Comprehensive Guidelines for PCB Solder Mask in 2023

Printed circuit boards (PCBs) used in electronic products are usually composed of important components such as solder mask, silk screen layer, wiring, core, etc. Each manufacturing process needs to go through tedious procedures and strict checks. With the development of the electronics industry, manufacturers’ demand for PCBA is moving towards reliability and high density. Therefore, PCBA factories continue to optimize high-density interconnection (HDI) technology and solder mask technology. This article FS Technology will focus on the part that provides protection and insulation for the circuit board-PCB solder mask.

What is solder mask?

PCB solder mask layer (SM) is also called window layer and green oil layer, refers to the part covered by ink on the PCB, and the protective layer for patterning the finished printed circuit board in the subsequent finishing layer and group soldering process of the PCB. Since the solder mask layer of the circuit board uses a negative film output, when the shape of the solder mask layer is mapped to the bare board, the green oil solder mask is not applied, but the copper skin is exposed. 

The negative output of the PCB solder mask layer means that it is completely opposite to the actual situation, that is, the part marked with green oil during design is actually a non-green oil area, which is the so-called window opening. Therefore, the real meaning of the solder mask layer is to open a window on the entire piece of solder mask green oil to allow normal welding of the non-green oil parts.

Since the base of the PCB is composed of glass fiber and epoxy resin, the board does not have sufficient heat resistance at the temperatures of HAL processes and surface mount soldering, and without a solder mask, long-term and high-temperature exposure may cause surface destruction of the dielectric material. 

The difference between PCB solder mask layer and solder flux layer

Let’s take a two-layer board as an example. A simple two-layer PCB consists of a core (two breadboards (top and bottom)), a middle part of prepreg (pp), two solder mask layers (top and bottom) and two silkscreen layers (top and bottom) . As shown below:

Internal structure of double-layer PCB

Due to reasons such as soldering and heat dissipation, manufacturers do not coat the entire solder mask layer with green oil during PCB manufacturing. Therefore, we will habitually call it a “window” where copper is exposed.

As shown in the figure below, it is more intuitive!

Details of PCB soldering layer

The soldering layer (paste mask) of PCB refers to the layer on the printed circuit board that is not coated with green oil. In fact, it refers to the stencil, and many engineers understand it as the top layer of the PCB, which is just the same as the data on the top layer of the PCB. Carefully observe the double-layer PCB structure diagram above and you will find that the soldering layer is not in the PCB structure. The function of the soldering layer is different from that of the solder mask layer. It does not exist to realize the soldering process, but to assist SMD SMT assembly. During SMT assembly, the solder paste is accurately dripped from the holes of the stencil to the PCB pad, so the soldering layer is also called the upper tin layer.

The picture below shows the soldering layer (paste mask).

PCB flux layer

Although both the paste mask and the solder mask are used for tinning, there are essential differences. As far as the inside of the circuit board is concerned, the solder mask PCB layer also plays the role of preventing salt spray and moisture for the PCB, while the paste mask focuses on the production of SMT steel mesh. The significance of the existence of the solder mask is to allow soldering. By default, the areas without the solder mask are painted with green oil. The paste mask is used to design the stencil, the purpose is for patch packaging, which is usually used when wholesale PCBA.

The influence of PCB solder mask layer on PCBA

With the Chinese government’s support for the PCBA industry, China’s PCB manufacturing and PCBA processing technology has achieved unprecedented development, and the PCB solder mask process has less and less impact on the PCBA processing process. The following is the possible impact of the solder mask process on the PBCA process:

SMT Soldering Difficulty

For some SMD components with small pin spacing, unreasonable minimum solder mask spacing design will seriously affect the difficulty of SMT assembly, increase the factory damage rate, and increase customer assembly costs.

Solution: Combined with the actual PCB and PCBA process level of the factory, optimize the component packaging process to avoid manufacturability risks. Specific optimization methods: Strictly review the customer’s PCB design documents and strengthen the company’s internal design department.

Solder mask thickness

Controlling the thickness of the solder mask is one of the key points for PCB manufacturers to manufacture the solder mask. Too thin solder mask layer may lead to copper leakage in line traces, serious IC false soldering problem; too thick solder mask layer may lead to suspension bridge and open circuit phenomenon during reflow soldering.

Solution: Control the thickness of the solder mask to make it smaller than the thickness of the copper foil pad; the solder mask thickness of the circuit board at the sharp corner position is greater than 10Um; the solder mask thickness on the traces and copper foil is less than 35um.

Solder mask processing and pad matching

When non-standard SMT assembly factories print solder paste, the surface of the pad will be contaminated, which will eventually lead to poor solder joints or solder balls, which are caused by the wrong match between the solder mask and the pad.

Solution: When designing the solder mask layer, the PCB designer should strictly implement the solder mask design specifications to minimize the spacing or air gap around the pad features.

Solder shrinkage

Too many SMDs on the PCB and unreasonable board design lead to adjacent SMDs sharing a common wire, which will cause stress when the solder is shrunken by heat to displace or break the SMD.

Solution: Timely communication with PCB assembly service provider on component and board design issues and reasonable installation of components.

Important Note During the Application of Solder Mask

When designing and applying a solder mask, it is necessary to ensure the isolation of groups of contact pads (for example, under microcircuit leads) from other conductive elements of the printed circuit board – vias, contact pads, conductors.

This isolation reduces the time and laboriousness of the soldering process. The fact is that in the process of mounting components on a printed circuit board, between adjacent contact pads (if they are not isolated), the thinnest solder jumpers can form. 

And it takes more time to find and eliminate them. If for one reason or another, such a jumper is not detected, then a PCB short circuit will result. This can lead to incorrect operation of the device or even to failure of the elements.

Also, the solder mask should protect the surface of printed circuit boards from the effects of aggressive technological environments during the application of some soldered coatings by chemical and chemical-technological methods.

 It should be noted that the solder mask does not protect the board from moisture in the harsh operating conditions of the board (since it is quite hygroscopic) and special organic coatings are used for moisture protection, which is sometimes called conformal in the technical literature.

However, did you know which of the various PCB solder mask types you ought to be by using? Yes, there are various sorts with various applications. Let’s look at it.

4 Types of PCB Solder Mask

Solder masks are available in a variety of materials and manufacturers. The application determines how and which solder mask will be used.

Top and Bottom Side Masks

Top-side and bottom-side solder masks are frequently used by electronic engineers to detect problems in the green solder mask layer. This PCB layer is added using film or epoxy methods. Following that, the component pins are soldered onto the board using the openings that the masks registered for them.

The conductive trace pattern on a board’s top side is known as top traces. Like the top-side mask, the bottom-side mask is used for the opposing side of the board.

Epoxy Liquid Solder Masks

Epoxy is a polymer that can be used as a cheaper liquid solder mask option. This is a printing technique called silk-screening uses a woven mesh to support the ink-blocking designs. The mesh makes it possible to locate empty spaces where ink can transfer.

Liquid Photoimageable Solder Masks-LPI/LPISM

With the evolution of demand, the PCB solder mask layer is no longer a single dark green, and manufacturers introduce colorful (red, blue, purple, black, pink) LPI masks.

Liquid photoimaging solder mask, also known as LPI or LPISM, is a relatively economical product among the four solder mask PCB layers, and is prepared from a variety of compositions (UV-curable resins, UV-reactive acrylic monomers, epoxy resins, photopolymerization initiators). Due to the instability of the compound, which can lead to deterioration if not used for an extended period of time after mixing, manufacturers often start mixing shortly before starting to manufacture LPOs.

When faced with the choice of solder resist suitable, if you have a complex surface for the circuit board, it is recommended that you choose this LPISM made for spray, curtain coating or screen printing applications, as it can fit more closely to copper traces and laminates. As a result, a thin coating that adheres to the target area’s surface can be extracted. 

Masks typically achieve the original purpose of solder mask by coating areas of the board that do not require any of the various final plating finishes available today.

The sensitivity of LPI solder resist ink to ultraviolet rays is unmatched by any other ink. 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.” It should be noted that this step needs to ensure a clean environment and check whether the PCB panel is clean and whether there is a redox reaction on the surface before starting.  We can use suspended pumice or aluminum oxide solution to clean PCB.

Contact printer and film tools are two of the most popular ways to expose the panel surface to UV light. The emulsion is used to print the top and bottom sheets of the film, blocking the soldering-sensitive portions. Tooling on the printer is used to fix the production panels and the film in place. The panel is then simultaneously subjected to a UV light source after that.

Another method makes use of laser-based direct imaging. However, this method eliminates the requirement for a film or tooling because the laser is controlled by fiduciary markings on the copper template of the panel.

Dry Film Solder Mask-DFSM

Vacuum lamination is used when using dry film solder masks. Next, the dry film is exposed and processed. A pattern is made by locating openings after the film has been processed. The components are then soldered to the copper pads after that. The copper is then layered onto the board using electrochemical processing.

Both on the trace areas and inside the holes, copper is layered. The copper circuits are finally protected by the application of tin. The film is taken off in the final phase, exposing the etched marks. Thermal curing is also used in this process.

High-density wire boards typically use dry film solder masks. It won’t, therefore, pour into the through holes. The advantages of employing dry film solder masks include some of these.

PCB Solder Mask Process Guidelines

In fact, regardless of the PCB design tools you like using, solder masks are optional. The 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 evaluate 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 fail. 

It takes a while to confirm whether these problems are the result of negligence or design file alteration. And some even trigger catastrophes. Your design files are therefore much worth a careful examination.

Solder Mask Design Guidelines

Via cap oil

Via cover oil refers to when the vias of the circuit board are covered by the solder mask so that the vias cannot be exposed. Unlike fill, via cap oil only the ring circumference is covered by solder mask. When the via is completely covered, it is called filling or plugging.

Via cap oil is currently the solder mask process used by Chinese PCB manufacturers to protect circuit boards. Usually used for epoxy filling or mask plugging due to PCB manufacturing cost impact. Among the methods of via bumping, LPISM bumping is the most cost-effective.

Tented vias created with soldermask
Tented vias created with soldermask

Solder Mask Dam

Solder dam refers to the solder mask gap, the purpose of which is to allow sufficient spacing between solder surface features to prevent the formation of solder bridges. Typically the solder dam distance is half the width of the conductor pitch, this rule can be relaxed when fine conductive patterns below 100µm are used.

solder mask dam
Solder Mask DAM

Solder mask opening

In order to expose the circuit to the tin material during solder paste printing, the outer layer of the PCB without a solder mask layer, that is, a solder mask opening is usually provided. This opening must be accurate, otherwise it will expose the copper that does not need to be printed with solder paste, resulting in PCB short circuit, corrosion or even damage to the line.

Solder mask opening
Solder mask opening

Solder mask coverage or extension

PCB design software can quickly design the spacing between PCB surface properties and solder mask. We call this specification solder mask extension, which can be positive, negative or zero.

  • Forward Solder Mask Extension: When there is a spacing between the end of the solder mask and the uncovered outer circumference of the pad, it is called positive solder mask swell.
  • Zero Solder Mask Expansion: It is zero value when there is no spacing or gap between the solder mask and the pad.
  • Negative Solder Resistance Extension: It is negative when the solder mask overlaps an area of the pad.
Solder Mask Coverage

Dry or liquid solder mask?

The main criterion for choosing a solder mask is the tenting (overlapping) of the holes.

Hole tinting goals:

  • Insulation during installation to prevent contact with the conductive pattern with hinged components that have conductive (metal) surfaces.
  • Protection of the copper column of the plating of vias from the impact of aggressive technological environments (pickling solutions, washing liquids, fluxes, etc.), if it is not protected by finishing coatings.

Benefits of Liquid Solder Mask:

  • cheapness of the technological process due to the low specific consumption of material associated with the possibility of forming thin protective layers of ≈30 µm (even on a relatively high relief up to 70 µm);
  • Comparatively higher adhesion than a dry soldermask, because the material is applied in a liquid state;
  • The possibility of forming jumpers with a width of up to 0.15 mm or less;
  • Resistance after final polymerization to hot and concentrated alkaline solutions (first of all, to solutions of the immersion gilding process at all its stages).