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August 2014 • The PCB Design Magazine 15 properties degrade rapidly—strength and bonds in the material. A high T g guards against barrel cracking and pad fracture during reflow. Stan- dard FR-4 has a T g of 135–170°C, whereas the high-speed materials are generally over 200°C. There a basically two types of dielectric ma- terial: 1. Woven fiberglass reinforced dielectric 2. Fiberglass free dielectric At high frequencies, a non-uniform dielec- tric in the substrate can cause skew in differen- tial signals. The inconsistency of the dielectric material comes from the fact that the fiberglass and the epoxy resin, that make up typical PCB core (laminate) and prepreg materials, have a different dielectric constant. And because the fabricator cannot guarantee the placement of the fiberglass with respect to the location of the traces, this results is uncontrolled differential skew. A fiberglass-free material can be used to eliminate differential skew. However, fiberglass- free materials come at a price. So for a cost-ef- fective solution, let's eliminate the fiberglass- free dielectric. Close attention should also be paid to the skew associated with the fiber weave effect. For high-speed data rates of 5 Gbps and above, this skew significantly cuts into the available jitter unit interval (UI) budget and leads to a reduc- tion in the observed eye width at the receiver. If the flexibility exists, specify a denser weave material (2113, 2116, 1652 or 7628) compared to a sparse weave (106 and 1080). Figure 2 com- pares the different types of fiberglass weaves to a 4/4 mil differential pair. Notice that one side of the pair can be routed over the fiberglass and the other over the gap (resin), depending on the placement. The different dielectric constants create skew. However, routing the differential signals diagonally across the weave can reduce this skew considerably. Typically, when the impedance of a substrate it first calculated, "virtual materials" are used as the basis. In other words, we choose a round number to represent the dielectric constant, di- electric thickness, and the attributes of the trace thickness and width to establish a solution. However, these are not the attributes, of the ac- tual materials, used by the fab shop to manu- facture the board and are inherently inaccurate. I'm not saying that the use of virtual materials should be avoided but rather, the numbers need to be in the ball park to begin with. In order to select the correct dielectric mate- rials and variables for your substrate, you need to consider the following: 1. Dielectric loss needs to be low. 2. Dielectric constant needs to be low. 3. Glass transition temperature needs to be high (= >180 o C). 4. Dielectric thickness needs to be low. MATERIAL SELECTIoN FoR DIGITAL DESIGN continues Figure 2: Different types of fiberglass weaves compared to a differential pair. (source: Altera) beyond design