Aug 27, 2020|General

What is LPI Solder Mask?

If you’ve heard this term come up and you’re wondering, “what is LPI solder mask?” we cover all you need to know in our comprehensive guide.

Discover what LPI stands for and how it works, including LPI application. Find out what colors of solder mask are available, its role in assembly, and how Gerber (CAD) data defines it.

What Does LPI Stand For?

LPI stands for Liquid Photo Imageable (a liquid solder mask) and is the industry standard for solder mask or solder resist.

Besides LPI, you can also opt for a Dry-Film Photo Imageable solder mask (DFSM) or epoxy liquid. 

However, LPI outshines DFSM and epoxy liquid for several reasons. It encourages accurate printing, forms better contact with the PCB, and is consequently more durable. 

How is LPI Solder Mask Applied?

In this section, we delve into LPI solder mask application. Note that the general process remains similar for all types of solder mask, exempting application techniques:

1. Clean PCB

The PCB must undergo thorough cleaning to remove any trace of contaminants or oxidation, and then dried. It can be physically scrubbed or covered in a cleansing solution.

2. Apply LPI

The exact method of application depends on one question: what material is solder mask?

A Dry Photo Imageable solder mask can only be applied via vacuum lamination. In contrast, LPI ink is more versatile due to the nature of the solution. 

There are four choices: curtain coating, electrostatic spray, air spray, and silk screen printing. 

In curtain coating, the PCB travels through a “curtain” of ink falling through a lot. This method is ideal for more complex PCBs, as it enables rapid application with little ink loss.

Electrostatic spray application entails the ink getting atomized in a rotating bell. As the LPI develops a negative charge, it is attracted to the PCB, which is earthed. Unfortunately, this technique tends to result in a less uniform coating.

Air spray is straightforward: the LPI solder mask emerges from one or more spray nozzles. A drawback is that overlap (and LPI waste) can occur with multiple spray guns.

With silk screen printing, a squeegee blade deposits ink on the PCB through a tensioned mesh. Even though these printers are advanced, a successful application depends on correctly controlling the settings (e.g., speed, pressure, etc.).

3. Tack Dry

The mask-coated PCB gets placed into an oven to tack-dry. This step allows for easier handling during the remainder of the process.

4. Add Protective Film 

Once dry, a film is printed with emulsion to cover the areas from which the solder mask will be removed. After all, you don’t want to get the mask on your solder pads. 

The transparent film indicates areas where the solder mask should adhere to, whereas black film shields the areas you don’t want masked. 

5. Cure LPI

The second-to-last step is UV light curing to affix the solder mask to the PCB permanently. If you want to go the DIY route, you’re probably asking: how do you use UV solder mask?

Don’t worry; you can follow the same approach for UV light curing. Use the applicator to smear the LPI solder mask on your PCB and allow it to cure for the specified period. 

6. Remove Excess Ink

Now that the LPI solder mask is firmly bonded to the PBC, you can wash off residual undeveloped ink.

Solder Mask Colors

The solder mask is what gives your PCB its distinctive pigment. There are various color options to choose from in either gloss or matte finish, or both.

A quality PCB manufacturer should offer at least some of these choices:

Green

Green is a popular standby for PCBs—for a long time, it was the only available choice. The color reduces eye fatigue and provides a sharp contrast for high visibility. 

White

White solder mask gives your circuit board a clean, sleek look. However, it can be challenging to discern components under bright lights—although it pairs well with black silkscreens.

Black

Black is practical in terms of contrast, but you may encounter visibility struggles under the wrong lighting (shadows can pose a problem). Much like white, a black circuit board is striking to look at.

Red

Red is both visually appealing and user-friendly as it provides decent clarity, though not quite as good as green. Silkscreens tend to stand out against red PCBs.

Yellow

Yellow solder mask is another alternative if you want a unique circuit board. Critical parts are easily viewable against this vibrant color. 

Blue 

Blue is ideal for LCD mounting and offers an impressive contrast against silkscreens. It might not be quite as exciting as the other shades on the list, but it’s perfectly functional.

How Is Solder Mask Defined in Gerber (CAD) Data?

Where Gerber (CAD) data is concerned, the solder mask is akin to any other layer, such as silkscreen or copper. 

The difference is that it presents like a negative image: the file displays areas that will not be covered by the solder mask.

Solder Mask and Assembly

A solder mask, also known as solder resist, is a vital stage in the PCB manufacturing process. It shields the copper traces against oxidation and prevents the formation of solder bridges between solder pads during manufacturing. 

So, is solder mask an insulator?

Although that may seem like the case, a solder mask is not precisely an insulator—it does have insulating capabilities, but it also offers much more. Solder masks have evolved from solely isolating solderable surfaces to become a critical facet of circuit board design.

The solder mask layer serves as a barrier against general contaminants such as dust and dirt and allows for safe handling. An LPI mask can work to prevent solder starvation by plugging vias near the surface mount pads. Finally, it protects copper tracing against rust over time. 

All of these features contribute to the lifespan of the circuit board. They also save rework efforts after assembly by lowering the chances of short-circuiting and accidental solder bridges.

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