Selecting proper PCB material is a good start of PCB manufacturing. Typical PCB consists of one or more layers of copper laminated between sheet layers of a non-conductive substrate. The PCB serves as the physical foundation upon which electrical components, conductive traces, pads, and other features will reside.
In this passage, we will focus on the selection of PCB material and if you want get more information about it, then check the content below.
PCB Material Selection
Working through the PCB material necessities will have any engineer considering thermal, mechanical, electrical, and chemical properties involved in the process.
For thermal properties, you’ll want to consider temperature durability of valuable components, as well as the distribution of peak operating temperatures throughout your circuit design. Mechanical properties include enclosure and form factor management, as well as things like density or flexibility particularly for rigid-flex boards.
Electrical properties get a bit more involved with inclusion of dielectric necessities like impedance, signal integrity, and resistances that the design engineer will have to balance throughout the board (or system). Chemical properties like moisture absorption (necessary for humid environments like sensors in agricultural IoT), and flammability are invaluable to consider for the longevity of your product; however, environmental and sustainability constraints can also affect your PCB material choices.
Silkscreen
There’s nothing more iconic than the white silkscreen contrasted against the backdrop of the classic green soldermask of a PCB. Silkscreen gets its name from the stenciling technique used to print markings, letters, symbols, numbers and other information onto a board. This layer’s primary purpose is to provide information for PCB assembly and identification. Things such as polarity, the location of components, and other details can be vital for proper assembly. The two most common silkscreen PCB printing techniques are:
Liquid Photo Imaging (LPI): Time intensive, high-resolution print, Ultra Violet (UV) cured ink
Direct Legend Printing (DLP): Faster, simple inkjet print, acrylic ink
From labeling pins with reference designators to applying UL certification numbers, the general rule of thumb is to apply ink that contrasts nicely with the color of the substrate. While white is the most common, silkscreen is available in a variety of colors. It is uncommon for more than one color of silkscreen to be used on the same board.
Silkscreen on a green pcb with components
Silkscreen materials on your PCB can determine manufacturing costs.
Soldermask
Soldermask is the polymer-layer on top of the copper foil that gives the PCB its iconic green color (although any color may be used). Like its name suggests, it prevents solder bridges from forming by stopping solder migration and encouraging the manufacturer to solder to the correct places via exposed features such as silver rings and SMD pads. The soldermask also physically insulates conductive copper tracers from contact with solder, metal, and other conductive bits, while protecting against oxidation. Soldermask material is determined by the method used to apply it to the copper layer:
Epoxy Liquid: The cheapest type of solder mask is a thermosetting epoxy applied via a silk screening method.
Liquid Photoimageable Solder Masks (LPSM): For boards with unusual topography, a UV curable ink formulation can be applied using various coating techniques, exposed to a pattern and developed. While LPSM doesn’t yield a perfectly consistent layer, it provides better coverage and contact with the copper traces for PCBs with complex surface features.
Dry Film Photoimageable Solder Masks (DPSM): For flat boards with a uniform topography, a dry film may be applied via vacuum lamination before exposure and development. Dry films will give you a uniform surface thickness, but can only be applied to boards that are flat.
Copper Clad Laminate (CCL)
Underneath the soldermask is a base PCB material called copper clad laminate (CCL) that consists of two parts:
Copper Foil: A thin layer of conductive copper. Common manufacturing standards for copper foil include STD-Type E (electrodeposited), ANN-Type E (annealed electrodeposited), and the AR-Type W (as rolled-wrought).
Substrate: A non-conductive layer that provides mechanical strength and support. The substrate is typically composed of FR-4, a glass-reinforced epoxy laminate material made up of fiberglass cloth and a flame retardant epoxy resin binder. The FR stands for fire retardant, and there are many different classification standards available depending on performance, flammability, and the reinforcing material used.
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