Solder mask is an often-overlooked component of an RF/microwave printed-circuit board (PCB). It provides protection for a circuit but can also have an effect on final performance, especially at higher frequencies. Solder mask may not always be used in RF/microwave circuits but, when it is part of a circuit, it should be accounted for electrically as well, for the most accurate modeling and simulation. Knowing more about the material properties of solder mask can help boost an understanding of how these circuit layers can impact performance at RF/microwave frequencies.
A solder mask is essentially a thin polymer layer that protects copper conductors from oxidation and helps to minimize the creation of short circuits by means of bridges formed by excess solder.
Solder mask provides protection for areas of a PCB that do not require any final plating finish. Traditionally, solder masks have provided their green color to PCBs but solder masks are now available in many other colors as well as in lead-free versions. The requirements for a particular solder mask will be determined by PCB conductor thickness, circuit density, viaholes, and types of components to be attached to the PCB, such as surface-mount-technology (SMT) components.
Solder mask can be applied in dry (film) or liquid forms. Because of the more complete coverage they provide on practical, three-dimensional (3D) surfaces, most solder mask is applied in liquid form. Liquid solder mask is typically formed from a two-component mixture of liquid photoimageable (LPI) polymers and solvents. The liquid components are mixed and used to form a thin coating that will adhere to the different surfaces and materials of a PCB, notably the conductive traces. An LPI solder mask provides protection for the PCB’s circuitry and dielectric materials, and the solder mask must be patterned to form openings (solder mask voids) for electrical connections of circuit components. When using LPI solder-resist inks, openings and other patterns can be formed in the solder mask by exposure to an ultraviolet (UV) light source, using either photolithography or direct laser imaging to create desired openings and patterns.
Most LPI solder mask is based on either an epoxy or acrylic formulation. Each solder mask approach brings its own contributions to a PCB, such as high dissipation factor and poor moisture absorption, that do not necessarily add to excellent high-frequency performance. For example, in contrast to circuit laminates intended for RF/microwave applications and often characterized at 10 GHz, the typical dissipation factor (Df) of an LPI solder mask is 0.025 at 1 GHz, with a dielectric constant (Dk) of around 3.3 to 3.8 at the same frequency, depending upon formulation.
The moisture absorption of an LPI solder mask also depends on the solder mask formulation, and is typically around 1% to 2%. Compare this to moisture absorption of less than 0.3% for most RF/microwave circuit laminates, and it is clear that both parameters of a PCB laminate will be affected by the contributions of an LPI solder mask. Because of the negative impact on RF/microwave performance, solder mask is often omitted from the RF/microwave portion of a PCB even if it can provide protection that enhances reliability.
In high-frequency circuits with microstrip or grounded-coplanar-waveguide (GCPW) transmission lines fabricated on low-loss circuit laminates, the addition of solder mask will increase the dielectric losses and the effective Dk of the circuit compared to a circuit without solder mask. Whether in double-copper-layer or multilayer designs, the characteristics of the solder mask must be included in any computer-aided-engineering (CAE) modeling and simulation performed to predict circuit performance, especially if a design goal is concerned with minimizing circuit losses.
Often, small patches of solder mask are used in RF/microwave circuit designs as “dams” for those areas where solder will be applied and SMT components assembled. In contrast to full circuit solder mask, these smaller patches or dams tends to have negligible impact on electrical performance. In general, if a solder mask patch is less than one-tenth wavelength of the operating frequency, it will not have a significant impact on the performance at that frequency. Provided that such solder mask patches are sufficiently small, they will have negligible effects on a high-frequency PCB. However, the use of multiple solder mask patches within a relatively small section of a PCB can result in a change in the material properties within that area that can have resulting electrical effects, such as higher loss.
In selecting solder mask for PCBs, a number of characteristics should be considered. These include long shelf life, high adhesion, high electrical insulation, good heat resistance, high plating resistance to all forms of plating, including with electroless nickel and gold, and compliant with halogen-free requirements. For applications where performance is critical, the choice of solder mask color can influence the material’s Df and Dk characteristics, where a difference in color can mean a difference, although slight, in both parameters. Proper cleaning and preparation of a PCB surface can also go a long way in ensuring that solder mask adheres strongly to the laminate surface when applied.
Additional information on solder mask is available from the global trade association, Association Connecting Electronics Industries, and its Institute of Printed Circuits (IPC), in the form of publication IPC-SM-840C, which reviews material types, adhesion characteristics, and other parameters of interest. By the nature of the LPI solder mask materials, and the need to form precise openings and other features typically by photolithography, they will be limited in terms of electrical performance compared to modern low-loss, high-performance circuit laminates. But when the protection of solder mask is needed for a design, and through proper planning and simulation with modern CAE circuit modeling software, solder mask can be added to RF/microwave double-copper-layer and multilayer circuits with minimal or no penalties in performance.
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