Solder Mask Vs Paste Mask: Differences And Functions

What is PCB Solder Mask?

Solder mask is an essential process in PCB manufacturing. It refers to the thin, protective polymer layer applied over the copper traces of a printed circuit board (PCB). This layer is typically seen as a green coating, although other colors such as red, blue, black, white, and even pink or transparent are also available. When you see a colored layer on the PCB, that is the solder mask. It’s applied during the final stages of PCB fabrication to protect the circuitry from environmental damage.

Function of Solder Masks

Solder masks serve multiple critical roles in ensuring the durability and performance of PCBs:

Protection of the Printed Circuit Board against oxidation

It offers protection from external heat.

The solder mask prevents accidental circuit bridging due to soldering

It offers protection from electrostatic discharge from the Printed Circuit Board.

Prevention of dust from interfering with essential components in the circuit.

Types Of Solder Mask

Epoxy Liquid Solder Mask

This kind of Solder mask is the cheapest kind available in the market. It uses epoxy resin and is applied using silkscreen printing methods. Silkscreen allows ink to be transferred to the open areas on the circuit board.

Top and Bottom Solder Mask

PCB manufacturers employ this solder mask to identify the kind applied to the top and bottom of the Printed Circuit Board. These are typically made from epoxy-based materials.

Liquid Photo-imageable Solder Mask

Also known as LPSM, this type uses a liquid resist that is sprayed onto the PCB surface. Usually called LPSM, it comes from ink formulation using a liquid solder resist. After application, it undergoes UV exposure through a photomask and is then developed to reveal desired patterns.

Dry Film Photo-imageable Solder Mask (DFSM)

The DFSM is pretty similar to the LPSM. However, the dry film is applied using a vacuum lamination process instead of liquid solder resist. This method ensures no air bubbles are trapped under the film and provides high resolution for fine-pitch components.

Pros

It prevents oxidation, dust, and dirt on the circuit board.

It is primarily green; hence they cut across varied applications of green PCB.

prevention of accidental bridging during assembly.

Cons

It is not applicable in hand-soldered PCB assemblies.

During exposure, ink usually overflows on the Circuit board.

High temperature in the reflow oven affects it.

What is PCB Paste Mask?

Paste Mask is also referred to as solder paste. It represents a stencil pattern used during Surface Mount Technology (SMT) assembly. The paste mask is actually a stencil used for surface mount component packaging and corresponds to the pads of the SMD components.

During SMT processing, a stainless steel stencil with holes aligned to component pads allows solder paste to be deposited accurately. When the PCB is placed under the steel plate, the solder paste will flow down and evenly cover each pad with solder.

Function Of PCB Paste Masks

Paste masks play vital roles in SMT-based assembly:

Paste masks enable easy component soldering on the printed circuit board.

To allow for possible reflow soldering process during PCB Assembly.

It makes surface mount technology possible during PCB assembly

Types Of Paste Mask

Top Paste Mask

Top paste masks correspond to openings for surface-mount pads located on the top side of PCBs. This Paste mask is found on the top side part of the printed circuit board. Top Paste Mask is also known as the component side.

Bottom Paste Mask

Bottom paste masks are used when components are mounted on both sides. The bottom Paste mask is found on the bottom side of the printed circuit board. It is also known as the solder side.

Pros

It reduces the reflection of light in PCBs

Paste masks are made on the circuit board's top and bottom sides.

Cons

It is not liquid, as it would require extra heat to melt it before being applied to the Printed Circuit Board.

PCB Assembly expertise is required for accurate solder deposition.

Differences Between Solder Mask and Paste Mask

The distinction between these two layers lies in their function and placement:

· The openings on solder mask layer have no solder mask ink, but the openings on paste mask layer have paste.

· The solder mask layer is one part on the PCB, but the paste mask layer is not. Paste mask layer is just for the stencil.

· Solder mask is used for applying solder mask ink, but paste mask is used for applying paste.

· Solder mask is used in the PCB manufacturing process, while paste mask is used in PCB assembly phase.

· Solder mask has many available colors, while paste mask is usually gray.

These differences emphasize why understanding "solder mask vs paste mask” matters when designing or assembling PCBs.

FAQ

Q: Can I use only one type between solder or paste masks?
No. Each serves distinct purposes—solder masks protect copper traces during manufacturing while paste masks guide precise application during assembly.

Q: Are both layers included in Gerber files?
Yes. The Gerber file includes separate layers for both; however, only paste layers are needed for stencil creation.

Q: What happens if my paste layer size exceeds pad size?
This can lead to excess solder bridging or tombstoning issues. Therefore, the paste mask should not be larger than the actual pad size, and it is best to be smaller than or equal to it.

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