Introduction

Printed circuit boards (PCBs) are components in electronic devices and usually consist of distinct layers, including solder masks and paste masks.

As solder masks and paste masks serve different purposes, this guide will offer an in-depth exploration of the differences between solder masks and paste masks for your understanding.

Solder Mask and PCB Production

The application of a solder mask is essential for enhancing the longevity of a PCB. During PCB assembly, copper traces are susceptible to oxidation when exposed to air, necessitating the use of a layer, like the solder mask, as a manufacturing step.

A solder mask is a layer applied on copper traces to prevent rust when exposed to air, moisture, and dust.

What Does A Solder Mask Do?

The functions of solder masks on printed circuit boards are varied serving purposes:

● It safeguards the PCB from oxidation.
● It shields the PCB from heat sources.
● It protects the PCB from accidental circuit bridging during soldering.
● In cases of discharge, the solder mask acts as a barrier for the PCB.
● The solder mask prevents dirt and dust from harming the circuit components.

Types of Solder Masks

Different types of solder masks are used in PCB production. Here are some common ones:

Top and Bottom Solder Mask: Made with epoxy, this type covers both the bottom traces of a PCB.

Epoxy Solder Mask: Applied onto the PCB to facilitate ink transfer to areas on the circuit board. It's a readily accessible type of solder mask.

Liquid Photo Imageable Solder Mask (LPSM): A liquid photo imageable solder mask is an ink mixture sprayed onto the PCB. It's quite simple and quick to put on, making it a cost-effective option.

Dry Film Photo Imageable Solder Mask (DFSM): Dry film photo imageable solder mask and liquid photo imageable solder mask are quite similar, with the main difference being that DFSM involves vacuum lamination while LPSM uses liquid solder resist. It's crucial to ensure no bubbles are trapped underneath once the solder mask is applied.

When the mask layer is fully applied, it forms a pattern that allows components to be mounted on the PCB.

Solder Mask Application Process

The process of applying the solder mask can be carried out by following these steps: 

Clean the circuit board to remove all dirt and contaminants before beginning.

Apply the solder mask ink by placing the circuit board in the coating equipment containing solder mask ink. The thickness of the solder mask may vary based on how the PCB is used, the components involved, and the capability of the equipment to coat the PCB.

During the hardening phase, the coating on the PCB solidifies and helps remove any excess coating from the PCB during development.

If there are any solder mask residues on the PCB, they are dipped into a developer solution to remove them.

Afterward, the board undergoes hardening and cleaning processes before moving on to development after applying the solder mask.

Paste masks are stencils with perforations that facilitate solder paste application onto the PCB.

Stencils are used to apply solder paste, and attaching components onto the printed circuit board becomes simpler. For a fit, the size of the paste mask must match that of the circuit board.

The layer of solder paste aids in identifying copper areas. The stencil file utilized for application contains solder paste traces. A single layer is sufficient to coat one side of a PCB using one stencil file.
Two stencil files are necessary when applying paste to both sides of the PCB.

Functions of PCB Paste Masks

The primary functions of a paste mask for PCBs include:

● Facilitating soldering of components on the PCB.
● Allowing for reflow during soldering when assembling the PCB.
● Enabling the use of surface mount technology in the PCB assembly process.

Types of Paste Masks

There are two types of paste masks used in PCB assembly;

Top Paste Mask: This type coats the side of the PCB, where components are attached using this mask.

Bottom Paste Mask: A bottom paste mask is applied to the side of the PCB, which is also incorporated through hole technology.

Applying Paste Masks

To apply a paste mask to a PCB, follow these steps:

● Begin by preparing the PCB through an etching process.
● Choose a method for applying the paste mask.
● To achieve the desired pattern, you can use a stencil or a spray gun to apply the mask. Alternatively, you can move the curtain system to apply the mask.
● Afterward, expose the PCB to UV light to solidify the mask and inspect its quality.
● Remove any mask from the PCB to achieve the desired pattern.

Solder Mask vs. Paste Mask: What are the Differences

It's crucial to understand the differences between solder and paste masks. Here's an explanation of how solder and paste masks differ in function and application.

Opening Layout

The solder mask layer lacks ink on its opening, while the paste mask layer contains paste on its opening.

Circuit Component

The paste mask is not part of the circuit; it's solely part of the stencil. On the other hand, the solder mask layer is a circuit board coating.

Layer Application

The paste mask assists in applying a layer of paste, while the solder mask is responsible for applying solder mask ink.

Application Stage

The application stage differs for both masks. The solder mask is applied during PCB manufacturing.

However, the application of the paste mask occurs during the assembly process.

Variety of Colors

While solder masks can be found in various colors, the paste mask typically comes in gray.

Conclusion

Although both masks serve as layers on printed circuit boards, it's crucial to understand their distinctions.

These masks not only differ in name but also vary significantly in their application methods, stages, colors, properties, and functions.

If you aim to have assembled PCBs with the mask applied at the correct stage, reach out to PCBasic. They ensure top-notch quality and meticulous attention to detail throughout PCBs' manufacturing and assembly stages.