During my 15-year career in the electronics and fiber-optic industries, I've noted that the debate over "who the designer is" and "who the engineer is" has a different answer for each distinct industry. What ultimately matters is a product's performance. In the electronics industry, there are electrical engineers, mechanical engineers and designers, but the designer generally does the PCB layout. There also are mechanical designers who do a great deal of the mechanical packaging. However, when the word "designer" is spoken, it usually refers to the PCB layout person.
While our mechanical group focuses on thermal analysis and our electrical computer-aided design (ECAD) designers concentrate exclusively on the PCB layout, the truth is that both groups need to understand thermal issues and constraints. Why? We design and build PCBs and card cage assemblies for our optical networking equipment, and both the electrical and mechanical engineers are equally responsible for the performance of a PCB. The electrical engineer works closely with the designer to ensure that the components are placed properly and according to the constraints given by the mechanical engineer. While the designer is working on the PCB layout, the mechanical engineer will be working on the thermal analysis. The mechanical engineer will determine which components will require special attention in the layout in order to assure the best possible dissipation of heat on the PCB, and will instruct the designer.
Roles irrespective of title
Conceptual design work begins with an initial planning process that defines a proposed product's functionality. Once the user interfaces are settled, the actual design work starts with the mechanical engineer creating the board's general size and shape while the electrical engineer begins the schematic drafting and component selection.
The mechanical engineer is responsible for the packaging design, determining the constraints of the board and the location of high-powered components and connectors. As PCB designers well know, the main goal at this stage is to understand general cooling performance by evaluating general airflow patterns around components and through the chassis. With the optical components industry becoming even more focused on time-to-market, this places an even greater emphasis on executing a rapid-turn iterative design workflow.
In our department structure, the mechanical engineer can do this effortlessly because the analysis software we use runs directly in our design software. It is very easy to do multiple iterations because we can simply relocate solid models of components and see how the changes affect airflow. Once the mechanical engineer arrives at a preliminary design with thermal characteristics, the electrical engineer verifies the PCB layout, builds the PCB and tests the assembled board, focusing on functionality. (This is the basic division of responsibility in the design process that we use very successfully at Sorrento Networks.)
Thermal design Q&A
Where and when does thermal analysis enter the design workflow? The designer must overcome thermal design challenges in many places and ways. Some are overcome procedurally through standard analysis sessions, while others call on the designer's experience and intuition. In all cases, there will be questions for the electrical engineer, the layout person and the manufacturing department. Some of those questions follow.
Who carries decision-making authority as it relates to thermal issues within the overall product design? The mechanical engineer has the final say but must collaborate closely with the electrical engineer on cooling options such as heat sink, and component type and placement.
Where does the thermal environment fit in the priority list of design criteria? This is an issue for mechanical and electrical engineers alike. Obviously, if the design doesn't work, it is worthless. Similarly, if something doesn't fit, the PCB can't work. And if thermal issues affect the long-term viability of a product design, then the product will be a commercial failure. In the end, all three issues are concerns and arguably have similar importance.
How much thermal detail does the designer have to have before moving forward? The minimum requirement is a very rough sketch of the placement of the components. This includes power numbers and the location of all components greater than one watt. I use one watt as a cut-off point regarding defining components as "hot." (Additionally, I know that some engineers worry about transient heat but I don't worry about it at all because our boards are designed to run 24 hours a day at a constant temperature. Therefore, transient numbers are irrelevant in our design criteria).
Does the designer wait until all of the preliminary thermal design work has been completed before beginning his/her work? No, because in the beginning, two things happen in parallel. The electrical engineer begins work on his schematic. As this schematic is taking shape, the mechanical engineer needs to obtain a list from the electrical engineer of all proposed components that have a relatively high power. (This information is absolutely vital for the mechanical engineer.) With this list, and based on personal experience, the mechanical engineer places the components on the board in a best-guess layout. (If the components are really hot, the first choice is obviously near an air source). Then he or she will create a drawing that indicates the board size and placement of components and connectors for the PCB layout person. While the electrical engineer reviews the schematic with the layout designer, the mechanical engineer starts running the thermal analysis. If the thermal analysis doesn't reveal any hot spots and the electrical layout is correct, the project is finished.
What type of programmed components should we use in our thermal analysis? I do not maintain a library of programmed thermal components since there is such a wide variation of what is used on our assemblies. A small percentage of our components tend to contribute a very large percentage of the overall thermal budget, so I will model only this small set of components and average the rest of the estimated heat over the surface of the PCB. In addition, our analysis software has a very useful library of preprogrammed fans that can also be user-configured.
What does the designer need to understand about turbulence and radiation exchange? Unless the PCB will be used in space, radiation is a very small contributor. One of the great benefits of modern analysis programs is that mechanical engineers do not need to worry about turbulence. Analysis software tools not only identify turbulence, they also will calculate heat and heat flow using this information.
At what moment should it become obvious or clear to the designer that the thermal option on the computer model is the best solution? This is a relative issue but it is generally obvious that the design is acceptable from a thermal perspective when all of the components are within their rated temperatures, thus minimizing the board's mean-time-before-failure rate.
How does a designer decide what constitutes the best thermal solution out of a number of good ones? What are the criteria for selecting the very best one? The best thermal solution is whenever your maximum component temperature is the lowest. For example, if a board design had 10 hot components, reducing the temperature value of the largest one as low as possible would produce the best thermal solution. That is not to say, reduce one value as much as possible; instead, the result with the lowest maximum temperature component is usually a better design.
How much time does thermal modeling require on the average PCB design? In my experience, thermal analysis of a fairly simple model (comprised of 150-200 total components of which 5-7 are hot) will probably last a week, in which time the designer will conduct three separate analyses. If I set up an analysis session before leaving at night, it is usually done in the morning. For situations where the geometry is much more complex, I will set it up on Friday night and arrive on Monday morning to find my results.
Do designers typically run thermal analysis of complete layouts or on grouped components? At Sorrento, we typically analyze complete layouts, mainly due to the relatively small number of hot components.
Does the engineer have to ask the designer which cooling solutions were used? In our situation, the answer is no. The reason is that he or she will already know which were used because we work as a very close team. As department manager, I ensure that the electrical engineers, the mechanical engineers and the designers work very closely and talk frequently when collaborating on design projects. If the mechanical engineer wants to alter anything or modify the cooling design, it is imperative that the electrical engineer knows immediately. This is especially important when considering the use of heat sinks. They can have electrical as well as mechanical impacts on a PCB assembly design.
Is the designer responsible for evaluating manufacturability, and what does the designer need to understand about the relationship between thermal design and manufacturability? In our design/build process, the mechanical engineer, the PCB designer and the operations department are collectively responsible for evaluating the difficulty and cost of manufacturing a PCB design. Thermal design and manufacturability are not directly related in most cases, except for designs that require a lot of heat sinks or intricate heat sinks. If such a design raises the manufactured cost of a PCB too high, then the design needs to be revisited and other thermal options investigated.
The most important question
During more than 15 years of experience designing PCBs, I have learned that the most important question a designer or engineer can ask is: How can I do my job better and more efficiently? That question should be closely followed by: What new information, process, material technology or IT hardware and software has arrived and obviated whatever came before?
For individuals who conduct thermal analysis, the most important new technological advancement has been the continued enhancement of analysis software. Analysis software such as FloWorks clearly reveals where the airflow is and where it goes and its flow pattern dramatically helps the engineer. With fluid analysis programs becoming ever simpler to use and their results easier to interpret, their graphical analyses quickly tell our mechanical team where we need to place components. At some level, airflow may seem obvious (especially after so many years of experience); however, I am finding out (more often than I would like) that airflows are not as obvious as originally thought. Consequently, I am still learning and still posing questions to my peers, my staff and industry contacts.
Wayne Cottle is a 15-year veteran of the PCB industry. He is currently a manager for Sorrento Networks in San Diego, CA. Before joining Sorrento Networks, he worked for several electronics companies is the San Diego area as a mechanical engineer, working in packaging of electrical systems and various types of design analysis.
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7/1/01, Issue # 1807, page 10.
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