What are the best practices for applying solder mask to flexible PCBs?

1 Factors to consider

Before applying solder mask to flexible PCBs, you need to consider some factors that affect the performance and durability of the solder mask. These include the flexibility, thickness, adhesion, and curing of the solder mask. You want to choose a solder mask that can withstand the bending and twisting of the flexible PCB without cracking, peeling, or delaminating. You also want to ensure that the solder mask has a suitable thickness that does not interfere with the electrical or mechanical properties of the flexible PCB. Moreover, you want to ensure that the solder mask has a strong adhesion to the substrate and the copper, and that it can be cured properly without damaging the flexible PCB.

2 Types of solder mask

There are different types of solder mask available for flexible PCBs, each with its own advantages and disadvantages. The most common types are liquid photoimageable (LPI) solder mask, dry film solder mask, and screen-printed solder mask. LPI solder mask is applied as a liquid and then exposed to UV light and developed to create the desired pattern. It offers high resolution, good coverage, and low cost, but it may not be flexible enough for some applications. Dry film solder mask is applied as a pre-cut film and then laminated and exposed to UV light and developed. It offers high flexibility, good adhesion, and uniform thickness, but it may be more expensive and difficult to handle. Screen-printed solder mask is applied as a paste and then cured by heat or UV light. It offers high flexibility, low cost, and easy application, but it may have lower resolution, coverage, and durability.

3 Methods to apply solder mask

Depending on the type of solder mask and the design of the flexible PCB, there are different methods to apply solder mask to flexible PCBs. The most common methods are spray coating, curtain coating, roller coating, and screen printing. Spray coating involves spraying the liquid solder mask onto the flexible PCB using a nozzle or an airbrush. It offers high flexibility, speed, and uniformity, but it may require masking and cleaning of the unwanted areas. Curtain coating involves passing the flexible PCB through a curtain of liquid solder mask that falls from a slot. It offers high speed, uniformity, and coverage, but it may require precise alignment and control of the curtain. Roller coating involves passing the flexible PCB through a pair of rollers that apply the liquid or film solder mask onto the surface. It offers high speed, simplicity, and low cost, but it may require careful adjustment of the roller pressure and gap. Screen printing involves pressing the paste solder mask through a screen with the desired pattern onto the flexible PCB. It offers high flexibility, simplicity, and low waste, but it may require frequent cleaning and maintenance of the screen.