What is Thermal Profiling in PCB Assembly

In the production and assembly of electronic products, precise control of process parameters is essential. This is done by continuous monitoring and recording of process parameters which includes thermal profiling. It involves controlling the temperatures during the component soldering on the PCB.

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Thermal profiling or temperature profiling is very important for any manufacturing process with heat treatment. It involves controlling 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 for different processes like peak, soak or slope, and reflow. This is a significant part of efficient design services that require advanced board manufacturing and assembly. Whenever a fabrication entails heat-processing, it is important to use a specific method that can ensure the product is heated to a specified temperature and duration. Maintaining the precise temperature for a particular period has a significant effect on product quality. While setting the thermal profile we should consider the following points:

• Solder paste type and manufacturer guidelines/datasheet.
• RoHS or leaded components to be used.
• Component peak temperature and withstand capacity as mentioned in the datasheet.
• Circuit board size and thickness.

Significance of thermal profiling in PCB assembly

At the beginning of the reflow process, it is necessary to apply solder paste (a combination of powdered metal solder and flux medium) on the board using a stencil (a thin sheet of material having apertures matching the footprint of a board). Later, components are placed using a pick-and-place machine.  At Sierra, a Juki pick-and-place assembly machine is used that can place 15000 parts per hour. This machine is highly reliable and accurately positions all components. After mounting the parts, the entire board assembly is passed through a reflow oven. 

A typical reflow profile consists of four stages that include preheating, thermal soak, reflow, and cooling zone. Each zone has different characteristics based on temperature requirements. It is necessary 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.

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Process window and its index of thermal profile

Process window is the essential temperature range.  A thermal profile is classified on the basis of 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 a process window index of −60%.

Explanation of the mean being 200 °C: Maximum deviation is 200-180=20 or 200-220=-20. Therefore the maximum deviation is ±20 °C. For 188 °C the deviation is 12 °C. 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, 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

1. Assures a proper wetting and formation of intermetallic compounds by ensuring the desired soldering temperature is achieved. 
2. Confirms the adhesive curing temperature is reached, thus gaining adequate adhesive strength.
3. Ensures that components and PCBs do not suffer damage due to excessive temperatures or ramp rates. 
4. Provides reliable data to optimize the process and make adjustments wherever relevant. Presently, compliance and quality assurance are critical issues.

There are two main types of thermal profiles: The ramp-soak-spike (RSS) and the ramp to spike (RTS).

Ramp-Soak-Spike (RSS) and Ramp to Spike (RTS)

A typical reflow profile consists of four stages that include preheating, thermal soak, reflow, and cooling zones. In reflow soldering, a sufficient amount of heat melts the solder and forms joints without causing any damage to components or the circuit board.

Ramp refers to the rate at which temperature changes over time. It is expressed in degrees per second. Usually, the process limit is set at 4°C/s, but some manufacturers specify 2°C/s.  In the soak segment, the solder paste approaches a phase change. Most of the flux evaporates from the solder paste. The soak time for different pastes varies, but typically it is between 60 and 120 seconds. Heat transfer at high temperatures can cause spattering and solder balling, as well as oxidation of pastes, attachment pads, and component terminations. However, too slow heat transfer can cause the fluxes to not fully activate and result in cold solder joints, voids, and incomplete reflow. 

Following the soak phase, the profile enters the ramp-to-peak phase. In this segment, the temperature and time exceed the melting point of an alloy. Cooling is the final area of this profile. Typical requirements for the cooling segment are 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 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 two dissimilar electrical conductors that form electrical junctions at differing temperatures. The change in temperature at the junction produces a temperature-dependent voltage. Thermocouple probes must have sufficient length to accommodate the profiler. Additionally, they should be able to withstand the typical oven temperature. 

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.

High-temperature solder is not the best choice for TC attachment. This is because 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 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 inaccurate readings that lead 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.

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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 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 performance and reductions in production costs.

The key elements to consider for smart profiling in PCB assembly:

• Component types
• Type of board material
• Circuit board thickness
• Number of layers
• Dimensions
• Air pressure applied

At Sierra circuits, a custom thermal profile for solder reflow is developed for each board. It is done by attaching thermocouples to the areas that will be most prone to absorb heat.

Later, the samples are sent to the reflow oven, and the temperatures at each selected area with respect to time are recorded. After uploading the data, profiling software optimizes the reflow oven for ramp-up, steady-state, and cool down for that particular job. This ensures solder joint reliability. Thermal profiling at regular intervals gives reliable information to improve your process and make necessary adjustments wherever possible.

It is crucial to follow all the specifications to avoid a poorly developed thermal profile, as it leads to the creation of voids, de-wetting, non-wetting, and solder joint fractures.

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 that 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.

Virtual profiling

The complexity and value of electronic assemblies are increasing with the long-term reliability of safety-critical products. It has resulted in the need for continuous monitoring of production processes. Thus, it is necessary to implement an automated system for monitoring the reflow oven at regular intervals. This will indicate if the process is heading out of control before it does. 

The virtual profiling process eliminates the need to physically attach thermocouples to the same production board each time. A virtual profile gathers all the typical data that is measured by instrumented profiles. It is possible to make virtual profiles automatically for both reflow and wave soldering. Initially, a recipe is needed for demonstrating purposes, but afterward, virtual profiling can be performed. The framework allows profiles to be generated at regular intervals or continuously for each assembly. 

The quality and reliability of the finished product depend upon the performance of the thermal profiling. Having a properly optimized reflow profile is one of the most critical factors for achieving quality solder joints on a printed circuit board assembly. 

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