Picking Materials For Multilayer PCBs

Multilayer printed-circuit boards become attractive for designs that require high functional density in a small space, although substrate materials should be carefully chosen.

Multilayer printed-circuit boards (PCBs) offer an opportunity to save space by stacking circuits. once associated more with digital circuit cards, multilayer PCBs are now commonly used in commercial, military, and test applications requiring a large amount of functionality in a small space. With those tightly packed circuits, however, come some stiff requirements for the circuitboard materialssuch as laminates and prepregsused in building the boards.

High-frequency circuit-board materials for single-layer or double-layer designs are usually specified according to various parameters, such as dielectric constant, dissipation factor, and thermal conductivity. For multilayer circuits, parameters related to dimensional and electrical stability are critical.

Because the impedance of transmission lines is a function of substrate dielectric constant, essential requirements for a multilayer-circuit-board material is extremely tight tolerance and consistency in the dielectric constant and in the thermal coefficient of dielectric constant (amount of change in the dielectric constant as a function of temperature). In addition, the coefficient of thermal expansion (cte), especially in the z-axis (through the thickness of the material), is of particular importance in multilayer designs where plated through holes (PTHS) are used to make connections between different circuit-board layers. the cte is a yardstick for expected ptH reliability.

A material traditionally chosen for its low dielectric constant and low loss, polytetrafluoroethylene (PTFE), is well suited for low-loss microwave circuits, but less than ideal for multilayer circuits because of its dimensional and dielectric changes with temperature. PTFE is considered a "soft" dielectric material, and can require special processing steps when drilling and preparing PTHS. although pure PTFE substrates are available from some suppliers, including Polyflon, most PTFE-based laminates are structurally reinforced by such "fillers" as woven fiberglass, glass fiber, or ceramic materials. these fillers help to control temperature-related expansion and contraction of PTFE in the x and y directions (the length and width of a panel). but as a result, PTFE-based substrates tend to have relative high cte values in the z direction (through the thickness of the material). any movement in the z axis can affect ptH reliability.

As an example, consider a laminate with an excellent z-axis cte of +50 ppm/c, and the fact that the expansion of a copper conductor and plating for the ptH is about +18 ppm/c. because the copper and the dielectric material expand and contract with temperature by different amounts, stress is placed on the copper plating in the holes drilled through the dielectric layer. using more realistic z-axis cte values for PTFEsuch as +150 or +200 ppm/cit is easy to see that the stress on PTHS increases, especially over operating temperatures.

PCBs must endure temperature cycling during various processing steps, including etching and plating, and PTHS can be subject to a variety of different stresses even before reaching a customer. because the dielectric material expands and contracts at a significantly higher rate than copper during these processing steps, the copper plating in the PTHS can suffer from metal fatigue and conductive barrels forming the PTHS can crack. reliable multilayer PCBs must handle wide temperature swings during these processing steps with minimal stress on the copper conductors and PTH plating.

To take advantage of the electrical properties of PTFE in multilayer PCBs, but also improve upon its structural integrity, materials developers experimented with their PTFE "recipes" to control z-axis movement. by adding fiberglass, glass, or ceramic to base materials such as PTFE, loss can be maintained at low levels while the structural integrity of the material can be improved.

For example, the tlc and tle substrates from the advanced dielectric division of taconic combine the electrical characteristics of PTFE and the mechanical properties of polyimide materials, making them more suitable for multilayer PCBs than traditional PTFE-based microwave materials. TLC and TCE substrates, which incorporate woven-glass filler material for structural integrity, exhibit dielectric constants of 2.95 and in the range from 2.75 to 3.20, respectively.

To assist designers working on multilayer PCBs, the firm also offers its model HT1.5 bonding material to combine layers of these substrates. HT1.5 is effectively the glue between layers, a thermoplastic film. It has a somewhat different dielectric constant that either the TLC or TCE laminates, at 2.35, and this must be taken into account when making connections (impedance matches) between circuit layers. The firm also offers its lower-cost TRF-41, TRF-43, and TRF-45 ceramic-filled substrate materials with dielectric constants of 4.1, 4.3, and 4.5, respectively, which place them in the range of FR-4 materials.

To provide the ease of FR-4 processing in a high-performance material suitable for multilayer assemblies, the Advanced Circuit Materials Division of Rogers Corporation developed their RO4000 Series microwave laminates. These thermoset materials feature low temperature coefficient of dielectric constant to minimize phase variations, as well as z-axis CTE closely matched to copper. The company offers a tutorial on selecting high-frequency laminates for single- and multilayer applications, "Considerations When Choosing High-Frequency Laminates." Written by the firm's John Coonrod, the article reviews laminate choices at different frequencies and the importance of various parameters, such as CTE, on different applications. It is available for free download from the Rogers website.

Traditionally, FR-4 and higher-performance substrate materials have been mixed in multilayer PCBs to take advantage of the low cost of epoxy-based substrates while delivering higher pefformance where needed. This "hybrid" multilayer PCB approach allows a blending of higher-cost laminates (e.g., PTFEbased materials) for critical circuits and functions, such as RF/microwave transmission lines, and lower-cost laminates (e.g., FR-4) for functions such as powersupply lines.

Multilayer PCBs typically sidestep the use of discrete resistors by incorporating embedded resistors, such as resistive foils. Known as planar or buried resistors, they are formed of thin resistive foils laminated on a dielectric layer. These resistive foils must provide consistent resistor values with temperature, and withstand the temperature extremes of typical PCB processing steps.

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