA). The software is used mainly by electronic design engineers and electronic technicians to ...

), MT.PHO/MB.PHO (

PADS Pads (also called leg guards) are a type of protective equipment used in a number of sports and serve to protect the legs from the impact of a hard ball, puck, or other object of play travelling at high speed which could otherwise cause injuries t ...

), LSMVS/LSMRS ( WEdirekt) or GTS/GBS (Gerber and many others).

Notes

References

Further reading