In the production and assembly of electronic products, precise control of process parameters is essential. This is done by continuous monitoring and record of process parameters which includes thermal profiling. Thermal profiling involves controlling the temperatures during the component soldering on the PCB.
This article will cover:
- Thermal Profiling
- Significance of Thermal Profiling in PCB assembly
- Process Window and Process Window Index
- Ramp-Soak-Spike (RSS) And Ramp to Spike (RTS)
- Thermocouples (TC) In Thermal Profile
- Smart Profiling: The Best Practices for A Precise Thermal Profiling
- Virtual Profiling
Thermal profiling or temperature profiling is very important for any manufacturing process with heat treatment. It involves plotting the temperature at numerous points on a given product as it passes through a thermal process. Typically, a thermal profile is a complex set of time-temperature data. Therefore, the thermal profile is all about controlling all the dimensions like peak, soak or slope, etc. affecting the process. This is a significant part of efficient PCB design services that require advanced PCB manufacturing and assembly equipment.
Significance of thermal profiling in PCB assembly
The process of components assembly on the PCB involves applying soft solder paste on the board and then precisely placing the components on the PCB using a CNC machine. To firmly connect the components to the PCB, you need to melt the solder paste slowly and then slowly cooled down. You can do this by placing the PCB with components on paste on a conveyor which passes through an oven. This oven will have different zones of temperature starting with the low temperature, slowly increasing to high temperature where the solder paste melts. Then you bring down the temperature slowly to ambient temperature – where solder solidifies and holds the components to the PCB. You need to control the temperature in the oven precisely to avoid thermal shocks to the components.
This control of temperature in an oven is called thermal profiling.
What Is Exactly Thermal Profiling?
Before we start defining the process, we need to understand the basic principle behind thermal profiling. It involves a simple question, “how hot and for how long?” Whenever a fabrication entails a heat-processing step, you would need a method that can ensure the product is heated to a specified temperature for a specified period. Attaining the precise temperature and for the precise time should significantly affect the quality of the product.
Process Window and Process Window Index of Thermal Profile
Typically, the essential temperature is defined as a range. This range is called the “process window”. It is of utmost importance that this process window is distinct. A thermal profile can be classified on “how it fits in a process window”.
The center of the process window is usually declared as zero, and the extreme edges as ±99%. Any process window index (PWI) which is greater than or equal to 100% is as obvious out of the window. Raw temperature values are normalized in terms of a percentage comparative to both the process mean as well as the window parameters. For example, if the process ‘mean’ is set at 200 °C with the process window standardized at 180 °C and 220 °C respectively, then a measured value of 188 °C implies to a process window index of −60%.
Explanation of the mean being 200 deg : Maximum deviation is 200-180=20 or 200-220=-20. Therefore the maximum deviation is +/- 20 deg. For 188 deg the deviation is 12 deg. As a percentage of maximum deviation, this works out to be 12/20= 0.6 or 60%.
It tells you the exact range of your process window the given profile uses, and thus how robust your profile is. The PWI also assists you to find the single best profile that your process is capable of attaining.
Why should you know about thermal profile?
If this thermal profile is that critical then why don’t all facilities have it defined? The oven or furnace has also been set up by a technician from the oven manufacturer. Also, the furnace is running without any form of process control provided there is a problem, which would require troubleshooting.
It is good to know what happens to the board inside your reflow oven or furnace. Thermal profiling at regular intervals provides reliable data to optimize your process and make adjustments where relevant. That being said, compliance and quality assurance are critical issues presently. Exact compliance with the manufacturing specifications is not yet a prerequisite, but it is most likely to become a key selling point for your product.
One of the major uses of this method is the soldering of electronic assemblies. There are two main types of profiles in use these days: The ramp-soak-spike (RSS) and the ramp to spike (RTS). These profiles are mapped using data from the ramp soak temperature controller or the ramp soak PID controller.
Ramp-Soak-Spike (RSS) and Ramp to Spike (RTS)
We know already about the soak segment of reflow soldering. In that segment of the profile, the solder paste kind of approaches a phase change. Most of the flux evaporates from the solder paste in this stage. Different pastes have different soak durations, but typically it is a 60 to 120-second exposure. The duration also considers the mass of the given PCB. Too high a temperature or a rapid heat transfer can lead to solder spattering or balling even oxidation of the paste, the attachment pads, and the component terminations. Similarly, too slow, heat transfer may not fully activate the fluxes and hence result in cold solder joints, voids, and incomplete reflow.
Many components have a specification where the rise in temperature should not exceed a specified temperature per second, for example, 2 °C/s. The ramp is the rate of change in temperature over time, expressed in degrees per second.
The most commonly used process limit is 4 °C/s, though many components and solder paste manufacturers specify the value as 2 °C/s. After the soak stage, the profile enters the ramp-to-peak segment, it is a given temperature range and time exceeding the melting temperature of the alloy.
The next and final area of this profile is the cooling section. A typical requirement for the cooling segment is less than −6 °C/s.
While the ramp-soak-spike permits for about 4 °C/s, the specifications for the ramp to spike is about 1–2 °C/s. Graphically, the RTS profile is almost linear, starting from the beginning of the process until the peak segment. The cooling stage has a greater change in temperature. Again, these values depend on what solder paste you use. The Soak region of RTS is a part of the ramp and is not as distinguishable as in RSS.
Thermocouples (TC) in thermal profile
Since the process has been defined and the process limits have been set, we need instrumentation to measure the heat. We use a thermocouple for this purpose. A thermocouple is a sensor that measures temperature. It is an electrical device that comprises of two dissimilar electrical conductors that form electrical junctions at differing temperatures. This change in temperature change at the junction produces a temperature-dependent voltage. Then this voltage can be interpreted to measure temperature. Therefore we use these thermocouples to measure the thermal profile.
Thermocouple attachment methods include using epoxy, high-temperature solder, aluminum tape, Kapton tape, etc. Epoxies are a very common method of attaching the TCs to the profiler. It operates in a wide range of temperature tolerances. You also need to note that epoxies come in both insulator and conductor formulations. The specs of the process must be considered before using any epoxy, otherwise, it might have a negative impact on the data collection. Also, the properties and specifications of the epoxy should be considered. Again, one main problem with epoxy attachment is its adherence quantity changes from place to place. Hence, it decreases reproducibility.
High-temperature solder is not the best choice for TC attachment. One of the reasons for this is the same as that of the epoxy. The amount of solder required to adhere to the TC to a substrate varies from location to location. Also, the solder is conductive hence can short-circuit the TCs.
Kapton tape is a widely used adhesive for the attachment of TCs and the substrates. The main disadvantage of Kapton tape is that at temperatures above 200 °C, the tape shows the properties of an elastic. Hence, the TCs tend to lift the substrate surface. The result is erroneous readings leading to the plotting of jagged lines in the profile.
As we know, no process can be fool-proof. Hence, you need to consider both your substrate as well as the method for optimum results. Each of them yields varying success for different methods.
Smart Profiling: The best practices for a precise thermal profiling
There is a range of factors affecting the accuracy of your profile. The most common of them is an improper thermocouple attachment. It can cause noise, i.e., zigzagging or erratic thermocouple reading on a profile graph. For precise charting, you need to attach thermocouples to areas that vary in terms of mass, location, and known trouble spots. Good thermocouple attachment involves solid contact with the surface of the product sans any air current.
Simulating output as closely as possible during profiling is useful. For this, insert the board into the oven, in the same manner, as you would during production. It is best practice to profile the product with a fully loaded oven, you should duly note that the characteristics of an oven usually vary significantly between loaded and unloaded. You must allow the product to cool down to the room temperature between the profile runs. Otherwise, it will most likely cause some exponential errors.
Utilizing state of the art thermal process setup and monitoring apparatus leads to significant improvements in the performance and reductions in production costs.
The key elements to consider for smart profiling in PCB assembly:
- Component types
- Type of PCB material
- PCB thickness
- Number of PCB layers
- PCB dimensions
- Air pressure applied
For example, a 93-mil thick board will not have the same thermal characteristics as a 62 or 125-mil thick board. The same applies to the stack-up. The number of power and ground planes in the stack-up determines the amount of heat the board requires to absorb in any given cycle to create a perfect thermal reflow.
A largely SMT populated board will require more heat than a board which is mostly populated with through-hole components and a few surface mounts. Also, you must monitor the air pressure in a reflow oven with enough caution to avoid blowing away smaller packaged devices. If the specifications related to thermal profiling aren’t followed to the letter, a poorly developed thermal profile can result in the creation of voids and de-wetting if too hot and if too cold, non-wetting, and solder joint fractures.
There is a noted increase in electronic assembly complexity and values, combined with long term reliability of safety-critical products. This has resulted in the need for continuous monitoring of production processes has become commonplace. In turn, this calls for an automated management system that can monitor the reflow oven at regular intervals. This will indicate if the process is drifting out of control before it actually does so. Hence, you verify the profile of each and every board.
Setting up instrumentation for PCB manufacturing for every new profile is time-consuming which is not desirable. It is a method of creating profiles without attaching the thermocouples or having to physically instrument a PCB each and every time a profile is run for the same production board. All the typical profile data that are measured by instrumented profiles are gathered by using virtual profiles. Virtual profiles are made automatically, for both reflow or wave solder machines. An initial recipe is required for demonstrating purposes, however once done, profiling can be made virtual. As the framework is programmed, profiles can be produced at regular intervals or constantly for every single assembly.
Your oven, the furnace is the heart of your business. The quality and reliability of your finished product depend upon the performance of your thermal profiling. You know a lot about every other manufacturing step in your operation, so why not about thermal profiling? Understanding what is happening to your product as it travels through an oven or furnace is the first step to controlling and optimizing that heat-treatment process. Thermal profiling is a critical tool used by thousands to help obtain that understanding.
Soldering is a complex process, and temperature profiling plays a major role in the soldering process. While setting the thermal profile we should consider the following points:
- Solder paste type and solder paste manufacturer guidelines/datasheet.
- RoHS components or leaded components to be used
- Component peak temperature and withstand capacity as mentioned in the Components Data Sheet.
- PCB Size & Thickness.
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