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Extra lately, physics-based reliability prediction has associated failure charges of digital assemblies to the magnitude of temperature change over an operational cycle (power-on, power-off, power-on…) and fee of temperature change, each of that are influenced by steady-state working temperature.
Electronics failures are sometimes attributed to solder joint fatigue on the package-board interconnect. In some functions, like computing, the place efficiency is vital, temperature adversely impacts velocity. In different circumstances, parts need to run at very comparable temperatures to keep away from timing points. Excessive temperatures could cause operational points, akin to latchup. Whether or not the intention is to extend reliability, enhance efficiency, or keep away from issues throughout operation, correct prediction of element temperatures helps thermal designers obtain their targets.
Maximize certainty in element temperature prediction
Dependable, correct prediction of element temperatures permits designers to grasp how shut the design values come to the utmost allowable* temperature. Listed below are 5 suggestions for reaching high-fidelity element temperature prediction throughout the design circulation and gaining elevated confidence in your ultimate simulation outcomes.
Tip 1: Mannequin key parts explicitly
To precisely predict a key element’s temperature, the element must be modeled explicitly as a part of the thermal simulation. Nevertheless, not all parts should be modeled, and it’s typically impractical to take action. Small parts with a low energy density that aren’t significantly thermally delicate will be considered thermally benign and don’t should be represented discretely. Warmth from these parts will be added as a background warmth supply utilized over the entire board or as a footprint warmth supply on the board. Electronics cooling simulation software program ought to present filtering choices to do that routinely in late design when the populated board structure is imported from the EDA system.
Determine 1 Thermal mannequin of an insulin pump, with parts modeled in several ranges of element
Bigger parts might disrupt the airflow, requiring them to be represented instantly as 3D objects. One class of element that may fall into this class is electrolytic capacitors, used, for instance, in energy provides. These are thermally delicate, with a low most allowable temperature. Modeling electrolytic capacitors explicitly might help guarantee their most temperature shouldn’t be exceeded.
Giant, high-power parts and parts with excessive energy density will should be modeled discretely, as their thermal administration and affect on neighboring parts are vital to the general thermal design of the product.
Tip 2: Use good energy estimates
As famous above, a part of the choice concerning whether or not it’s essential to signify a element instantly will depend on its energy density, which is the element energy divided by its footprint space.
It’s price re-checking selections about which parts to mannequin discretely because the design evolves and extra info turns into accessible. In early design, it might solely be potential to make use of the utmost rated energy for the element as an alternative of estimating the possible energy consumption. Energy budgets for particular person parts and the board as a complete can change over the course of the design, so these should be re-checked commonly.
Determine 2 Instance energy vs. time profile
Tip 3: Use the appropriate bundle thermal mannequin
The golden rule is to start out early and begin easy. The mechanical engineer liable for the thermal integrity of the product ought to goal to supply as a lot helpful suggestions as potential to the digital engineers to information the design in regards to the thermal impression of their decisions, particularly throughout early design.
From the mechanical engineer’s perspective, on the PCB degree, this entails serving to with bundle choice and the most effective positioning of parts to make the most of system airflow for cooling. Inevitably, each structure and bundle choice are pushed primarily by a mixture of digital efficiency and price concerns. Nonetheless, the results of these decisions on thermal efficiency must be made as clear as potential, as temperature and cooling additionally have an effect on efficiency and price. The selection of element thermal mannequin will depend on a number of components.
In early design, earlier than the board is routed or the variety of layers within the board is thought, correct element temperature prediction is solely not potential, so a thermally subtle mannequin of the element shouldn’t be known as for. Later within the design, when the PCB mannequin will be refined, the element thermal mannequin also needs to be refined. Decisions about probably the most applicable element thermal mannequin are iterative, as parts predicted to be scorching* self-indicate the necessity to refine the element’s thermal mannequin and maybe take into account a component-specific thermal administration answer. The board design can type a part of that thermal administration answer, for instance, utilizing thermal vias to conduct warmth to a buried floor airplane.
Tip 4: Use compact thermal fashions from early design
It’s vital to mannequin parts precisely and use a 3D illustration of the element within the thermal design earlier than bundle choice. 2-Resistor and DELPHI compact thermal fashions have been launched. Right here, we talk about in additional element the predictive accuracy of those and different thermal fashions.
2-Resistor Fashions
As famous, a 2-Resistor compact thermal mannequin (CTM) is the bottom constancy mannequin able to predicting each case and junction temperatures. One profit is that it doesn’t require any extra mesh than a easy conducting block, so utilizing 2-Resistor fashions has no destructive impression on simulation time. Though this has the bottom computational burden, the worst-case error in junction temperature prediction will be as excessive as ±30% and varies with each bundle sort and bundle dimension. The junction-to-case resistance and junction-to-board resistance thermal metrics this mannequin is predicated on are measured beneath standardized circumstances.
The JEDEC Commonplace JESD15-3 requires the junction-to-board resistance to be measured on a 2s2p board with steady energy and floor planes. The junction-to-case resistance is measured by urgent the highest of the bundle in opposition to a chilly plate. Because of this, the predictive accuracy of the 2-Resitor mannequin is greater the extra intently the applying resembles the take a look at circumstances.
For the junction-to-case resistance, the applying atmosphere that the majority intently corresponds to the take a look at atmosphere is when the element has a heatsink protecting the entire bundle floor. Because of this 2-Resistor fashions can be utilized to initially assess the dimensions of heatsink required.
Observe that the highest floor of a 2-Resistor mannequin is an isothermal node representing the case in order that the bottom of the heatsink can be held near isothermal. A 2-Resistor mannequin can subsequently be used to find out the variety of fins, fin thickness, and fin top wanted to scale back the air-side thermal resistance of the heatsink, however not the bottom thickness wanted to adequately unfold the warmth to make sure that warmth passing to the outer fins shouldn’t be overly restricted.
RC-Ladder Fashions
For packages which have a single warmth circulation path, akin to LEDs and TO-style packages, there’s a JEDEC normal strategy 3 for measuring a thermal resistor-capacitor mannequin of the warmth circulation path from the junction right down to the bundle tab. Observe that this methodology doesn’t instantly present thermal resistance to the uncovered prime floor of the bundle. Nevertheless, supplied this may be estimated someway, Simcenter Micred T3STER {hardware} can be utilized to create an RC-ladder thermal mannequin that takes this under consideration.
Simcenter Micred T3STER is the industry-leading answer for measuring packaged ICs to create these thermal fashions, which can be utilized instantly as a Community Meeting in Simcenter Flotherm. In contrast to 2-Resistor fashions, which solely include thermal resistors, these fashions embody thermal capacitors and can be utilized for transient simulations. These fashions can provide glorious outcomes when the applying atmosphere is near that of the take a look at chilly plate atmosphere, e.g., when the bundle is soldered to an MCPCB or a copper pad on a high-conductivity board.
DELPHI Fashions
DELPHI fashions are so-named as they originated from the DELPHI Undertaking coordinated by Flomerics Ltd. within the late Nineties. These fashions have partitioned prime and backside surfaces, with a matrix of thermal resistors to attach these surfaces to the junction and/or to 1 one other. These further inside thermal resistors permit the warmth circulation by these paths throughout the bundle to regulate relying on the boundary circumstances, and in lots of functions, the mannequin will predict the junction temperature accuracy to effectively inside ±10%, being a worst-case determine. On the whole, DELPHI fashions are enough for detailed thermal design work of all however probably the most thermally-critical packages, stacked or 3D ICs, or the place further info is required from the simulation, e.g., the temperature distribution on the die floor. Like 2-Resistor fashions additionally they solely include resistors, so can solely be used for steady-state simulation.
Detailed Fashions
Detailed fashions are thermal fashions that discretely mannequin all of the thermally related options of the bundle internals. Observe that these fashions typically include some extent of approximation, as options akin to particular person bond wires and solder balls are sometimes lumped collectively. Nevertheless, such fashions goal to permit the temperature distribution throughout the bundle to be precisely represented. If the geometry and materials properties are right, such fashions provide the very best constancy.
Determine 3 Detailed thermal mannequin of a chip bundle
Elements that require particular thermal administration options, akin to a heatsink, fansink, or thermal pad must be modeled intimately to appropriately optimize the cooling answer. For instance, within the case of a warmth sink, it has lengthy been recognized that the temperature distribution within the bundle influences the temperature distribution within the heatsink base and vice versa. Consequently, detailed bundle thermal fashions are beneficial for such functions.
One other benefit of detailed fashions is that they permit the temperature of the solder interconnect to be predicted. Thermomechanical shear, coupled with temperature change, is the first stressor affecting solder joint life.
Determine 4 Temperature distribution on underside of a BGA bundle exhibiting particular person soler balls
BCI-ROMs
A latest advance in predicting element temperatures is thru the usage of decreased order fashions, or ROMs. ROMs can now be created independently of their boundary circumstances (BCI) reasonably than particular for a given thermal atmosphere. That signifies that BCI-ROMs will be created by bundle distributors, impartial of their thermal atmosphere, and supplied to finish customers to be used in simulating a selected thermal atmosphere. They’re accessible in quite a lot of codecs, akin to uncooked matrices, SPICE, VHDL-AMS, and FMU. There at the moment are a spread of authoring choices for BCI-ROMs inside Simcenter.
BCI-ROMs produce other very fascinating traits:
They’re extremely correct, with the accuracy outlined as a part of the creation course of (sometimes > 98%)
Help a number of warmth sources
Help all transient timescales
Conceal delicate IP as the interior geometry of the mother or father detailed mannequin from which they’re derived can’t be reverse-engineered from the ROM
Report the suitable junction temperature outlined by the seller with out the seller having to disclose the place throughout the mannequin that’s.
Clear up orders of magnitude quicker than detailed fashions
A serious benefit of that is that they are often included in circuit simulators, like Xpedition AMS and PartQuest Discover, permitting them to be ‘temperature conscious,’ which is vital to getting correct energy estimates in early design.
Use of BCI-ROMs inside 3D CFD simulators has the potential to be a game-changer within the provide chain for bundle thermal fashions, and BCI-ROMs may also be created for complete boards.
Tip 5: Create your fashions as required
In follow, the selection of thermal mannequin might largely rely upon what is offered from the seller. Even at the moment, we discover distributors might solely provide info within the type of a datasheet, e.g., as a PDF, and these might not include the data obligatory for even primary thermal design. For instance, the information sheet might include solely a junction-to-ambient thermal resistance, which may’t be used for design, and solely efficiency comparability. JEDEC has printed JEP1817, an ordinary file format for thermal simulation information change. It’s XML-based, utilizing ECXML developed by Siemens, brief for ‘electronics cooling extensible mark-up language’.
We hope that the following tips have began your journey to enhance element temperature predictions. For a further 5 suggestions, please obtain this free whitepaper.
Creator
Dr. John Parry, Simcenter Trade Director, Electronics & Semiconductor, Siemens Digital Industries Software program
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