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Zulki's PCB Nuggets: Critical Aspects of a Thermal Profile
December 24, 2012 |Estimated reading time: 7 minutes
Editor's Note: This column originally appeared in the November 2012 issue of SMT Magazine.In this day and age, the technologies and know-how going into PCB assembly are dramatically escalating and, for some assemblers, it’s difficult to keep up with those advancements. Take, for example, the thermal profile, one of the most important yet constantly changing elements in the PCB assembly process. Unfortunately, there’s still a belief in our industry that one size fits all when it comes to the thermal profile. We have to remember, for the sake of our OEMs and their products, that all thermal profiles are not created equal.
However, the significance of a good thermal profile goes far beyond that fact. Many critical aspects of the thermal profile should be embraced by OEMs as a way to effectively track the reliability of their products. To start, let’s define a thermal profile. An effective thermal profile is created to make solder joints the perfect connections they need to be and to ensure they comply with associated solder paste manufacturer specifications, the types of components on the PCB, the type of PCB, and the thermal mass a particular board carries.
For each new assembly, thermocouples are attached to each PCB and then it’s run through the reflow oven to determine the hot and cold spots on the board. The temperature is measured to ensure the board is not overheated, which will damage the components, or too cold, to create perfect solder joints.
A Keen Eye on Critical Aspects
At least a half-dozen key and critical aspects are associated with creating an effective thermal profile. These include:
- Reflow oven calibration;
- The number of zones in the reflow oven;
- The oven design itself;
- The thermal profiler;
- Whether or not it is a panelized or individual board;
- Component material mixture; and
- Thermocouple attachment to components.
Top-notch calibration of the reflow oven means complying with the specifications of the reflow oven manufacturer. Compliance also includes performing regular preventive maintenance on the reflow oven and keeping a disciplined record of maintenance.
The number of heating zones in a reflow oven differs. In a small tabletop reflow oven, there can be as few as four to six zones. In a full-size oven, there can be 12 to 16. As shown in Figure 1, generally, the more zones the better to permit incremental temperature increases or temperature rise. Thus, it’s always better to have 12 zones versus six to eight because the objective is to find the coolest spot on the board versus the hottest and assure all components are properly assembled. Figure 1: Reflow oven zones.
Meanwhile, the oven design itself plays a significant role in thermal profiling. The temperature above liquidus (TAL) must be maintained at certain levels. Extra caution must be applied here to avoid ramping up the temperature too high, too quickly. A temperature cushion must be factored in to keep from damaging components and the board itself. On the other hand, if the oven is not effectively maintaining TAL, there won’t be sufficient wetting and soaking which can lead to improper assembly joints, creating open joints and intermittent connections.
As shown in Figure 2, the thermal profiler, a rectangular device about the size of TV remote control, has thermocouple wires attached to a board, monitors thermocouple temperatures, and records the PCB’s thermal profile. Assuring that this device is properly calibrated is of utmost importance. Today’s profilers offer a number of advanced features, among them accurate data acquisition, which is important to collect such crucial details as oven and zone temperatures, how the zones are being measured, and how precise the zones are. Figure 2: Thermal profiler.
Moreover, profilers provide a wealth of historical profiling and process data. That data can be extracted from the past so it can be carefully studied before making a new profile. Basically, profilers offer a library of different sets of boards with innumerable considerations as the basis for the right process for creating a brand new thermal recipe or profile.
Process window index (PWI), associated with the profiler, is another key element in the creation of a thermal profile. Whatever the process window is, the profile must fit into it. With the advent of lead-free, the process window has narrowed because temperatures are high. You can only keep those temperatures high for components for a certain amount of time. For instance, the solder paste manufacturer’s specifications might say, “Keep the temperature at 250° for no more than 20 seconds.” This means the assembler must strictly adhere to specifications and guidelines to create the correct profile or else risk damaging the component. Another factor to consider is whether it is an individual or panelized board. An individual board has different thermal mass characteristics compared to a panelized one. Panelized boards are normally smaller with air gaps in between. As a result, it behaves differently when it goes through the reflow oven versus individual boards. Therefore, you must make sure your profile is created accordingly.
You’ve also got to consider the mixture of component material in the creation of a thermal profile. Most components are plastic-, ceramic-, glass-, or tantalum-based. These different types of components come with different thermal and heating or cooling characteristics. This means your thermal profile should be aligned properly with those characteristics.
Also, on the subject of materials, another critical aspect is the method used to attach thermocouples to a component and whether it is horizontal, perpendicular, horizontal parallel, or at an angle. This aspect is generally ignored in the industry, but it plays a significant role. The different attachment methods include Kapton-based polyimide tape, aluminum, and thermal adhesive. Figure 3 shows a small PCB using Kapton-based polyimide tape as the attachment method. Figure 3: Small PCB with Kapton polymide tape attachment.
Regardless, the attach material used has thermal characteristics and coefficient of thermal expansion (CTE). This affects the type of readings taken. In some cases, adjustments are necessary based on the material used. Another intricate point is the fact that there is an air gap between the component and the attachment device. It can be aluminum, Kapton polyimide tape, or adhesive. But, in each case, the temperature at that attachment spot while the board is in the reflow oven is the air temperature, not the joint temperature, which is what needs to be measured. Attention must be placed on the joint temperature of the ball or lead of a component and the PCB’s surface and not on the air temperature.
Juggling Act
When you take into consideration these critical aspects, thermal profiling takes on a new meaning and there’s even more to this story. What it comes down to is thermal profiling is a juggling act due to multiple conflicting characteristics and data points, as well as manufacturers’ point of views that the process engineer has to contend with to develop the perfect recipe.
For example, you’ve got the solder paste manufacturer, who carefully controls the ramp and pre-heat cycle. He also wants the TAL as high as possible. But, you’ve got the component manufacturer who wants the TAL to peak as low as possible to prevent component damage. We’re not done yet. Here comes the oven manufacturer into the mix. He wants to make sure his oven can make a repeatable and quality profile time after time and his contributions, in some instances, may run against the grain of the other two guys.
The process engineer creating the thermal profile is caught right in the middle. In effect, like the old saying goes, he or she is “between a rock and a hard place,” and is definitely forced to perform a juggling act by having to make intelligent trade-offs if necessary.
It Comes Back To Experience
Experienced process engineers are well equipped to deal with this rapidly-evolving assembly step simply because they’ve successfully handled changes related to creating a thermal profile over the last 10 years. But when you bring an inexperienced process engineer into this situation, it’s a different story.
They’re not quite ready to create a quality thermal recipe that’s repeatable and without defects. Inexperience brings about such common defects as tombstoning (Figure 4), delineations, or tilting of components; plus, at times, there’s not enough heat, resulting in intermittent connections or head-on-pillow for BGA components, for example. In addition, CSP and LGA devices have their own issues. Figure 4: An example of tombstoning.
If the process engineer isn’t sufficiently experienced, he or she can create an inefficient profile. This means lots of back and forth QC and a lot of rework. It costs time and money and sometimes there’s damage to components. Here, we’re talking about the probability of damaging a high-ticket component costing $200 to $500 and, in some cases, a few thousand dollars. Zulki Khan is the founder and president of NexLogic Technologies, Inc., in San Jose, California, an ISO 9001:2008-certified company, ISO 13485-certified for manufacturing medical devices and a RoHS-compliant EMS provider. Prior to NexLogic, Khan was general manager for Imagineering, Inc. in Schaumburg, Illinois. He has also worked on high-speed PCB designs with signal integrity analysis. He holds a B.S. in EE from NED University in Karachi, Pakistan, and an M.B.A. from the University of Iowa. He is a frequent author of contributed articles to EMS industry publications.