Thermal Profiling: The PCB Assembly Unsung Hero
A few days back, I came across a really cool quote about data. “In God we trust. All others must bring data.” – W. Edwards Deming, statistician. Since then, I have been wondering what could be the best way to share, incorporate and explain this. What’s better than our today’s topic thermal profiling?
Thermal profiling or temperature profiling is very important for any manufacturing process with heat treatment. It is 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. But prior to gaining control of any process, it must be defined.
What Is Exactly Thermal Profiling?
Before I start defining the process, what is the basic thing about thermal profiling? According to me, it’s a simple question, “how hot and for how long?” Whenever a fabrication entails a heat-processing step, you would need something, some 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. No matter if it is the correct cure for incinerated wastes or are the physical properties of a heat-treated PCB part. This is called thermal profiling.
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%. 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?
But if this thermal profile is that critical then why don’t all facilities have it defined? Your oven or furnace has also been set up by a technician from the oven manufacturer. Since then the product has been running successfully. Also, the furnace is running without any form of process control provided there is a problem.
Alright, that’s true. But do you really understand the situation inside your oven or furnace? Your production is going well? Perfect! But can you optimize it? What if it is not going well, could you immediately take remedial action? Do you even have any relevant data to avoid complications? Wouldn’t you like to? 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).
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 out of the solder paste in this stage. Different pastes have different soak durations, but usually, it is typically a 60 to 120-second exposure. The duration also considers the mass of the given PCB. Too high a temperature or an over-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. Soak region of RTS is a part of the ramp and is not as distinguishable as in RSS.
Thermocouples (TC) In Thermal Profile
Already the process has been defined and process limits set, so now? No, I cannot read your mind from this side of the connection, it’s just instinct. But the heat cannot be measured by instinct hence we need something that can.
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. And yes, we use these thermocouples to measure the thermal profile.
Various attachment methods are used to attach the thermocouples: 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. But mind you 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 also 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 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 properties of an elastic. Hence, the TCs tend to lift up 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 full-proof. Hence, you need to consider both your substrate as well as the method for optimum results. Each of them yields different 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, thermocouples are attached to areas that are unlike in terms of mass, location and known trouble spots. Good thermocouple attachment is a solid contact with the surface of the product sans any air current. Another point that you should note and remember is the wearing of the thermocouples.
Simulating output as closely as possible during profiling is useful. Insert the board into the oven, in the same manner, as you expect it during production. Preferably profile the product with a fully loaded oven, you should duly note that 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
As for example, a 93-mil thick board will not have the same thermal characteristics when compared to 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.
Again, 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, air pressure in a reflow oven must be monitored 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.
With increasing electronic assembly complexity and values, combined with long term reliability or safety-critical products, the need for continuous monitoring of production processes has become commonplace. The current emphasis is on a process that builds in rather than inspecting in quality. This calls for an automated management system that can vigorously 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.
Now, imagine instrumenting your PCBs before every new profile. Tough, right? Hence, virtual profiling. 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.
So, what are you waiting for? Start your haul for an optimized and controlled production!
DOWNLOAD OUR DFM HANDBOOK:pcb assembly, pcb design, PCB Manufacturing