Solder Mask is the PCB protective coating applied to the surface of most circuit boards, usually leaving only the solderable areas exposed. It is typically GREEN, although other colors are available. Requirements for solder mask are defined in IPC-SM-840 QUALIFICATION AND PERFORMANCE SPECIFICATION OF PERMANENT SOLDER MASK AND FLEXIBLE COVER MATERIALS. There are different categories of requirements based on the intended application, described as:
T - TELECOMMUNICATION
This includes computers, telecommunication equipment, sophisticated business machines, PCB instruments, and certain non-critical military applications. Solder mask and cover material on printed boards in this class is suitable for high performance commercial and industrial products in which extended performance life is required but for which interrupted service is not life threatening.
H - HIGH RELIABILITY/MILITARY
This includes equipment where continued performance is critical, equipment down-time cannot be tolerated and/or the equipment is a life support item. Solder mask and cover PCB material on printed boards of this class is PCB suitable for applications where high levels of assurance are required and uninterrupted service is essential.
FT - FLEXIBLE PRINTED BOARD APPLICATIONS (TELECOMMUNICATIONS)
This applies to cover materials for flexible printed board applications used in Telecommunications applications.
FH - FLEXIBLE PRINTED BOARD APPLICATIONS (HIGH RELIABILITY/MILITARY)
This applies to cover materials for flexible printed board applications used in High Reliability/Military Applications.
The Flexible Cover Materials were added to revision "E" of the PCB document. Requirements for solder mask materials correspond to the three "classes" of bare boards:
Class 1 circuit boards have no requirement for solder mask.
Class 2 circuit boards correspond to Class T/FT (Telecommunications) mask requirements.
Class 3 circuit boards correspond to Class H/FH (Military/high reliability) mask requirements.
Solder Mask requirements specify parameters such as flammability, hardness, PCB adhesion, shelf life, and resistance to soldering, moisture, biological growth, and to chemicals such as solvents, fluxes, and cleaning agents.
If conformance to IPC-6012 is specified on a fabrication drawing or in a procurement document, Solder Mask is automatically covered by IPC-6012 SECTION 3.7 which states:
"When solder mask is required on printed boards, PCB it shall meet the qualification/conformance requirements of IPC-SM-840. If a solder mask performance class is PCB not specified for Class 1 or 2, IPC-SM-840 Class T shall be used. For Class 3, IPC-SM-840 Class H shall be used."
Regardless, designers quite often specify the mask in a separate note on the drawing. Here is an example:
SOLDERMASK OVER BARE COPPER PER IPC-SM-840 CLASS T, COLOR GREEN, MATTE OR SATIN FINISH. NO MASK ALLOWED IN HOLES, ON FIDUCIALS OR ON SOLDERABLE SURFACES. RESIZING FOR MINIMAL LAND TO MASK CLEAR-ANCE IS ALLOWED.
One other consideration I've heard people discuss PCB is the preference for MATTE or SATIN finish over GLOSS or SEMI-GLOSS. Some don't like the glare for manual processes and inspection, others say it reduces solder beading. You may want to ask your manufacturing partners what they prefer.
