Thick Copper PCB SMT PCB Assembly

A Guide to PCB Material Selection

Published Date August 30, 2018 Author OrCAD PCB Solutions

CC BY 2.0 by David Lenker

So you want to build your own PCB (Printed Circuit Board)? A material selection guide is as good a place as any to start. Your 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 your electrical components, conductive traces, pads, and other features will reside. In this article we’ll dive into the materials and design considerations that go into each layer of a PCB: silkscreen, soldermask, copper, and substrate.

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.

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.

When people talk about a two-layer or double sided board, they are referring to a CCL with copper foil attached to both the top and bottom of the substrate. Standard multilayer boards are typically made by stacking two or more double-sided boards together with insulating layers in between. As a result, your standard multilayer boards are sold as even numbers. More layers give you more ground planes to help with power distribution and noise. The only real drawback to adding layers is cost.

The Future of PCB Materials

While minor changes in the ink used for silkscreen or the materials in your soldermas aren’t likely to make any headlines, the bulk of innovation effort seems to be focused on improving the CCL.

• The need for smaller, lighter boards for applications such as mobile devices has increased the demand for multilayer architectures which can pack more functionality into a smaller form factor.
• Enhanced epoxy systems such as Getek, Megtron, 4000-13 and FR-408, which cite better performance than FR-4 with reduced Dk values and loss tangents.
• High performance materials such as A-PPE by Asahi Glass, Nelco 600-21 Si, and Rogers 4350, which offer significant improvements in lower Dk, better impedance control, and reduced jitter.
• Flexible CCLs made of PI or polyester.
• Stricter RoHS (Restriction of Hazardous Substances) regulations have encouraged higher heat resistance and reliability. Halogen-free CCLs limit chlorine and bromine content to within 900 ppm. Lead-free CCLs remove lead by swapping the DICY curing system of standard FR-4’s with a PN curing system using phenol-formaldehyde resin curing agent.

In this article, we took a look at the materials and design considerations that go into each layer of a PCB, before finishing off with a glimpse of the future of PCB materials. Eager to put theory into practice? Click to learn how our suite of PCB design and analysis tools can help you get started on your next project.

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