Basic materials and boards build-up‘s


Selecting material that meets performance characteristics and minimizes manufacturability issues such as bow, twist, and mis-registration is very critical. Raw laminate is a major cost component in a multilayer board. To optimize cost, laminate construction around standard base materials is used to achieve maximum material utilization based on the usable area available. When specifying dielectric thickness, required for impedance reasons, the designer should know the construction.

Here are some outlines of material parameters that affect circuit performance and give some direction about how to decide when to choose the correct material for any specific application.

Material selection will play a role in trace width control and a key role in dielectric thickness control and PCB manufacturer is also a major contributor to proper control of both, especially in high layer count boards. Each PCB design is taken with serious consideration in terms of choice of the most suitable base materials.

There are three main components that make up the PCB.

One is copper foil. Sheets of copper foil are incorporated into the outer layer of the PCB placing it on the prepreg to create the outer layers.

Second is core, commonly called C-stage or laminate. This is fully cured glass-epoxy material with copper laminated to both sides. This is used for internal layers. Prepreg is used to separate multiple layers. This is commonly called B-stage or bonding sheet. This consists of glass and uncured resin that is able to flow when heated. There is no copper attached to this material. It is used to provide isolation and thickness between core layers and outer layer foil layers and to flow around the copper features on the adjacent . The new trend toward environmentally conscious manufacturing (ECM) to utilize designs and processes that are less wasteful often limits the choice of design engineers for selecting material. However, there are certain practices that help the manufacturers and do not impact cost. One is the choice of solder mask that affects the amount and toxicity of the solvent used and emitted. The choice of anti-tarnish instead of hot air solder leveling (HASL) reduces the use of lead and emission from flux and fusing oil. Another is the use of the lightest copper weight, 0.5 oz, which results in the least use of chemicals and generation of waste byproducts.

Material Choices- Materials are basically divided into two groups, those designed for digital circuits and those designed for analog circuits. Based on operating parameters, the major differences between the materials are:

1) Analog materials generally have a much lower Er that’s more stable over frequency and changing temperature.

2) Analog materials generally have tighter tolerance on dielectric thickness.

3) Analog materials are often offered with Rolled or ED copper. Digital materials come with ED copper only.

4) Only a few analog materials are designed for multilayer applications. All digital materials are designed for multilayer applications.

5) Loss tangent varies a great deal across the material choices, but is generally much lower in analog materials, often by a factor of 10 or more.

Some of the materials aimed at the digital arena could be used in an analog circuit and in many cases that might be a good use of resources. In fact many of the materials now considered to be high end digital materials were developed for the commercial end of the high frequency analog arena. Looking through the specs of these materials it will be apparent which are well suited to analog applications, even at very high frequencies.



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