I’ll then walk you through a thorough comprehension of the information pertinent to the solder mask layer. The following is among the content:
Solder Mask Manufacturing Process
Sloppy solder mask creation is unacceptable for professional PCB producers. For starters, it must adhere to tight regulations like ISO9001, UL, or RoHS. For example, the production of solder masks is broken down into a number of processes, each of which calls for extensive manufacturing expertise, cutting-edge machinery, and fully developed technology. Fortunately, PCBShare meets all of these requirements; our 18+ years of PCB manufacturing expertise and top-notch equipment guarantee some easy progress. To see the PCBshare production facility, click.
The general process for fabricating solder masks goes as follows:
Step 1: Clean The Circuit Board.
This technique involves washing the PCB board surface to remove debris and stop color fading while maintaining a dry surface.
Step 2: Solder Mask Ink Coating.
After that, insert the well cleaned circuit board into the vertical coating device for solder resist ink coating. The reliability requirement of the PCB, the area served by the PCB, the thickness of the PCB board, and other criteria determine the coating’s thickness. What’s worse is that the PCB board’s surface is not as smooth as it could be. When positioned on various areas of the board, such as traces, a substrate, or copper foil, solder mask inks have varying thicknesses. Based on the performance of the device and their previous production experience, skilled PCB producers frequently specify particular coating thicknesses.
Step 3: Pre-Hardening.
Total hardening is distinct from pre-hardening. Make the board’s coating stable. In this manner, the undesirable layer is eliminated from the board during the development phase.
The panel needs to be pre-cured after being completely covered. This crucial step eliminates all of the solvent from the ink. Registration won’t be done correctly without it.
Step 4: Imaging and Hardening.
Hardening and imaging. Transparencies with some circuit pictures are put on the board at this point and exposed to UV light. The solder mask covered by the clear film segments is hardened during this process, but the solder mask covered by the circuit diagram segments is left pre-hardened. As a result, when hardening is carried out, accurate alignment must be guaranteed to stop the exposure of unidentified copper foils from causing short circuits or negatively impacting the board’s final performance.
Step 5: Developing.
In order to adequately expose the designated copper foil, the PCB is subsequently submerged in developer to remove any unnecessary solder mask.
Step 6: Final Hardening and Cleaning.
The final hardening must be implemented in the last stage. This is what I do to make the solder mask ink accessible as it is mounted on the PCB surface. These boards need to be cleaned before being processed further. This is done either for surface finish or assembly.
Types of PCB Solder Mask
The four primary categories of PCB solder masks on the market now are listed below.
- Liquid Epoxy Solder Mask
- Liquid Photoimageable Solder Mask (LPI)
- Dry Film photoimageable Solder Mask
- Top and Bottom side Masks
Liquid Epoxy Solder Mask:
Epoxy is the solder mask choice that is the most affordable. Silkscreening is used to apply a polymer on the PCB. The term “silkscreening” describes a printing method that supports ink-blocking patterns on a woven mesh. For the purpose of the ink transfer procedure, the mesh makes it possible to locate the open areas. Thermal curing is the next stage after this.
Liquid Photoimageable Solder Mask (LPI)
LPI, or liquid photoimageable solder masks, are a combination of two distinct liquids. To provide a longer shelf life, the liquid components are combined shortly before application. One of the more affordable options among the four varieties of PCB solder mask is this one.
Screen printing, curtain coating, and spraying are all applications for LPI. The mask is a concoction of several solvents and polymers. As a result, a thin coating that adheres to the target area’s surface can be retrieved. Although the PCB does not need any of the final plating finishes that are typically available nowadays, these types of masks nevertheless fulfill the original purpose of solder masks.
In contrast to earlier epoxy inks, LPI is UV-sensitive. The mask must be worn over the panel. The board is then subjected to UV light using photolithography or a UV laser after a brief “tack curing cycle.”
The panels must be thoroughly cleaned and checked for oxidation before the mask is put on. This is accomplished with the use of a unique chemical mixture. The panel can also be scrubbed with a suspended piece of pumice or with an aluminum oxide solution.
There are many different colors available for LPI masks, including green (matte or semi-gloss), white, blue, red, yellow, and many others.
Manufacturers and designers have been inspired to create more robust white and black materials by the LED business and laser uses in the electronics industry.
Dry Film photoimageable Solder Mask
The final hardening must be implemented in the last stage. This is what I do to make the solder mask ink accessible as it is mounted on the PCB surface. These boards need to be cleaned before being processed further. This is done either for surface finish or assembly.
Top And Bottom Side Masks
Vacuum lamination is necessary for the usage of dry film photoimageable solder masks. This dry film is exposed, and then it is processed. Openings are found after development to create a pattern, then the parts are soldered to the copper pads. Dry film solder masks are suitable for high-density wire boards since they don’t flood the through-holes.
What is the Standard Solder Stop Mask Thickness?
How To Measure The Thickness Of A Solder Mask?
Measure the solder mask to ensure it is not too thick, even if you want it to be thick enough to protect your circuitry. You need a thickness more than 7 micron on the conductor’s top and side edges. For finished copper parts with a thickness of up to 35 micrometers, the maximum solder mask thickness is 40 micrometers. The solder mask can be up to 80 micrometers thick if the copper is thicker.
Other wise guidelines for solder mask usage are as follows:
- If the solder resist encroaches on lands, meet the minimal annular ring requirements.
- Prevent exposed isolated pads.
- Only allow solder resist when plating through holes that aren’t meant for solder fill.
- Avoid letting the fingers of connectors or test points resist solder.
What Is The Effect Of Solder Mask Thickness?
If the solder mask is too thin, the copper would be exposed, which would cause issues with SMT. Corner lines are often larger than 8 microns, as per IPC requirements. Otherwise, when cooked, it will turn yellow and bleed copper.
The maximum solder mask thickness can go up to 40 M for completed copper thickness of 35 m. The solder mask thickness can reach 80 M for larger final copper thickness.
The pieces may tilt if the PCB solder mask layer is too thick, which may damage the quality of the assembly and lead to issues when welding IC components. Copper that is thick always resists welding well.
PCB Solder Mask Colors
The traces of the printed circuit board are covered with a thin polymer coating known as a solder mask during the manufacturing process. The copper traces are shielded from oxidation by the solder mask. One of the most important design factors for any printed circuit board is color. Green has long been the color of choice for solder masks.
Although there are many different colors available, green is the most typical color for solder masks. pcbshare also employs the following hues:
● Green ● Red ● Blue ● Yellow ● White ● Black ● Purple
Why Was The Early PCB Solder Mask Green?
1) The solder mask must be green, which is a throwback to earlier military regulations.
2) The reason why solder mask is green is because the basic resin, which is brownish-yellow in color, and the hardener, which is a very muddy brown color, were blended to make green when they were first produced. Additionally, most laminates at the time were green. The concept of a green solder mask was so simple to accept as well.
3) When tested in a variety of harsh environments, that specific shade of green generated the proper contrast with the white legend ink.
4) It has been established that the color that the human eye can see the best is green.
Red Solder Mask
Designs that highlight internal hardware frequently employ the visually arresting option of red solder masking. The hue has gained popularity as one of the alternate PCB colors, but there is a drawback: it has less contrast and visibility than a typical green PCB. Greater magnification could be necessary for service technicians working on a red PCB to fix problems on the board.
White Solder Mask
Small features cannot be resolved by white solder masks the same way that they can by regular green, blue, or red solder masks. This is due to the extremely heavy pigment loading required to make a White LPI solder mask. To produce a dazzling white solder mask that won't yellow, substantial amounts of special white pigments are employed in the formulations. Commonly utilized in LED PCB applications with metalcore is white solder mask. The obstacles that developers confront when creating printed circuit boards for LED components are numerous. One need is that the color that the LEDs produce be crisp and clear. On the other hand, they must make sure that the heat generated by the LED components is properly dispersed. LED systems are more brilliant when using a white solder mask. Notification: White solder masks cannot be utilized with white silkscreens. The characters can be shown using black, yellow, red, and other colors.
Blue Solder Mask
Blue solder masks can be found on Arduino boards. To achieve a high contrast, board assemblers employ blue against the silkscreen. The PCB's aesthetic appeal is also improved by the hue. Due to the fact that it doesn't provide vivid backgrounds or clearly contrasted edges, it is even appropriate for mounting on liquid crystal displays (LCDs). On a typical circuit board, blue's contrast is somewhat weak in comparison to the green and red solder masks. Therefore, if you plan to use blue, you might need to magnify the image to spot manufacturing-related flaws. The PCB color code for blue solder masks is #4990E2.
Yellow Solder Mask
Although yellow's silkscreen doesn't have as strong of a contrast with the board as green's, it accomplishes the same contrast between planes, no-copper areas, and traces. Although it might not produce the desired outcome, black silk printing can help the assembler resolve the silkscreen issue. When it comes to light-colored residues, yellow PCBs are simple to remove. Use a dark yellow solder board to draw attention to any pathways you want to stand out. The PCB color code for yellow solder masks is #F6A624.
Black Solder Mask
While there is little contrast, black solder masks are more visible than white ones, making it simpler to see bulky components and labeling. On LCD back panels, black also looks fantastic. The eyes aren't diverted by the details on the screen when the background is black. However, black solder masks have the potential to raise heat and cause the silkscreen to become discolored (turning it into light brown during the reflow process). If black is chosen, the board must have a temperature sensor installed. The matte and gloss finishes of the black solder masks are both available, with the former offering a somewhat better contrast. Black PCBs can be identified using the color #000000.
Purple Solder Mask
Purple is the ideal hue for creating a sharp contrast between the surface's planes and traces. PCBs for submarines frequently employ purple solder masks. However, they don't exhibit white silk printing well at all. Although it will cost more than other PCB colors because to its distinctive and colorful appearance, purple can be a nice alternative to consider if you're searching for a vibrant hue to compliment an immersion gold surface.
Why Solder Masks Use Different Colors?
The assembly equipment will be impacted by the light’s transmission, reflection, or absorption characteristics. To provide the necessary contrast or brightness, applications can also specify color requirements, with the exception of green. The identification of lead-free components can be considerably impacted by alternate colors, preventing mistakes in contexts with mixed component types. The properties of the PCB themselves are unaffected by the solder mask’s hue. The resolution power of the board will be different colors like red and blue, which phase match the resolution of the green PCB, but this is where different colors differ from one another. The clear solder mask has a better resolution than the black and yellow solder masks, which have a substantially lower resolution than the green ones.
How to Choosing The Right Color？
It pays to be aware that the color you choose can, to some extent, affect the performance of your solder mask, even though playing with different hues might make your project appear a lot more fascinating than with the traditional green.
For starters, because UV light doesn’t always get through the entire coating, black, white, and yellow are notorious for having poor resolution. Less tracks will be present on a PCB with these colors as a mask than on a board of comparable size with a green, red, or blue mask.
However, although offering the finest resolution, transparent solder masks are prone to color changes due to the numerous temperature excursions and chemical treatments a PCB goes through.
The caliber of the solder mask itself also affects how well a color type performs. Custom shades from top providers typically offer great resolution, as well as temperature and chemical resistance.
Designers frequently employ various colored solder masks for purely aesthetic reasons. Nevertheless, if you don’t have any specific color preferences, different tones can assist you distinguish your boards both during and after construction.
Red, blue, and black can be used to indicate various prototype stages of the design before deciding on green or white for the finished item.
What Is Peelable Solder Mask?
Peelable solder masks are a specific kind of solder mask that may be removed from a printed circuit board’s surface. During the reflow soldering, wave soldering, or surface finishing processes, this kind of solder mask is utilized to protect particular PCB sections. It prevents excess solder from building up on empty pads during the soldering process and is put over the pads or plated through holes prior to PCB assembly. Additionally, it shields gold-plated connections from dissolving in molten solder. Compared to standard solder masks, peelable solder masks are simpler to remove.
For the components that will be manually inserted following the automated assembly procedure, a peelable solder mask is also required.
A solder mask that can be peeled from the surface of a printed circuit board is known as a peelable solder mask. This kind of solder mask is used to shield particular PCB components during the reflow, wave, or surface finish processes. Prior to PCB assembly, it is put over the pads or plated through holes to prevent extra solder from building up on the empty pads. Additionally, it shields contacts with gold plating from dissolving in hot solder. Peelable solder masks are simpler to take off than traditional solder masks.
The pieces that will be manually inserted following the automated assembly process also require a peelable solder mask.
Peelable solder masks are typically applied by screen printing and removed following processing at the PCB assembly facility that has been hired. The PCB fabricator can apply the mask to one or more parts per side at once in any pattern or shape.
Solder Mask VS. Paste Mask
Due to their similarities, Solder Mask Layer and Paste Mask Layer may cause confusion for many beginners. To help beginners understand better, we will explain the distinctions between solder mask and paste mask in this post.
Except for the pads (surface pads, mount pads, plug-in pads, and vias) on PCB boards, which should be coated with solder resist ink, solder mask is the layer of solder. In order to avoid tin while wave soldering, these pads are exposed. Other names for it are solder mask layer and green layer.
What Is A Paste Mask layer ？
In the business, paste mask layer is frequently referred to as stencil. The hollow shape is typically somewhat smaller than SMD pads or the same size. In the SMD automatic assembly soldering procedure, this stencil is used to apply tin paste to the SMD pads. In the surface mount device (SMD) welding procedure, the stencil must first be adhered to the circuit board (aligned with the corresponding pads), after which paste must be applied, excess paste must be scraped off with a knife, and finally the stencil must be removed, allowing paste to be applied to the solder pads. In order to finish the SMD assembly, attach the components to the solder paste next (either manually or via an assembly machine).
Here are some differences between solder mask and paste mask
1) The openings on the paste mask layer have paste, while the apertures on the solder mask layer lack solder mask ink.
2) The paste mask layer is separate from the solder mask layer on the circuit board. Only the stencil is used on the paste mask layer.
3) Paste mask is used for applying paste, whereas solder mask ink is used for applying solder mask ink.
4) When creating PCB boards, solder mask is used; however, during assembly, paste mask is used.
5) While paste mask is typically gray, solder mask comes in a variety of hues.
How To Repair The Solder Mask?
PCB Solder Mask Repair Process:
- To achieve adequate adhesion and to have appropriate electrical qualities, remove extra solder from the pads.
- In the indicated regions, prepare and apply the coating replacement mask material.
- In some cases, colorants are also employed to match the coating to the board’s current color.
- Check the electrical continuity between the vias and pads, then let the mask dry in the oven.
How To Do Testing For PCB Solder Mask?
A lot of printed circuit boards employ soldermask. Sometimes it’s necessary to check for solder resist issues after the soldermask procedure.
1. Direct visual inspections and then 1.5 to 10 times magnification for the solder quality test.
2. Adhesion (tape test): Using 3M tape to assess peel strength in accordance with IPC-TM-650/TM2.4.28.1 Standard.
3. Hardness Test: In accordance with IPC-SM-840C 3.5.1 / TM 2.4.27.2 Standard, after baking, draw an inch-long line on a board using a JIS-standard pencil and around one Newton of force at a 45-degree angle. Check for scratches after erasing the toner using an eraser. When the copper is covered, everything is well.
4. To perform an acid and alkali resistance test, two 6PNL silver test panels of each kind should be dipped for 30 minutes in a 10% HCl, 10% H2SO4, and 10% NaOH solution, in accordance with IPC-SM-840C 3.6.1.1. Take two additional 6PNL OSP panels and submerge them for 60 minutes in a 10% HCl, 10% H2SO4, and 10% NaOH solution. Next, look for blisters, peeling, discolouration, and other issues on the surface.
5. Solvent resistance: in accordance with IPC-SM-840C 3.6.1.1, clean the coated surface before examining it to see whether any blistering, peeling, discoloration, or other issues have occurred.
6. Resistance to high temperatures
1）Welding: according per IPC-SM-840C 3.7 minimum
2）Resistance to soldering: in accordance with IPC-SM-840C 3.7.2 normative
3）Heat resistance of solder
4）Test for tin drift: 260 degrees Celsius, 10 seconds, and three tests.
7.To measure the Soldermask ink thickness, use a silver sheet and an OSP board to create a number of sections.
8.Undercu: To determine whether the undercut is deeper than 1.2 mil.
9. Resistance to thermal shock:
According to the IPC-SM-840C 3.9.3/TM2.6.7.1 standard, H degree, 65–125°C, 100 cycles, check for blistering, peeling, or discoloration on the surface.
10. Test Design for Solder Dams Negative film has solder dams of 2, 3, 4, and 5 mils (according to the various process capabilities). After development, baking, and FQC, determine if it is incorrect.
How is a solder mask applied?
The steps for applying a solder mask layer to a PCB are described below:
Step 1: Board cleaning
After being cleaned of dirt and other impurities, the board is dried on its surface.
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.
Step 3: Pre-hardening
Pre-hardening is different from total hardening because it aims to make the coating on the board somewhat firm. This makes it easier to remove the undesired coating, which in turn makes it simpler to remove the PCB during the development phase.
Step 4: Imaging and hardening
A picture film that has been laser plotted to define the solder mask region is used for imaging. The panel, which has previously been solder ink coated and tack dried, is aligned to this film. The film that is aligned to the panel is exposed to UV light during this imaging procedure. The opaque portion permits the UV light to pass through the film after being exposed to it, which causes the ink below it to polymerize (hardened).
Photo films are not necessary when using LDI imaging since the UV laser will immediately harden the board areas that must hold the solder mask ink.
Step 5: Developing
The unnecessary solder mask is subsequently removed from the circuit board by dipping it in developer, ensuring that the necessary copper foil is accurately exposed.
Step 6: Final hardening and cleaning
For the solder mask ink to be used when applied on the PCB surface, final hardening is required. Before continuing with other steps like applying the surface finish, the boards that have been covered in solder mask must first be cleaned.
What is solder mask clearance?
How closely the solder mask can be to the features on the PCB surface is determined by a tolerance called solder mask clearance. To provide sufficient distance between surface features that receive solder, or solder dams, is the goal of this tolerance or clearance. Solder bridge creation is stopped as a result.
In most cases, the solder mask clearance must be half as wide as the conductor spacing. The solder mask clearance can be as low as 50 m when utilizing small, conductive designs (less than 100 m).
Pads
In order for the solder mask to determine the size of the pad used in BGAs, the copper pad must have an opening that is smaller than the solder mask opening. The size of the copper pad will change as the mask clearance changes.
Printed circuit boards frequently employ soldermask. Sometimes you need to check the solder resist after the soldermask process to see if there are any issues.
1. Direct visual inspections and then 1.5 to 10 times magnification for the solder quality test.
2. Adhesion (tape test): Using 3M tape to assess peel strength in accordance with IPC-TM-650/TM2.4.28.1 Standard.
3. Hardness evaluation: using IPC-SM-840C 3.5.1 / TM 2.4.27.2 Standard, use a pencil with a JIS standard hardness and approximately one Newton of force at an angle of 45 °C to draw an inch-long line on a board after baking. Check for scratches after erasing the toner using an eraser. When the copper is covered, everything is well.
4. To perform an acid and alkali resistance test, two 6PNL silver test panels of each kind should be dipped for 30 minutes in a 10% HCl, 10% H2SO4, and 10% NaOH solution, in accordance with IPC-SM-840C 3.6.1.1. Take two additional 6PNL OSP panels and submerge them for 60 minutes in a 10% HCl, 10% H2SO4, and 10% NaOH solution. Next, look for blisters, peeling, discolouration, and other issues on the surface.
5. Solvent resistance: in accordance with IPC-SM-840C 3.6.1.1, clean the coated surface before examining it to see whether any blistering, peeling, discoloration, or other issues have occurred.
6. Resistance to high temperatures
1)Welding: in accordance with standard IPC-SM-840C 3.7
2) Soldering resistance: in accordance with specification IPC-SM-840C 3.7.2
3) Heat resistance of solder
4) Test for tin drift: 260 degrees Celsius, 10 seconds, and three tests.
7.To measure the Soldermask ink thickness, use a silver sheet and an OSP board to create a number of sections.
8.Undercu: To determine whether the undercut is deeper than 1.2 mil.
9. Resistance to thermal shock:
According to the IPC-SM-840C 3.9.3/TM2.6.7.1 standard, H degree, 65–125°C, 100 cycles, check for blistering, peeling, or discoloration on the surface.
10. Test Design for Solder Dams Negative film has solder dams of 2, 3, 4, and 5 mils (according to the various process capabilities). After development, baking, and FQC, determine if it is incorrect.
Solder Mask Design Tips
In fact, regardless of the PCB design software you like using, solder masks are optional. Simple design of a solder mask is possible by filling out a few parameters. Even automated solder masks can be provided by some software.
Before beginning the actual design, it is essential to speak with the PCB manufacturer that has been hired in order to accurately assess their capabilities with regard to solder mask thickness and minimal copper pad spacing, neither of which are universal solutions for all boards.
Simple solder mask problems, such as insufficient or excessive openings, or an imbalance between the number of openings and the number of copper pads in the circuit plane, will cause a circuit board to malfunction. It takes a while to ascertain whether these problems are the result of negligence or design file alteration. And some even trigger disasters. Your design files are therefore much worth a careful scrutiny.
Reach PCBShare to Get Your PCB Solder Mask Design Suggestions
Since it has been providing PCB manufacturing services to the electronics industry for more than ten years, PCBShare has a wealth of knowledge in recommending solder mask color and design options. For advice on PCB design, contact us here. Has the finalized design been prepared for production or a prototype? You can click the button below to get a PCB quote right away!
