Thermal cycling can be used to non-destructively screen PCBs for potential defects. The purpose of the following technical tips is to help with the setup of the thermal cycling chamber and its usage for non-destructive screening.

Thermal cycling chambers require an ample supply of cooling water, and it is important to follow the equipment requirements. Most thermal cycling chambers have a water chiller system that holds cooling water just above freezing, at approximately 5°C. Since these systems are typically kept in remote parts of the infrastructure, it is important that properly sized piping is kept throughout the line. For a large thermal cycler that has upwards of 33 ft³ of space, ¾" pipe is required at the input of the chamber. If the supply line is less than required, the chamber will not be kept full for optimum heat transfer. Flow meters are not used within the chamber. A pressure differential switch is used instead. The pressure differential switch monitors the pressure drop across the chamber as a voltage. For the particular chamber at the EMPF, the water pressure required to input to the system is a constant 10psi. A decrease in water flow, and hence the pressure, will cause the signal voltage to trigger an alarm and shut down the chamber. It is not recommended by the manufacturer to bypass the sensor, although this can be done by simply shorting the sensor signal lines together.

Thermal cycling chambers are typically used for 500 to 1000 cycles for product reliability testing, so both the electrical and water supply must be reliable over the duration. Breaks in the electrical supply will trigger an alarm and can cause a reset of the testing program and a stop condition unless it is specifically programmed to resume testing on revival of power.

Alarms that are triggered from various temperature and pressure sensors throughout the unit can also signal a more critical failure. One such sensor is a pressure sensor of the stage-two compressor. A low signal indicates loss of Freon coolant and requires repair from the manufacturer. It is important that the user contact the manufacturer and become familiar with the different alarms that can occur on the unit. Some of the alarms require only a simple reset to continue, while others indicate major malfunctions with the unit.

Thermal cycling is used to burn-in the PCB for potential defects that may have been present in the plated through holes. Boards are thermal cycled through a standard range from -55 to 70°C to screen for thin plating (Figure 5-1). The units have large volume, 33 ft³, and can cycle more than 150 large boards during a run.

During experimental planning and setup, it is important to consider the size of the boards and ensure that holders are appropriate and allow for proper clearance of components. The thermal cycler has forced air convection which can cause erratic movement of the boards on the board tray. It is important that the boards are positioned securely to avoid damage. The chambers typically have an internal light for easy viewing of any motion or vibration of the boards. If the boards have some margin area on the edge, placement in the board tray will allow for proper clearance for all components. A brief inspection for obvious defects, at handling stages such as loading of a chamber, is important to perform so that the root causes of any failures observed after thermal cycling are not misdiagnosed.

It is important to discriminate failures that occur from thermal cycling from those that occur from other causes. Boards should first be electrically tested to verify functionality prior to thermal cycle testing. From our experience, the EMPF has observed damage to circuit boards that occurred during shipping. A brief inspection for obvious defects at handling stages such as loading and unloading of a chamber is also important. Electrical connections within the chamber must also be correct and reliable in order to ensure that failures that occur are from thermal cycling and not from intermittent connections. These are important tips to consider so that the root causes of failures observed after thermal cycling are not misdiagnosed.

Thermal cycling is a useful method for non-destructive screening of PCBs. After thermal cycling, boards can be screened for electronic failures by contact probing. If failures are present within a via, they will indicate thin plating of the via. This can be confirmed by microsection analysis and measurement with a microscope.