PCB assemblers are becoming more aware of the importance of process monitoring and as such, there is an increase in the thermal profiling’s frequency. In the surface mount technology industry, soldering of thermocouples using high temperature is common. This soldering method is a reliable means to attach PCB thermocouples for thermal profiling.
However, soldering thermocouples with the use of high temperature is a time consuming process which damages the circuit board. Due to the limitations of this method, there is an alternative to thermocouple attachment; particularly those that don’t need any sacrificial circuit board can reduce the stress associated with thermal profiling. You need to get familiar with how thermocouples work.
What is a PCB Thermocouple?
A PCB thermocouple is a sensor designed for measuring temperature. This element is made of two types of metals which are joined at one end. There is voltage when the joint part of the two metals is cooled or heated. A thermocouple is usually used in a PCB as it is a cost-effective and simple way to contribute to a board’s performance.
There is a current flow in the thermoelectric circuit when two wires that comprise dissimilar metals are welded at both ends and heated. Thermocouples have their usefulness in circuit boards. This temperature sensor has a wide range of detecting temperatures. Therefore, they are ideal for use in circuit boards.
Thermocouples come in different metal combinations. The “Base Metal” thermocouples are the most common. There must be a direct thermal contact between the thermocouple junction and the PCB surface to achieve reliable data from a thermocouple PCB. Thermocouples function in a different way. They function in a different way from other temperature sensors as they generate voltage from two different metals welded together.
Therefore, this helps in measuring from zero to more than the melting point of steel and iron. Also, thermocouples don’t easily break like other sensors. Although thermocouples can’t offer as much accuracy as a resistance temperature detector, they are accurate enough for printed circuit boards.
Ways to Attach a Thermocouple to PCB and MOSFETS
There are several ways of attaching a thermocouple to a circuit board and MOSFET. These methods include high-temperature solder, aluminum tape, epoxy, and polyimide Kapton tape. MOSFET denotes metal oxide semiconductor field effect transistor. This transistor serves switching purposes.
High temperature soldering
High temperature soldering is the most reliable way of attaching thermocouples to PCB. When it is well performed, it forms a Golden Board which can be integrated between five and thirty profiles. Also, high temperature soldering can take time and also requires a high temperature soldering iron. When this method is being used, it can be very hard to achieve an excellent thermally conductive solder joint.
Unsoldering and soldering the thermocouple joint can cause damage to the circuit board. If such a circuit board is being integrated for multiple runs, you will have to scrape it.
Aluminum tape
This is another ideal method for attaching a thermocouple to a circuit board. The combination of Aluminum tape with Kapton tap offers excellent thermal conductivity and a secure connection. This process involves cutting the aluminum tape with scissors to a size of about 1/8 x 1/8 inch. Also, it can attach the thermocouple to the PCB firmly. After which you need to overlay the aluminum tape connection using a ¼ piece of Kapton tape to avoid any form of lifting.
It is very easy to apply the aluminum tape and as well remove it. This tape doesn’t damage your circuit board and leaves no residuals or dents after it has been removed.
Epoxy
Epoxy is a common method of attaching a thermocouple to a circuit board. There is a popular adhesive system used for this. This adhesive system comprises two parts which are accelerator and instant adhesive. The PCB manufacturer drops the instant adhesive over the thermocouple after which he sprays the accelerator over the adhesive. After a few seconds, the wire is attached firmly.
Kapton tape
This is an electrical tape that features excellent thermal conductivity. You need to use scissors to cut this tape to about 1/8 x 1/8 inch. This tape will attach the thermocouple to the circuit board firmly. However, Kapton tape can get loose when it gets to an extremely high temperature. Therefore, attaching the thermocouple to the PCB surface can be quite challenging.
How Reliable and Accurate are these Alternative Methods
Asides from high temperature soldering, there are alternative methods of attaching thermocouple to PCBs. These methods are reliable and accurate. However, it is important to have an idea of each method’s level of accuracy and reliability. The aluminum tape performs well when combined with Kapton Tape. Also, this tape is non-destructive.
The Kapton tape is the least expensive way of attaching thermocouple to circuit boards. This method isn’t only cost-effective, it is simple. During profiling, all thermocouples are attached using Kapton tape lifts. Therefore, this method of attaching thermocouple to a circuit board is unreliable.
Well, another inexpensive method of fixing thermocouple is the conductive epoxy. This method is suitable in cases where tape can’t be used in tight locations. However, conductive epoxy can break down after a profiling run.
When attaching thermocouples to circuit boards and MOSFETs, you don’t need to consider thermal conductivity. This is because thermocouples aren’t designed to act as heat sinks.
Importance of Thermocouple PCB
Thermocouple PCBs are very crucial in electronic devices. The integration of thermocouple in a circuit board helps to ensure proper functioning. It is crucial to monitor temperature during the process of SMT reflow, particularly for LEAD-free processes. This is because the components can experience thermal shock easily. You can attach a thermocouple to a printed circuit board in order to achieve thermal-time data.
Therefore, to achieve reliable data from a thermocouple, ensure the thermocouple junction has a direct thermal contact with the circuit board surface. If this isn’t done, the thermocouple will measure only hot air temperature where there is about 100C difference with the actual temperature of the board.
Thermocouple PCBs have found their use in several applications. Although thermocouples are very easy to integrate, they need a particular amount of observation and experimentation. Thermocouples are of great importance in space exploration since they produce electricity from temperature.
What are the Types of Thermocouple?
There are different types of thermocouples. This includes K type, E type, S type, B type, M type, J type, E type, D type, N type, G type. The nickel-alloy category are the K, N, E, T, M, and J types. However, the S, B, and R types are Noble Metal thermocouples.
The last category includes the G, D, and C types. This category comprises rhenium/tungsten alloys. However, the K-type thermocouple is the most common type.
How to Choose a Thermocouple
Since thermocouple are available in different forms and shapes, it is crucial to have a good understanding of how to choose the right sensor. The temperature range is the best criterion for choosing a thermocouple. However, there are other criteria include vibration and abrasion resistance, chemical resistance, and a temperature range.
There are various kinds of thermocouple. However, all kinds function according to the Seeback effect. This explains how a conductor’s electric potential builds up when a conductor goes through a temperature gradient from the end of a conductor to another.
Another criterion for selecting thermocouple is installation requirement. This will determine the type of thermocouple to choose. Thermocouples are available in different types and as such their applications vary. When there is a need for high response times, you will need an exposed thermocouple. However, in corrosive environments, an ungrounded thermocouple is a better choice.
Asides from the performance of a thermocouple, there are other factors you should put into consideration to ensure a high level of accuracy. Furthermore, you need to make use of the biggest wire diameter for the thermocouple.
More Facts about Thermocouple PCB
Thermocouple PCBs have found their use in several applications. Although thermocouples are very easy to integrate, they need a particular amount of observation and experimentation. Thermocouples are of great importance in space exploration since they produce electricity from temperature. Also, thermocouple PCBs are ideal for use in home appliances, automotive systems, combustion analysis, and aircraft engines.
High temperature soldering is the most effective and best way of attaching thermocouples to PCB. When it is well performed, it forms a Golden Board which can be integrated between five and thirty profiles. Also, high temperature soldering can take time and also requires a high temperature soldering iron. When this method is being used, it can be very hard to achieve an excellent thermally conductive solder joint.
Conclusion
There are different methods of attaching thermocouples to circuit boards. The best method is the high temperature solder. However, other methods proved to be useful. The repeatability and reliability of aluminum tape made it a great choice in some applications. The integration of aluminum tapes is a non-destructive and inexpensive way of attaching thermocouples to circuit boards. These aluminum tapes minimize the effort and expense needed to achieve an accurate thermal profile.
