After PCB fabrication, the copper traces on the board typically face the risk of oxidation and corrosion due to environmental exposure. The most reliable way to prevent this and increase the life of the PCB is to provide a protective coating known as a solder mask layer.
This article will cover:
What is solder mask layer?
Solder mask is a thin layer of polymer that is put on a circuit board to protect the copper from oxidation and shorts during operation. It also protects the PCB from environmental influences such as dust and several other contaminants that may lead to shorts in the long run.
Types of solder mask
Solder masks are of different types and the selection of them depends on the application for which it will be used. Given below are the various types:
These are typically used by engineers to identify openings in the green solder mask, already applied on the PCB. This particular layer of solder mask is added through epoxy or film methods. Next, the component pins are soldered to the board utilizing the openings that have been registered using the masks.
The traces on the top-side of the board are called top traces and the corresponding solder mask used is called a top-side mask. Identical to the top-side mask, the bottom mask is used for the bottom side of the board.
Epoxy liquid solder masks
The most cost-effective option among solder masks is epoxy. It is a polymer that is silkscreened on the PCB. Silkscreening refers to a printing technique that uses a woven mesh to support ink-blocking patterns. The mesh enables the open areas to be identified for the process of ink transfer. This step is then followed by thermal curing.
Liquid photoimageable (LPI) solder masks
LPI solder masks are actually a mix of two distinct liquids. These liquids are kept separately and mixed right before the application of the mask layer to enable longer shelf life. LPI is also one of the more cost-effective options among various other types.
LPI is employed for curtain coat, screen printing, and spray applications. This mask is a blend of various polymers and solvents. A thin coating can be formed using this mask that can stick to the target area surface. After using this mask, the PCB will generally not require any final surface finish that is currently available.
Unlike conventional epoxy inks, LPI is sensitive to UV light. The panel is required to be covered with the mask and after a short ‘tack cure cycle’, the PCB is exposed to UV light using a UV laser or photolithography process.
Before mask application, the panels are subjected to cleaning and are checked for any sign of oxidation. This is done using a chemical solution, aluminum oxide solution, or using suspended pumice to scrub the panels.
One of the most popular techniques used to expose the panel surface to UV light is through the use of film tools and contact printers. The top and bottom sheets of the film are printed using emulsion to block areas that need to be soldered. Production panels and film are then fixed in position, making use of tooling on the printer and exposed to a UV light source simultaneously.
One method utilizes direct imaging that makes use of a laser. This technique does not need tooling or film as the laser is controlled through the use of fiducial markers placed on the copper template of the panel.
LPI masks are available in various colors such as green, black, red, yellow, white, and blue.
Dry film photoimageable solder masks
The use of dry film photoimageable solder masks requires vacuum lamination. This dry film is subjected to exposure and then developed. Post-development, openings are identified to generate a pattern, after which the components are soldered to the copper pads. High-density wiring boards benefit most from dry film solder masks as it does not flood the through-holes.
Regardless of the PCB solder mask types used, the resulting mask will leave behind certain exposed areas of copper on the circuit board. These exposed areas need to be plated with a suitable surface finish to prevent oxidation. One of the most popular surface finishes available is hot air solder leveling (HASL). Depending on the requirement other surface finishes also can be used such as electroless nickel electroless palladium immersion gold (ENEPIG) and electroless nickel immersion gold (ENIG). When needed, additional holes in the mask layer are left for the paste mask. This paste mask is utilized to attach pads or other components to the board based on the manufacturing process used.
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Solder mask design guidelines
Solder mask design requires fulfillment of certain criteria. Here are a few such parameters in practice:
When a via is covered with solder masks so that it is not exposed, it’s called a tented via. Unlike via filling, here only the annular ring is covered by the solder mask. If the hole is closed completely, it is known as a filled via or mask plugged via, depending on the method used to close the hole.
Tenting vias is a popular process used to protect PCBs and is chosen over epoxy filling or mask plugging due to cost considerations. Within methods of via tenting, liquid photoimageable (LPI) solder mask tenting is most cost-effective. To ensure a better tented via you can also use resin filling which is higher in cost.
The purpose of via tenting is to leave fewer exposed conductive pads on the PCB surface. This would minimize the shorts that occur during solder bridging in the assembly process. Paste migration from SMT pads is also reduced which can happen when vias are on pad edges or in standard BGA “dog-bone” patterns. Tenting also minimizes the likelihood of a via being damaged through exposure to the operating environment.
What is solder mask clearance?
Solder mask clearance is a tolerance to decide how close the solder mask can be to the PCB surface features. The purpose of this tolerance/clearance is to provide adequate spacing known as solder dams between surface features that receive solder. This prevents the formation of solder bridges.
Typically solder mask clearance needs to be half the width of the conductor spacing. When using fine conductive patterns under 100µm, the solder mask clearance can go below 50µm.
Solder mask defined pads are those where the mask opening is smaller than the copper pad such that the solder mask will define the size of the pad used in BGAs. Changes in the mask clearance will decide the copper pad size.
Non-solder mask defined pads include a gap between the pad and the mask.
The solder mask process needs fitting tolerances to be considered such that the mask clearance should always be greater than the solder pads. This is required to keep the whole pad free of solder resist and ensure optimal soldering.
What is solder mask opening?
The outer layers of the PCB without the mask are called solder mask opening. Its purpose is to expose the circuit to tin (solder). This opening has to be accurate or it might lead to unnecessary exposure of copper on the board leading to corrosion and damage.
Typically, fabricators choose to have solder mask openings 1:1 with the copper pads that can be altered to the appropriate manufacturing process. The opening has to be customized to the requirements or it might lead to exposure of the ground plane next to the pad. This might lead to shorts and should be prevented.
Solder mask coverage or expansion
Electronic design automation (EDA) software typically enables one to set the spacing between the surface features and the solder mask of the board. This specification is conventionally called the solder mask expansion and can be a positive, negative, or zero value.
Positive solder mask expansion
When there is a spacing between the end of the solder mask and the outer circumference of the pad that is uncovered, it is called positive solder mask expansion.
Zero solder mask expansion
When there is no spacing or gap between the solder mask and the pad it is zero value.
Negative solder mask expansion
When the solder mask overlaps a region of the pad it is of negative value.
IPC standards for solder mask
Solder mask requirements are defined in the IPC-SM-840 qualification and performance specification of permanent solder mask and flexible cover materials.
The intended application of solder mask places its requirements into different categories:
T – telecommunication
This category consists of telecommunication devices, computers, and equipment used in non-critical military applications. Solder mask layer and coverlay on boards in this particular class are suited to high-performance industrial and commercial products. Such products would require extended performance life but interrupted service would not result in life-threatening situations.
H – high reliability/military
This consists of equipment where progressive performance requirements are of a critical nature and device downtime is unacceptable. Solder mask and coverlay on boards of this PCB class is meant for applications that are critical in nature and uninterrupted service is completely essential.
FT – flexible printed board applications (telecommunications)
This is applicable to cover materials that are used for flexible board applications in telecommunications equipment.
FH – flexible printed board applications (high reliability/military)
This is applicable to cover materials that are utilized for flexible PCB applications that are required in military applications which call for high-reliability requirements.
Flexible cover materials have been appended to revision “E” of the IPC standards. Solder mask material requirements fit into three IPC classes of bare boards:
- Class 1 PCBs that don’t have a requirement for solder mask
- Class 2 boards that fit class T/FT (telecommunications) mask needs
- Class 3 circuit boards that match class H/FH (high-reliability military) mask requirements
The solder mask requirements mentioned above are meant to state parameters such as hardness, shelf life, flammability, and board adhesion. It also states the resistance to soldering, biological growth, moisture, and chemicals such as fluxes, cleaning agents, and solvents. Another important consideration is choosing matte or satin finish over gloss or semi-gloss finish. It helps minimize solder beading and cuts down on glare during manual processes and inspection. Procurement documents or fabrication drawings usually specify if there is conformance to IPC-6012 standards. When this standard is stated for conformance, the solder mask is covered by IPC-6012 section 3.7.
Applications of solder mask
The solder mask layer serves many critical functions as stated below:
- Protects the PCB against oxidation and corrosion.
- Prevents solder bridges that are the undesired connection of solder joints on the board that might damage circuit boards and lead to shorts. The solder mask, in this case, serves as a barrier between the solder joints and other conductive sections of the PCB. This provides additional insulation for the board components.
- Stops the growth of metal whiskers that are thin filaments sticking out of the PCB which might lead to system failure. Metal whiskers can form in a similar way to the formation of solder bridges leading to shorts and malfunctions.
- Reduces the amount of solder paste required during the soldering process.
- Increases the breakdown voltage of the board dielectric material.
- Protects the circuit board from contaminants during handling.
- Enhances the shelf-life of the PCB.
How is a solder mask applied?
Given below is the process for application of solder mask layer onto a PCB:
Step 1: Board cleaning
The board is cleaned to remove dirt and other contaminants and then the board surface is dried.
Step 2: Solder mask ink coating
Next, the board is loaded into a vertical coater for solder mask ink coating. Coating thickness is decided by factors such as the reliability required for the PCB and the domain in which it will be used. The solder mask thickness will vary when it comes to being added on different sections of the circuit board such as the traces, the copper foil, or the substrate. This mask layer thickness will depend on the equipment capability and the PCB manufacturing capabilities.
Step 3: Pre-hardening
This phase varies from total hardening as pre-hardening looks toward making the coating relatively solid on the board. This facilitates the removal of the unwanted coating which in turn can be easily removed from the PCB in the developing stage.
Step 4: Imaging and hardening
Imaging is done using a photo film which is laser plotted to define the solder mask area. This film is aligned to the panel that is already solder ink coated and tack dried. During this imaging process, the film aligned to the panel is subjected to UV exposure. Upon receiving the UV light, the opaque area allows the UV light to transfer through the film, and thereby the ink below the opaque gets polymerized (hardened).
In the case of LDI imaging, the photo films are not required as the UV laser will directly harden the portions on the board which need to retain the solder mask ink.
Step 5: Developing
The circuit board is then dipped in the developer to clean away the unwanted solder mask to ensure the required copper foil is accurately exposed.
Step 6: Final hardening and cleaning
Final hardening is necessary to make the solder mask ink available when mounted on the PCB surface. Next, the boards that have been covered with solder mask need to be cleaned before further processes such as applying the surface finish.
Solder mask processing with inkjet
PCB manufacturing has predominantly used inkjet systems to create nomenclature markings in the last decade. In recent years, however, technology has evolved enabling fabricators to print solder mask directly onto the board surface or panel surface. This is also sometimes called direct jetting (DJ), with the mask ink applied on the PCB surface according to the design input. The direct jetting is done through the use of a piezoelectric head.
Compared to the method based on photolithography, the DJ method cuts down most of the process steps leading to these advantages:
- Minimized material usage
- Reduced capital equipment and process variables
- Eco-friendly processing
- Elimination of excess use of solder mask
Applying solder mask onto a circuit board is a process that requires significant technical expertise and skill. To prevent board rejects and failure due to issues such as inadequate or excess reliefs, you need to select a PCB manufacturer who can handle this process without errors.
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