Material Allocation

What does it take to build a circuit board? For starters you can’t build a printed circuit board without materials. What types of materials to be used are defined by the customer supplied documentation and or by the methods/release engineer. If requirements are not specified by the customer the methods/release engineer shall default to IPC standards. The various manufacturing operations used are a series of added and subtractive processes. All of which are determined by the methods/release engineer. It takes many different types of chemicals and materials to fabricate the printed circuit board. The most important material is what we start with and that’s the base copper clad laminate.

Base laminate is typically copper clad FR4. FR4 is a composite material consisting of woven fiberglass re-enforcement, epoxy resin and copper. The epoxy resin of conventional FR4 has a chemical agent made of bromine that acts as a flame retardant. If the printed circuit board catches fire, it is the bromine agent that out-gases and extinguishes the flame. The woven fiberglass, referred to as E-glass, acts as a strengthening agent similar to the iron re-bar found in a concrete foundation. The E-glass provides stability in the X (length) and Y (width) axis. The draw back to the woven fiberglass is that it does not add stability to the Z (thickness) axis. There is no weave in that direction. These factors do place limits upon what can be done to the printed circuit board at the assembly process. To understand the limits you need to understand what happens to the material when exposed to assembly temperatures.

When a printed circuit board is exposed to assembly temperatures it expands. The rate of expansion depends upon the materials within the printed circuit board. All of the materials expand when exposed to heat. However they do not expand at the same rate. The rate of expansion for materials is measured in parts per million per degrees centigrade (ppm/°C). The rate of expansion for copper is constant at 17 ppm/°C up to its melting point. E-Glass has a rate of 5 ppm/°C up to its melting point. The epoxy resin is the most unstable component and can be as high as 250 ppm/°C.

In the X and Y axis of the printed circuit board the expansion rate of the epoxy resin is restricted by the E-glass. The E-glass prevents the epoxy from expanding at such a wild rate. The restricting force applied by the E-glass adds stability to the printed circuit board. The combined rate of expansion is 16 to 17 ppm/°C. The combined rate is more or less the same as the rate of expansion of the copper. This adds a level of stability which is important for the assembly process. These factors may vary slightly from supplier to supplier.

Whereas the E-glass adds stability in the X and Y axis, there is no added stability in the Z axis. Limitations with the fabrication process prevent woven fiberglass in the Z axis direction. The rate of expansion in the Z axis is more or less at the rate of the epoxy resin. The addition of plated through holes into the design may add some resistance. However, copper is a soft metal and not known for its tensile strength. The effect that the rate of expansion in the Z axis has on the printed circuit board shall be discussed in later posts.

Single and double sided material can come in any thickness. Standard material thicknesses come in multiples of .031” (0.79mm). Standard thicknesses are .031” (0.79mm), .062” (1.57mm), .093” (2.36mm) and .124” (3.15mm). Standard starting copper weights are ½ Oz and 1 Oz copper. Two Oz copper was standard but due to the cost of copper is now considered non-standard. Any copper weight above 2 Oz is available in increments of 1 Oz. It is important to remember that non-standard items are not on a shelf waiting to be bought. Lead times and cost to manufacture non-standard materials shall vary from supplier to supplier.

Multilayer constructions may use one or several different types of of laminate in order to form the inner layers. These are referred to as cores or C-stage. Layers are typically processed in pairs. In a 4 layer board, layers 2 and 3 are usually built on the same core. Layer 2 on the top side and layer 3 on the bottom side. The thicker the core the cheaper the build. Four layer core material tends to be .028” (0.71mm) or .039” (0.99mm) thick. Thinner cores are available in .014” (0.36mm) or less. How laminate and cores are manufactured by the laminate suppliers are a topic for my next post.

From there I shall start detailing the steps required for processing the inner layers of a printed circuit board.

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