Printed Circuit Boards or PCBs are not spared from the intensive evolution in electronics technology. New developments in materials, fabrication and assembly methods happen daily as the need for efficient and tiny devices is pushing the PCB industry forward. Much technical efforts are spent on designing PCBs that are manufacturable, reliable and profitable.
The fabrication of printed circuit boards is comprised of several processes that are based on the agreed customer-specific requirements. One of the important processes in PBC fabrication is the application of a solder mask, which is commonly seen as the dominant green colour in the outer layer of the circuit board. As different products entail different requirements, several solder masks are available to choose from. PCB solder mask materials should be able to withstand high thermal stresses without degradation as much heat is generated when circuits and components are in functioning mode.
In this article, the primary focus is to impart knowledge about the solder masking process, as well as PCB solder mask material types. Furthermore, we will explore Liquid Photo Imageable (LPI) and dry film options in the succeeding discussions.
What is a Solder Mask?
A solder mask is a protective layer on the PCB that has numerous functions during manufacturing and end application of the boards. It is referred to as a ‘mask’ as it helps prevent the conductive circuits from bridging with the adjacent solder during assembly. Additionally, the solder mask also keeps the holes, substrate and tracks from accumulating contaminants and foreign materials that may reduce the service life of the PCBs during application.
Solder mask also provides separation of the insulative parts from the components and conductive traces. It sits on top of the conductive copper traces to help prolong the life of the PCB by shielding the copper from oxidizing. The extreme temperatures encountered during assembly and application make solder masks a significant part of PCB fabrication. PCB solder mask materials selection can have a direct effect on the overall PCB performance.
Solder mask design can be categorised into the following major types based on their opening:
- Non-Solder Mask Defined (NSMD). Non solder mask define is a type of solder mask wherein the opening is slightly away from the metallization pads. This allows the copper pad to form a solder fillet with the adhesive during the soldering process.
- Solder Mask Defined (SMD). Solder mask defined or SMD has its solder mask slightly overlapping the metallization. In SMD, the solder flow is restricted by the solder mask opening.
What are the Different Types of Solder Mask?
Solder masks come in various methods which are selected based on product requirements, level of reliability and cost. Below are two widely used solder masking methods for printed circuit boards.
1. Dry Film Solder Mask
Dry film is a type of PCB solder mask material consisting of organic solvents of photo-initiators and polymers. It has an epoxy chemistry that has excellent insulation and dimensional stability. Dry film solder mask offers better mask uniformity in terms of cross-sectional thickness and with minimal roughness. This type of solder mask is less prone to voiding as it is done in a vacuum environment.
How is Dry Film Solder Mask Applied?
Printed Circuit Boards with dry film PCB solder mask material should be stored in an area within acceptable temperature and humidity. Exposure to heat and humidity may impact the performance of the PCBs. Below are the essential processing steps in the application of dry film solder masks.
- The first step is to clean the printed circuit boards to ensure complete adhesion of the dry film. Without this step, contaminants and moisture on the PCB can lead to the delamination of the solder mask. Though the PCB solder mask material has separate protection for the PCB, it is not a panacea.
- The pre-cleaned PCB is laminated with a dry film which is a PCB solder mask material. This process is done inside a vacuum machine to draw out air away from the circuit. The vacuum lamination is also done in elevated temperatures to ensure adhesion.
- The substrates go through exposure to polymerize the film. After exposure, the PCB with PCB solder mask material is subjected to development in an aqueous solution. Water rinsing and drying post-development are necessary to remove ionic contaminants.
- To complete the cross-linking of polymer molecules, the solder mask is cured using UV light. Temperatures must be controlled within the allowable range to ensure sufficient bonding of the film with the PCB substrate. In some dry film solder masks, both UV and thermal curing steps are employed to fully polymerize the epoxy film.
2. Liquid Photo Imageable (LPI) Solder Mask
Liquid Photo Imageable or LPI solder mask is a type of solder mask that is popularly used in PCB manufacturing. This kind of PCB solder mask material is made of resin, a photoactive component, and a solvent vehicle. LPI is a more mature process than dry film options. This method can be done even without a vacuum environment.
How is the LPI Solder Mask Applied?
Liquid Photo Imageable (LPI) solder masks are applied using the following steps:
- The PCB surface is preconditioned by cleaning off any dust and unwanted particles that can interfere with the masking process. This can be done through acid cleaning, jet scrubbing or chemical etching.
- The PCB panels are applied with the liquid photo-imageable solder mask on both sides. The PCBs with LPI solder masks are loaded inside an oven to dry the chemical at a defined time and temperature.
- A film containing the circuit pattern is applied on both sides of the boards. The film contains clear portions on where the mask is intended to adhere to the board.
- The boards are subjected to a development process using UV light. The UV light will penetrate the clear portions of the film to fully cure the solder mask. The important parameters in the UV exposure process include exposure time and energy. After the this process, the PCB solder mask material is fixated in the PCB surface.
- The uncured mask is rinsed off leaving the solder mask pattern on the PCB.
