Understanding the Role of Solder Resist Materials in PCB Fabrication

Solder resist, also known as solder mask, Yes they are the same. Again two different names for the same thing but the solder mask one is adapted globally. The protective coating used on PCBs to stop solder from bridging undesired places is referred to by both designations. Although the term "solder mask" is frequently used, "solder resist" is more technically correct since the resist material opposes the solder as it is being assembled. Nonetheless, in the business, both phrases are frequently used interchangeably.

Solder resist materials are essential for preventing short circuits, while being frequently disregarded in favor of copper traces and substrates. It is located under the silkscreen layers and referred as the second top layer in PCBs. Typically, it decodes the PCB's color, which is seen as green, red, blue, and black. Those hues are precisely what this solder resist layer is made of. During the assembly process, this layer makes sure that solder only sticks to designated places. PCBs may have weak connections, short circuits, and general performance problems if solder resist is not used. The definition, kinds, materials, application technique, and reasons why solder resist is essential to contemporary PCB manufacture will all be covered in this article.

What Exactly is a Solder Resist?

With the exception of pads or vias where solder is necessary, a thin layer of protective polymer coating known as solder resist is put over the copper traces on the PCB. During the soldering process, its main purpose is to stop molten solder from bridging nearby conductors.

Key Functions of Solder Resist:

● Prevents Short Circuits

● Protecting Circuit Patterns from Dust, Heat, and Moisture

● Supports Manufacturing / Smooth Manufacturing Process

● Maintaining Electrical Insulation Between Circuit Patterns

Solder Resist Application Process:

1. Surface Preparation: Clean the bare PCB, bare PCB means the one which has tracks and holes. The cleaning is required to remove dust, oxidation, or other contaminants.

2. Apply of Solder Mask: It is basically a process of screen printing. In which we need a mask over which solder is sprayed or coated by squeezing. Based on different solder mask the application also varies such that:

● Epoxy: Screen printed.

● LPI: Sprayed or curtain coated

● Dry Film: Laminated

3. UV Exposure (For LPI and Dry Film): To cure the solder resist this step is performed, in which a photographic film (with circuit image) is overlaid. UV exposure polymerizes exposed areas, defining the solder mask openings.

4. Development: It consists of a developer solution basically, after UV exposure the solder mask layer cures properly. But in some regions, which are unexposed under UV are washed away, revealing pads and vias.

5. Thermal Curing: It is the final baking and curing step. The PCB undergoes baking to harden the mask. In this step PCB achieve full curing making it market ready now.

Type of Materials Used in Solder Resist Layer:

Solder resist materials come in a variety of forms, each with unique advantages for certain production procedures. The types of solder resists:

Liquid Solder Resist: Liquid Solder Resist: The most common form, this one is easy to apply and may be applied by spraying or traditional screen printing. Generally speaking, if clients have no specific needs, this sort of solder resist is used by default. It is not appropriate for fine-pitch SMT and has a reduced resolution.

Dry Film Solder Resist: Before the board is exposed to ultraviolet (UV) radiation, a laminated substance called dry film solder resist is placed. The resist's undesirable components are removed by washing after exposure. It is utilized in high-frequency, or RF, PCBs and provides outstanding edge definition.

Photo-sensitive Solder Resist: This unique kind responds to UV light, which makes it perfect for intricate, small-scale PCB designs. The procedure consists of UV exposure spray or curtain coating developer solution:

● Curtain coating or spray

● UV exposure

● Developer solution

Negative and Positive UV Exposure:

Negative UV Exposure:
The solder exposed to UV light hardens in negative UV exposure. After development, these hardened regions remain on the PCB. During the developing process, the unexposed portions are eliminated. Because it works well for tiny traces and delicate features, negative type solder resist is the most often used solder resist in PCB fabrication.

Positive UV Exposure
When exposed to positive UV light, the exposed portions grow and become soluble. This indicates that the exposed areas are broken down by the UV radiation and are washed away. The sections that are not exposed stay on the board. Instead of being utilized for solder masks, positive resists are often employed for high-precision photolithography, such as circuit patterning.

Color for Good Looks or Performance:

Selecting the right color is not only about good looks. In some situations, it can aid in thermal conductivity which is crucial for high-power circuit applications. Some hues reflect better in UV light, which facilitates the photolithographic process. For example blue solder resist tends to make the UV light exposure process more accurate. Which makes it useful for high-resolution applications. We've covered the specific features of PCB colors in our previous blogs:

The availability of historically green makes it the preferred option. However, green's great visibility, particularly in ultraviolet (UV) light, is one of the primary reasons it continues to dominate. Photolithography is used in the PCB manufacturing process to expose particular board regions to ultraviolet light. FQC (final quality check) inspectors can examine the board for flaws or faults in the soldering process since green solder resist is quite noticeable under UV light.

Thickness of Solder Resist?

Is it an important consideration? But how thickness affects the PCB performance？ The solder resist layer typically ranges in thickness from 16 to 30 micrometers. The solder mask can be as thin as 0.3 mils or may be less. Thin solder resist layer is required because we do not want any components misalignments. If it is too thin it will peel off easily, same is the case for too thick. Now we have a large gap between pad and components, so more paste is required for soldering. Which may cause soldering errors. Often thinner close to the trace edges due to fabrication processes.

Environmental Considerations

● RoHS (Restriction of Hazardous Substances): Modern solder masks are lead-free and made to work with lead-free soldering techniques. It should follow the standards like: RoHS (Restriction of Hazardous Substances).

● REACH Compliance: Verifies that solder mask materials don't include any dangerous compounds. It comes under the forbidden material list of the European Union.

Conclusion:

In conclusion, solder resist compounds are much more than just decorative PCB embellishments. They are essential to the functionality and durability of electrical gadgets. The function of solder resist in PCB manufacturing ranges from shielding against environmental damage to avoiding solder bridging during reflow. Additionally, we have seen a variety of kinds, materials, and much more. A person's perception of color is always subjective. Achieving high reliability and low failure rates in electronics production will depend on choosing the appropriate solder resist.