Get on board: solving thermal problems at board level: the early bird gets the worm. Instead of waiting to address thermal issues at the system level, tackle them at board level instead.Normally, thermal issues are not addressed until the latter stages of the design process, when component placement has been fixed and the board routed. The problem with this approach is that potential thermal issues arising at this point can be expensive to resolve, because substantial changes such as cabinet redesign re·de·sign
tr.v. re·de·signed, re·de·sign·ing, re·de·signs
To make a revision in the appearance or function of.
re , board placement changes or rerouting may be required. In a cost-sensitive product, the need for custom or exotic thermal solutions may even end up killing the design after considerable time and money have been expended ex·pend
tr.v. ex·pend·ed, ex·pend·ing, ex·pends
1. To lay out; spend: expending tax revenues on government operations. See Synonyms at spend.
That's why it makes sense to perform board-level thermal simulation very early in the design process. This analysis can help highlight potential thermal issues and provide engineers with more flexibility in resolving them, before hundreds of hours of engineering time are invested in unusable designs.
Thermal Density Increases
In the fairly recent past, board-level thermal simulation was considered an unnecessary luxury. Thermal management was usually addressed at the time the chassis was designed, normally by adding fans and cooling vents. Often, this design was based on a system-level simulation. The chassis designed by this method normally has a relatively long shelf life, typically three to five years. Once the thermal solution is fixed, it is expected to last through several generations of boards. In the past, board-level thermal solutions were typically limited to adding a heatsink to thermally critical components. In most cases, this approach was satisfactory because thermal densities were low enough that board redesigns did not have a major thermal impact.
But as more speed and functionality are packed into electronic systems, this old approach is becoming untenable. Thermal densities are rising at a rapid pace with each new product generation. Often a chassis design that worked well in the previous generation fails from a thermal standpoint today. In many cases it is necessary to add to the expense and weight of the product by adding a heatsink or fan. Another concern is that adding heatsinks can have secondary effects, such as blocking airflow to downstream components. Sometimes it will be preferable to go back and redesign the chassis or the board layout, adding to engineering costs and potentially causing the product to be delivered late. In any case, the cost of design changes generally increases by an order of magnitude A change in quantity or volume as measured by the decimal point. For example, from tens to hundreds is one order of magnitude. Tens to thousands is two orders of magnitude; tens to millions is three orders of magnitude, etc. or more as the design moves from conceptual to detailed to validation.
Why are thermal management problems typically addressed late in the design cycle system level? One reason is that system-level thermal simulation is the primary method for assessing thermal management. System-level thermal analysis Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Techniques include:
tr.v. o·ver·whelmed, o·ver·whelm·ing, o·ver·whelms
1. To surge over and submerge; engulf: waves overwhelming the rocky shoreline.
a. with work as more board upgrades require the thermal solution to be reconsidered. The fact that not every board design variation can be thermally vetted results in an increased risk of thermally unviable designs passing to the final design phase.
Electronic manufacturers are beginning to address these problems by paying more attention to thermal design at the board level. Often, when designing a new board for an existing enclosure, electrical engineers are simulating the board alone to identify hot spots hot spots
acute moist dermatitis. . Problems identified at this stage can often be addressed by layout changes that can be made nearly without cost at this stage of the process. Board-level simulation tools are usually much easier for EEs to use because they are designed around tools they already use, such as functional block diagrams A chart that contains squares and rectangles connected with arrows to depict hardware and software interconnections. For program flow charts, information system flow charts, circuit diagrams and communications networks, more elaborate graphical representations are usually used. and physical layouts.
In a typical board-level thermal simulation process flow, the systems architect will develop the initial concept design by creating a functional block diagram. The hardware design engineer then derives the first physical layout directly from the block diagram. At an early stage in the design process, long before the mechanical engineer gets involved, the electrical engineer can use board-level simulation to evaluate the new board design in an existing system. A 3D computation al fluid dynamics fluid dynamics
n. (used with a sing. verb)
The branch of applied science that is concerned with the movement of gases and liquids. solver predicts airflow and temperature for both sides of the board. Often the designer will identify hot spots, and cooling management can thus be considered from the earliest stages of the design process. Changes made to the functional block diagram are instantly reflected in the physical layout and thermal representation.
At this stage, far more alternatives exist to deal with thermal problems. Rather than being limited to expensive add-ons, engineers can consider a wide range of processes such as changing the board layout, adding copper inserts or changing the package style. The board-level model can also be imported into a system-level thermal model, such as the one that may have been created when the original system was designed. This saves the mechanical engineer time in updating the system-level model, if necessary, while reducing the chance of errors caused by miscommunications. The results from the system-level analysis can also be exported to the board-level simulation, making it possible for the electrical engineer to apply the system-level airflow and temperatures to the board being designed. This approach keeps all team members in sync and enables them to contribute to concept development in real time.
Layout Modification For Thermal Success
Let's take a look at how the process of solving board-level thermal issues early works in the real world. A 12.75" x 7" board was designed to add point-to-point optical distribution technology to an existing 1.2" slot pitch The distance between like-colored phosphor stripes in a CRT that uses a slot mask. This is slightly closer than the dot pitch in traditional shadow mask CRTs, which measures the diagonal distance between dots. See slot mask. subrack containing 14 cards cooled by a fan tray with eight fans (FIGURE 1). The chassis was designed for a previous generation product using system-level thermal simulation so engineers already knew the airflow that goes into the slot where this new board is being placed. When the new board was first conceived, a systems architect defined the functional blocks and connectivity using a board-level thermal simulation tool. The hardware engineer then took over and generated the initial component placement definition. With this information entered, the hardware engineer was then easily able to perform a thermal simulation.
[FIGURE 1 OMITTED]
The results showed that the 40 W processor and 6 W chipset A group of chips designed to work as a unit to perform a function. For example, a modem chipset contains all the primary circuits for transmitting and receiving. A PC chipset provides the electronic interfaces between all subsystems (see PC chipset for illustration). in the new design were much too hot. With these results in hand, the hardware engineer consulted a thermal engineer, who took possession of the board-level model and tried adding heatsinks to the two hot components. Then the thermal engineer re-ran the analysis and discovered that this approach did not solve the problem because the relatively tall capacitors upstream of the processor were blocking the flow into the processor heatsink. The hardware designer relocated the regulator functional group slightly to allow the airflow full access to the processor heatsink. This initial thermal optimization defined the floor plan as a starting point Noun 1. starting point - earliest limiting point
terminus a quo
commencement, get-go, offset, outset, showtime, starting time, beginning, start, kickoff, first - the time at which something is supposed to begin; "they got an early start"; "she knew from the for the rest of the design process.
Board-level simulation was used in the early stages of the design of an upgrade to an existing cPCI chassis (FIGURE 2). The board area was constrained con·strain
tr.v. con·strained, con·strain·ing, con·strains
1. To compel by physical, moral, or circumstantial force; oblige: felt constrained to object. See Synonyms at force.
2. by the form factor to 166 x 230 mm and restricted vertically by the standard 0.8" board pitch. The current platform was tested and shown to provide a slot flow rate of 200 1fm. The speed and the number of interconnects were driving factors in the performance restrictions on the relative component placement. This translated to the physical definition in the pinout and subsequent ability to route the board with the least number of layers. For this board, the most critical nets were those that connect the CPUs, MCH See Intel Hub Architecture. , ICH See Intel Hub Architecture. and memory. Cost restrictions limited the design to four layers, so it was necessary to keep a direct line of sight from the appropriate pin section to the target counterpart.
[FIGURE 2 OMITTED]
For the purposes of a placement study, engineers used rules of thumb to place the larger components and those that have a fixed position, such as connectors. These are often the same devices included in the functional block diagram. The initial placement put the components with the highest power density at the leading edge of the board. This was the limit of the thermal qualification done at the board design stage before the detailed design work was started on the mechanical design and routing. A board-level thermal tool was used at this stage to accurately evaluate the design from a thermal standpoint.
The results of the simulation exposed a drafting effect in which the CPU CPU
in full central processing unit
Principal component of a digital computer, composed of a control unit, an instruction-decoding unit, and an arithmetic-logic unit. on the left side preheated the air passing over the DIMM (Dual In-Line Memory Module) A printed circuit board that holds memory chips and plugs into a DIMM socket on the motherboard. See memory module.
DIMM - Dual In-Line Memory Module . If this were caught after the detailed design work had begun, the options would have been either to reduce performance in order to drop the power dissipated dis·si·pat·ed
1. Intemperate in the pursuit of pleasure; dissolute.
2. Wasted or squandered.
3. Irreversibly lost. Used of energy. by this device or to add cost by implementing a thermal solution for the DIMM. By evaluating this in the concept phase, engineers were able to explore other options in a matter of minutes A Matter of Minutes is an episode from the television series The New Twilight Zone. Cast
Although rules of thumb and intuition intuition, in philosophy, way of knowing directly; immediate apprehension. The Greeks understood intuition to be the grasp of universal principles by the intelligence (nous), as distinguished from the fleeting impressions of the senses. are beneficial, many second-order effects such as drafting of components or thermal saturation saturation, of an organic compound
saturation, of an organic compound, condition occurring when its molecules contain no double or triple bonds and thus cannot undergo addition reactions. of the board are difficult to anticipate without analysis. It turned out that in this application, slightly drafting the CPUs provided the ideal solution. Identifying this before investing in detailed routing made it possible to achieve the desired performance without introducing any additional costs. In high-volume applications such as this single board computer, removing the need for an expensive solution such as a heat spreader spreader,
n See condenser. can save millions of dollars in material, manufacturing and qualification costs.
Board-level Troubleshooting Troubleshooting is a form of problem solving. It is the systematic search for the source of a problem so that it can be solved. Troubleshooting is often a process of elimination - eliminating potential causes of a problem.
While board-level simulation is usually most effective when used at the early stages of the development process, it also provides an effective troubleshooting tool. Board-level simulation was used to troubleshoot To find out why something does not work and to fix the problem. Troubleshooting a computer often requires determining whether the problem is due to malfunctioning hardware or buggy or out-of-date software. See debug. an upgraded add-on card measuring 6.5" x 5.5" (FIGURE 3). The previous platform provided a uniform flow rate of 400 1fm at 35[degrees]C and the vertical heatsink volume was constrained by the 25 mm pitch between boards. Engineers modeled the new components using "2 resistor resistor, two-terminal electric circuit component that offers opposition to an electric current. Resistors are normally designed and operated so that, with varying levels of current, variations of their resistance values are negligible (see resistance). " or "Delphi resistor" models (simplified thermal component models capable of predicting accurate case and junction temperatures Junction temperature is the temperature of the actual semiconductor in an electronic device. In operation it is usually higher than case temperature, the temperature of the part's exterior. ), some provided by the vendor and others created by the engineers themselves using a Web service, and added them to a board-level model. The results indicated that two components exceeded their maximum junction temperature and heated up a section of the board, causing a third component to approach its specified maximum temperature.
[FIGURE 3 OMITTED]
In this case, board placement had been frozen so it could not be used to resolve the problem. In addition, the option of transferring the heat into the board with thermal vias or a local increase in copper content to improve heat spreading did not look like a viable solution. So engineers considered adding heatsinks. In an effort to reduce costs, they placed the same 21 x 21 mm pin fin heatsink on both of the two hot components. Using a thermal interface material A Thermal Interface Material (aka TIM) is used to fill the gaps between thermal transfer surfaces, such as between microprocessors and heatsinks, in order to increase thermal transfer efficiency. These gaps are normally filled with air which is a very poor conductor. reduced thermal resistance between the components and the heatsink. The results of a new thermal simulation with these design changes indicated that with the new design, all components were safely below the 90[degrees]C junction temperature limit.
Current thermal densities require engineers to consider the mechanical requirements and limitations of the thermal solution in the earliest stages of board design. Although some rules of thumb can be employed in regards to the required volume and drafting effects, they often compete and do not provide accurate enough information for tradeoffs to be considered. These applications demonstrate the benefits of implementing board-level analysis as early as possible in the design cycle. The analysis helps highlight potential thermal issues and provides engineers with more flexibility in resolving them before engineering time is invested in unusable designs. In most applications, the board will continue to evolve, but with collaborative analysis tools engineers can work in cross-functional teams In business, a cross-functional team is a group of people with different functional expertise working toward a common goal. It may include people from finance, marketing, operations, and human resources departments. addressing thermal, electromagnetic compatibility (hardware, testing) Electromagnetic Compatibility - (EMC) The extent to which a piece of hardware will tolerate electrical interference from other equipment, and will interfere with other equipment. , stress and reliability issues until the physical prototype is built and tested to confirm the results of the simulations.
ROBIN BORNOFF is a product manager at Flomerics, UK. She can be reached at email@example.com.