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AD8155 Datasheet(PDF) 31 Page - Analog Devices |
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AD8155 Datasheet(HTML) 31 Page - Analog Devices |
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31 / 36 page 
AD8155 Rev. 0 | Page 31 of 36 PRINTED CIRCUIT BOARD (PCB) LAYOUT GUIDELINES The high speed differential inputs and outputs should be routed with 100 Ω controlled impedance differential transmission lines. The transmission lines, either microstrip or stripline, should be referenced to a solid low impedance reference plane. An example of a PCB cross-section is shown in Figure 48. The trace width (W), differential spacing (S), height above reference plane (H), and dielectric constant of the PCB material determine the characteristic impedance. Adjacent channels should be kept apart by a distance greater than 3 W to minimize crosstalk. PCB DIELECTRIC SIGNAL (MICROSTRIP) SOLDERMASK PCB DIELECTRIC PCB DIELECTRIC PCB DIELECTRIC REFERENCE PLANE REFERENCE PLANE SIGNAL (STRIPLINE) W S W H W S W Figure 48. Example of a PCB Cross-Section Thermal Paddle Design The LFCSP is designed with an exposed thermal paddle to conduct heat away from the package and into the PCB. By incorporating thermal vias into the PCB thermal paddle, heat is dissipated more effectively into the inner metal layers of the PCB. To ensure device performance at elevated temperatures, it is important to have a sufficient number of thermal vias incorporated into the design. An insufficient number of thermal vias results in a θJA value larger than specified in Table 1. Additional PCB footprint and assembly guidelines are described in the AN-772 Application Note, A Design and Manufacturing Guide for the Lead Frame Chip Scale Package (LFCSP). It is recommended that a via array of 4 × 4 or 5 × 5 with a diameter of 0.3 mm to 0.33 mm be used to set a pitch between 1.0 mm and 1.2 mm. A representative of these arrays is shown in Figure 49. THERMAL VIA THERMAL PADDLE Figure 49. PCB Thermal Paddle and Via Stencil Design for the Thermal Paddle To effectively remove heat from the package and to enhance electrical performance, the thermal paddle must be soldered (bonded) to the PCB thermal paddle, preferably with minimum voids. However, eliminating voids may not be possible because of the presence of thermal vias and the large size of the thermal paddle for larger size packages. Also, outgassing during the reflow process may cause defects (splatter, solder balling) if the solder paste coverage is too big. It is recommended that smaller multiple openings in the stencil be used instead of one big opening for printing solder paste on the thermal paddle region. This typically results in 50% to 80% solder paste coverage. Figure 50 shows how to achieve these levels of coverage. Voids within solder joints under the exposed paddle can have an adverse affect on high speed and RF applications, as well as on thermal performance. Because the LFCSP package incor- porates a large center paddle, controlling solder voiding within this region can be difficult. Voids within this ground plane can increase the current path of the circuit. The maximum size for a void should be less than via pitch within the plane. This assures that any one via is not rendered ineffectual when any void increases the current path beyond the distance to the next available via. |
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