In the 1980’s, the Liquid Photo Imageable LPI Solder Masks was introduced. LPI is a type of solder mask used in printed circuit boards. Since this solder mask type was introduced, PCB manufacturers have been applying it on circuit boards like flexible boards, rigid or rigid-flex boards. Today, the commonest solder resist used in PCBs is the LPI solder mask. This is because LPI offers more reliability and accuracy when printed on circuit boards. Also, this solder resist enables better contact with the PCB’s surface and the copper features it protects.
There are different types of solder masks for circuit boards. However, the LPI solder mask stands out due to the features and benefits it offers. The LPI solder mask is simply an epoxy based material that offers a good level of durability. It is very difficult to remove this soldermask after curing. If you wish to understand and acquire more knowledge about LPI PCB (liquid photo-imageable(LPI)soldermask, you can read further.
What is LPI PCB (liquid photo-imageable (LPI) solder mask?
Liquid Photo-imageable LPI soldermask is a liquid ink made of two component which PCB manufacturers spray-coats or silkscreen on the circuit board. This solder mask is an economical product which comprises polymers and solvents. The combination of polymers and solvents produces a thin coating which stays on the PCB surfaces.
The mask coats the areas of the circuit boards and as such serves the purpose of soldermasks in general. The areas which are coated don’t need any final plating finishes.
LPI inks are known for their sensitivity to UV light. They are different from other epoxy inks applied with the help of a screen that shield pads that needed solder or other finishes.
The mask covers the panel completely after a while; the panel is exposed to a UV light source by photolithography or laser direct imaging using a UV laser. LPI solder mask technique is usually integrated with PCB surface finishes like Immersion Gold and Hot Air Solder Levelling (HASL) surface finishes. The application process of this solder masks needs to be free of particles and should be carried out in a very clean environment.
More on LPI PCB
After the application of an LPI solder mask, the manufacturer uses soldermask to cover both sides of the PCB. The next step is usually curing. There are various ways of applying liquid Photo-imageable (LPI) solder mask. This includes electrostatic spray, screen printing, air spray, and curtain coating among others.
Curing is a crucial process in Liquid Photo-imageable (LPI) solder mask. This process permanently holds the solder mask in place, making it very difficult to get rid of. Therefore, this gives LPI solder mask its long shelf life.
It is very important that the LPI mask is properly cured in the appropriate locations. To achieve this, a contact printer prints negative film stencils of the bottom and top solder mask. Black sections which match with any uncoated PCB areas are used in printing the film sheets.
LPI PCB Liquid Photo-imageable (LPI) Solder Mask Application
The application process of LPI soldermask determines a lot. It determines its performance and shelf life. Here, we will explain the application process of this solder mask in details. Below are steps involved in LPI solder mask application process.
Cleaning
You need to clean the printed circuit board thoroughly to get rid of any oxidation or contaminants. The application of the solder mask will be ruined if the board is not properly cleaned. The board can be dipped in a cleansing solution or scrubbed physically. After cleaning, ensure the board gets dried.
LPI application
There are several methods of applying LPI solder mask. However, this depends on the material of the solder mask.
Vacuum lamination can apply a dry Photo Imageable solder mask. The nature of LPI ink makes it a more versatile option.
There are four different options which are silk screen printing, curtain coating, air spray, and electrostatic spray.
The silk screen printing option involves depositing ink on the circuit board with a squeegee blade. Although, silk screen printers perform well, controlling settings like pressure and speed determines successful application.
In the curtain coating option, the circuit board passes through a “curtain” of ink. This is a more suitable option for complex boards, as it enables you to easily apply ink with little or no loss.
The Air spray method is very easy to perform. Spray nozzles are used in applying LPI solder mask. One major drawback of this method is that there can be excess waste with multiple spray guns.
Electrostatic spray method involves atomizing the ink in a rotating bell. The LPI becomes attracted to the circuit board as it generates a negative charge. The disadvantage of this technique is that it could lead to a less uniform coating.
Tack Dry
This step involves placing the already coated circuit board in an oven to tack-dry. Track drying enables easier handling.
Addition of Protective Film
Immediately the circuit board gets dried. A film is used in covering the areas where you will be removing the solder mask. This will prevent the mask from getting on your solder pads.
Also, there is a transparent film that reveals areas the solder mask must stay. On the other hand, a black film protects the areas you don’t want to mask.
Cure LPI
At this stage, we are almost done. The curing process is the second-to-last step. It involves using UV light to affix the solder mask to the circuit board.
Also, if you are trying the DIY process, you might be curious about how to integrate UV solder mask. You can simply follow the same method for UV light curing. Smear the LPI solder mask on your board with the applicator and then leave it to cure for a particular period.
Get rid of excess Ink
Since the LPI solder mask has permanently bonded to the circuit board, it is time to get rid of any residual undeveloped ink. You can achieve this by washing.
Importance of LPI Solder Mask on PCB
Solder Mask is very crucial in a circuit board. It is an important layer that offers protection against any form of corrosion and oxidation. This layer is usually added before silkscreen. As earlier mentioned, solder masks are available in different types. Here, we are looking at the benefits of LPI PCB solder mask.
Another importance of LPI solder mask is the prevention of solder bridges. Solder bridges occur when solder joints connect on your circuit board. This can result in short circuits and PCB damage. Solder masks help in creating a dam between other conductive parts of the board and the solder joints to enable insulation of the components the board.
Also, metal whiskers can be formed on your PCB, resulting in short circuits or malfunctions in circuit boards. LPI solder masks prevent metal whiskers from forming on your board. Metal whiskers are thin filaments that generate from the circuit and result in system failure. They are usually found on tin plating.
Another advantage of LPI solder mask is that it offers more accuracy and reliability than other types of solder mask. Furthermore, Liquid Photo-imageable (LPI) solder masks ensure better contact with the surface of the circuit board.
In general, LPI solder masks are crucial maintaining the shelf life. While some solder mask options are stylistic, you need to understand the needs and application of your PCB before choosing this solder mask option. LPI solder mask is long lasting. The thickness of this solder mask ensures that, for most designs, breakdown of solder mask will not be a problem.
An LPI solder mask can also prevent starvation of solder by plugging vias close to the SMT pads.
Other Requirements of LPI Solder mask
LPI solder mask used in printed circuit boards today are made to offer more benefits other than just specifying where solderable surfaces are exposed. They need to undergo strict testing to ensure they meet IPC requirements.
Also, LPI solder mask must withstand processes and chemicals used in plating different surface finishes like immersion silver, ENIG, Immersion Tin, and more. The materials that make up Liquid Photo-imageable (LPI) solder mask must pass the flammability test. They must get the 94-V0 rating from UL.
In addition, just like other solder masks that are available in various colors, LPI solder mask comes in a wide range of colors and finishes. The green color is the most commonly used color. However, other colors like yellow, white, black, red, and blue are available. The use of LED and for circuit boards impacted the solder mask market to make more resilient white and black materials.
Also, LPI solder masks have really developed more than the original capability requirements and have become a highly preferred solder mask among PCB manufacturers.
Conclusion
Liquid Photo-imageable (LPI) solder mask is a commonly used solder mask type in PCB manufacturing. Also, this solder mask option is preferred to other solder mask types since it is more advanced. However, the application of this solder mask on a printed circuit board requires professional expertise and skill.
