A customer called into the EMPF Helpline after experiencing intermittent electrical performance in their circuit cards and were interested in design for manufacturing assistance.
The EMPF Helpline received a call from a control and telemetry electronics integrator involving PC board assembly (PCBA) failures in a radio transceiver. This PCBA module is used to control unmanned ground and air vehicles and other robotic systems. The customer had been experiencing intermittent electrical performance in their circuit cards and were interested in designfor- manufacturing assistance. The customer also requested that the EMPF review their PCB layout documentation, in order to resolve any fundamental design issues. The customer was able to obtain assistance with electronics manufacturing issues, RF design, layout, and packaging.
Some of the customer’s RF design issues and questions included the following:
- Frequency drift in a crystal oscillator
- Via placement and connection to ground layers
- Trace (width and spacing) issues
- Antena connector footprint
- Additional mounting holes for an EMI shield
- Determine necessity for impedance matching networks
- Interference between the customer’s device and an adjacent wireless transmitter
The customer’s circuitry was experiencing drifts in the fundamental frequency of an on-board crystal oscillator (which was a 4 pad surface mount package). The cause of the drift was traced to excessive heat being applied to the component during the assembly process. As it turned out, excessive heat relieved stress within the crystal, causing it to “speed” up. Excessive heat also caused unwanted material internal to the package to adhere to the crystal, thereby “slowing” it down. Ultimately, it was determined that there was no adequate way to solder this part by hand with a soldering iron. (Figure 3-1) To improve solder attachment, thermal relief connections were added to the component lands on all layers between ground pins and the surrounding ground plane. This improved some of the overheating issues that the customer was experiencing. The EMPF determined that the only acceptable method of crystal attachment was using solder paste and hot air reflow or a hot air pencil used for repair of SMT boards.
When designing RF PCBs there are many important factors to consider such as: minimization of signal loss, impedance matching, maintaining low layer counts, and dielectric control. Engineers at the EMPF performed RF simulation to verify the appropriate PCB dielectric thickness between signal layer and ground layer and to determine if the trace widths and spacings were optimized for this high frequency application. This was done to ensure that all of the connections between RF components were true 50 ohm transmission lines. If the traces are 50 ohm lines, it ensures optimal transfer of RF power between components on the circuit card. Overall, RF traces were shortened to minimize signal loss. It was also found that RF signals were being routed through a via hole. Since this would cause the RF signals to couple onto other layers, the EMPF recommended placing the antenna connector on the same side of the PCB as the RF components to eliminate the unwanted via hole connection.
To guarantee protection of the circuitry from moisture and corrosion, the EMPF assisted the customer with selection of conformal coating products. There are two types of conformal coating: translucent and opaque. Since certain conformal coatings containing urethane resin will interfere with RF signals, they were ruled out as a possible selection for this application. Encapsulating (“potting”) the boards was also ruled out as an option since the customer would have to incur expensive tooling charges to fabricate a mold.
The EMPF’s final recommendation to the customer was the use of an opaque conformal coating which also happened to be Mil-Spec approved. Based on testing and analysis of the radio module, which was performed at the EMPF, it was determined that manufacturing issues were the root cause of the customer’s problems.
Also, due to the customer following key design guidelines (listed above) and manufacturing recommendations of the EMPF, the performance of their radio transceiver module was greatly enhanced. If you have any questions regarding the diagnosis of assembly related manufacturing problems or require design assistance and verification for RF circuit assemblies, please contact the EMPF Helpline at (610) 362-1320.
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