MDEV-GPS-F4 datasheet(23/25 Pages) LINX

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Microstrip Details

A transmission line is a medium whereby RF energy is transferred from one

place to another with minimal loss. This is a critical factor, especially in

high-frequency products like Linx RF modules, because the trace leading

to the module’s antenna can effectively contribute to the length of the

antenna, changing its resonant bandwidth. In order to minimize loss and

detuning, some form of transmission line between the antenna and the

module should be used, unless the antenna can be placed very close

(<1⁄8in) to the module. One common form of transmission line is a coax

cable; another is the microstrip. This term refers to a PCB trace running

over a ground plane that is designed to serve as a transmission line

between the module and the antenna. The width is based on the desired

characteristic impedance of the line, the thickness of the PCB, and the

dielectric constant of the board material. For standard 0.062" thick FR-4

board material, the trace width would be 111 mils. The correct trace width

can be calculated for other widths and materials using the information

below. Handy software for calculating microstrip lines is also available on

the Linx website, www.linxtechnologies.com.

Trace

Board

Ground plane

Example Microstrip Calculations

Dielectric Constant

Width/Height

Ratio (W/d)

Effective Dielectric

Constant

Characteristic

Impedance (Ω)

4.80

1.8

3.59

50.0

4.00

2.0

3.07

51.0

2.55

3.0

2.12

48.0

Figure 51: Microstrip Formulas

Figure 52: Example Microstrip Calculations

Production Guidelines

The module is housed in a hybrid SMD package that supports hand and

automated assembly techniques. Since the modules contain discrete

components internally, the assembly procedures are critical to ensuring

the reliable function of the modules. The following procedures should be

reviewed with and practiced by all assembly personnel.

Hand Assembly

Pads located on the bottom

of the module are the primary

mounting surface (Figure 53).

Since these pads are inaccessible

during mounting, castellations

that run up the side of the

module have been provided to

facilitate solder wicking to the

module’s underside. This allows

for very quick hand soldering

for prototyping and small volume production. If the recommended pad

guidelines have been followed, the pads will protrude slightly past the

edge of the module. Use a fine soldering tip to heat the board pad and the

castellation, then introduce solder to the pad at the module’s edge. The

solder will wick underneath the module, providing reliable attachment. Tack

one module corner first and then work around the device, taking care not

to exceed the times in Figure 54.

Automated Assembly

For high-volume assembly, the modules are generally auto-placed.

The modules have been designed to maintain compatibility with reflow

processing techniques; however, due to their hybrid nature, certain aspects

of the assembly process are far more critical than for other component

types. Following are brief discussions of the three primary areas where

caution must be observed.

Castellations

PCB Pads

Soldering Iron

Tip

Solder

Figure 53: Soldering Technique

Figure 54: Absolute Maximum Solder Times

Warning: Pay attention to the absolute maximum solder times.

Absolute Maximum Solder Times

Hand Solder Temperature: +427ºC for 10 seconds for lead-free alloys

Reflow Oven: +240°C max (see Figure 55)

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