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PCBD-June2015

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24 The PCB Design Magazine • June 2015 cores. The copper thickness is typically 1/3 to 2 oz. (17 to 70 µm). The prepreg (B-stage) material is comprised of thin sheets of fiberglass impregnated with uncured epoxy resin which hardens, when heated and pressed, during the PCB fabrica- tion process. Isola's FR406 materials include 1.7, 2.3, 3.9 and 7.1 mil prepregs that may be combined to achieve thicker prepreg. The most common stackup is called the foil method. This features prepreg with copper foils bonded to the exterior on the outermost layers (top and bottom). Core then alternates with prepreg throughout the substrate. An alternate stackup is known as the capped method, which is the opposite of the foil method and was used by old-school military contractors. FR-4 has industry approvals of IPC-4101B and is Underwriter Laboratories (UL) rec- ognized for product safety. FR-4 has a glass transition temperature (Tg) of 170°C (the temperature at which the resin begins to flow and the substrate changes to a viscous state) and a decomposition temperature (Td) of 294°C (the temperature at which the sub- strate breaks down or decomposes). The peak reflow temperature for lead-free solder is 260°C, which is only held for 20 seconds, to reflow solder the surface mount components to the substrate. RF-4 can be used for designs up to 1 GHz. The Rogers materials (RO4350 & RO4003) are another common dielectric that can with- stand higher temperatures (Tg >280°C and Td = 425°C), and they are ideal for high-speed de- signs up to 10 GHz. But this is somewhat more expensive than FR-4. The total substrate thickness is general- ly 62 mil (1.6 mm), but may vary according to the application: 20, 31, 40, 47, 62, 93 and 125 mil are a few other not-so-typical thick- nesses. Backplanes, for instance, will typically use the thicker substrate to ensure mechanical support. One of the steps of the PCB fabrication process is lamination. Core materials are pinned together in a lamination book with sheets of prepreg separating copper layers. The prepreg basically glues the core materi- als together. Outer layers are made of a foil of copper, which is etched last in the process, so the outer layers of prepreg act as cured core. Horizontal alignment is critical. The stack is pinned between two heavy metal plates and put in a heated hydraulic press for about two hours, until cured. In Figure 3, the left stackup has a total thickness of 9 mil. However, when the board is cured the resin in the prepreg (green) flows around the signal traces below (as in the right diagram). This envelopes the trace completely, and also thins the prepreg material. As the sig- nal trace becomes closer to the above plane, the impedance drops. So, here are a few effects of the prepreg be- ing cured: a) The total board thickness reduces, by the thickness of the signal layer copper, as the trace is totally enveloped in resin from the pre- preg. The resin also flows into the antipads of the nearby planes and oozes from the sides of the lamination book. STACkuP PLANNING, PART 1 continues beyond design Figure 2: The PCB lamination process.

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