Category Archives: heat sink

Optimizing Heat Sink Base Spreading Resistance to Enhance Thermal Performance

Posted on August 13, 2018 by | 2 comments

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

Heat sinks are routinely used in electronics cooling applications to keep critical components below a recommended maximum junction temperature. The total resistance to heat transfer from junction to air, Rja, can be expressed as a sum of the following resistance values as shown in Equation 1 and displayed in Figure 1.

Where, Rjc is the internal thermal resistance from junction to the case of the component. RTIM is the thermal resistance of the thermal interface material. Rf is the total thermal resistance through the fins. The final term in Equation 1 represents the resistance of the fluid, e.g. air, going through the heat sink where m is the mass flow rate and Cp is the heat capacity of the fluid. As Equation 2 shows, Rcond and Rconv are the conduction and convection resistance respectively through the heat sink fins respectively.

Heat Sinks

Fig. 1 – Resistance network of a typical heat sink in electronics cooling. [1]

Fig. 2 – Heat source on a heat sink base. [2]

Rs stands for the spreading resistance that is non-zero when the heat sink base is larger than the component. The next few sections show the full analytical solution for calculating spreading resistance, followed by an approximate simplified solution and the amount of error from the full solution and finally the use of these solutions to model and optimize a heat sink.

Analytical Solution of Spreading Resistance

Lee et al. [2] derived an analytical solution for the spreading resistance. Figure 2 shows a cross-section of a circular heat source with radius a on the base with radius b and thickness t. The heat, q, originates from the source, spreads out over the base and dissipates into the fluid on the other side with heat transfer coefficient, h. For heat transfer through finned heat sinks, the effective heat transfer coefficient is related to thermal resistance of the fins, Rf as shown in Equation 3. For square heat source and plates, the values of a and b can be approximated by finding an effective radius as shown in equations 4 and 5.

Where,
h = heat transfer coefficient [W/m2K]
a = effective radius of the heater [m]
Aheater = area of the heater [m2]
b = effective radius of the heat sink base [m]
Abase = total area of the heat sink base [m2]

The derivation of the analytical solution starts with the Laplace equation for conduction heat transfer and applying the boundary conditions. Equation 6 shows the final analytical solution for spreading resistance. The values for the eigenvalue can be computed by using the Bessel function of the first kind at the outer edge of the plate, r=b as shown in Equation 7.

Where,
k = Thermal Conductivity of the plate or heat sink [W/mK]
J1 = Bessel function of the first kind
λn = Eigenvalue that can be computed using Equation (3) at r = b
t = thickness of the heat sink base [m]

Lee et al. [2] also offered an approximation as shown in Equation 8 along with the approximation for the eigenvalues as shown in Equation 9. This approximation eliminates the need for calculating complex formulas that involve the Bessel functions and can be computed by a simple calculator.

Approximation vs. Full Solution

Simons [3] compared the full solution (Equations 6 and 7) with the approximations shown in (Equations 8 and 9). The problem contained a 10 mm square heat source on a 2.5 mm thick plate with a conductivity of 25 W/mK, 20 mm width and varying length, L as shown in Figure 3. Figure 4 shows that the percentage error increases with length but stays relatively low. Less than 10% error is expected for lengths up to 50 mm; five times the length of the heater. This is acceptable for most engineering problems since analytical solutions are first-cut approximations that should later be verified through empirical testing and/or CFD simulations. However, the full analytical solution should be used if the heater-to-heat sink base area difference gets much larger or if a more accurate solution is desired.

Fig. 3 – Example problem for comparing analytical and approximate solutions for spreading resistance. [3]

Fig. 4 – Percent error between the analytical and the approximate solution of spreading resistance for the example shown in Figure 3. [3]

Optimizing Heat Sink Performance

The goal of any electronic cooling solution is to lower the component junction temperature, Tj. For a given Rjc and RTIM, the objective is to maximize heat sink performance by reducing the spreading resistance, Rs, and the fin resistance Rf.

The spreading resistance can be reduced by increasing base thickness. However, most electronics applications are limited by total heat sink height and thus any increase in base thickness leads to shorter fins which reduce the total area of the fins Afins. For a fixed heat transfer coefficient (the heat transfer coefficient is a function of fin design and air velocity and we can assume it is fixed for this exercise) a reduction in the fin area increases Rf as shown in Equation 2. Equation 10 shows this combined heat sink resistance, Rhs, as a function of the spreading and fin resistance.

Thus, for a given fin design, the thermal engineer must choose the appropriate heat sink base thickness to optimize heat sink performance. To illustrate this point, let’s take an example of an application with the parameters as shown in Table 1.

Table 1 – Example Heat Sink Application

Figure 5 shows a graph of the total thermal resistance of the heat sink, Rhs and spreading resistance, Rs as a function of base thickness for copper and aluminum material. (Note that the final term from Equations 1 and 10 is ignored because it is constant and does not contribute to the understanding of spreading resistance). The graph shows that spreading resistance improves monotonically with increased base thickness. However, the total thermal resistance has an optimal point between 2-4 mm base thicknesses. For base thicknesses less than 2 mm, there is a sharp increase in spreading resistance which leads to a higher overall thermal resistance.

Fig. 5 – Total and spreading resistance of the example shown in Table 1 for a 50 mm heat sink.

On the other hand, increasing the base thickness above 4 or 5 mm gives diminishing marginal returns; the improvement in spreading resistance is minimal compared to the increase in thermal resistance due to the reduced fin area. Additionally, the graph also shows that higher conductivity materials such as copper, improves thermal performance across the entire domain.

Conclusion

The heat spreading resistance is an important factor when designing a heat sink for cooling electronics components. The full analytical solution for calculating the spreading resistance, shown in Equations 6 and 7, can be substituted with the approximations shown in Equations 8 and 9 with minimal error. The error increases with increased difference between the heat sink base and heater size and the complete analytical model should be used if needed. The analytical model can be used to choose the right heat sink base thickness that optimizes heat sink performance as shown in Figure 5.

Techniques such as higher conductivity materials, embedded heat pipes, vapor chambers etc. are available if the spreading resistance is major obstacle in the cooling. Thermal engineers must balance the increased weight and cost of such techniques against the benefits for each application.

References
1. “Spreading Resistance of Single and Multiple Heat Sources,” Qpedia. September 2010
2. Seri Lee et al. “Constriction/Spreading Resistance Model for Electronics Packaging,” 1995.
3. Simons, Robert http://www.electronics-cooling.com/2004/05/simple-formulas-for-estimating-thermal-spreading-resistance

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].

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Posted in heat sink, Heat Sinks, Thermal Design, Thermal resistance

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Heat Sink Design: ATS Engineers Bring Ideas to Life

Posted on June 29, 2017 by | 1 comment

Marketing Communications Specialist Josh Perry sat down with Product Engineering Manager Greg Wong to discuss the process that Advanced Thermal Solutions, Inc. (ATS) engineers go through to create a heat sink and find a thermal solution for customers.

Watch the full conversation in the video below and scroll down to read the transcript of the interview.

JP: Greg, thanks again for joining us here in marketing to explain what it is that goes into designing a heat sink for a customer. So, how does that process begin?
GW: We usually start with a few basic parameters; we call them boundary conditions. So, we start with a few boundary conditions, basics like how much airflow we have, how much space constraint we have around a heat sink, and how much power we’re dissipating, as well as the ambient temperature of the air coming into the heat sink.

So, those are the real basic parts that we need to start out with and sometimes the customer has that information and they give it to us, and usually we double-check too, and then other times the customer has parts of the information, like they know what fan they want to use and they know what kind of chassis they’re putting it in and we take that information and we come up with some rough calculations so we can arrive at those things like air flow and stuff like that.

JP: When you get the data from the customer, how do you determine what the problem is, so that way you can move forward?
GW: We usually start out with an analytical analysis. So, we put pen to paper and we start out with basic principles of heat transfer and thermal resistance and stuff like that so we can understand if what we’re trying to achieve is even feasible and we can come up with some basic parameters just using that analytical analysis.

Like we can calculate what kind of heat sink thermal resistance we need or we can calculate how much air flow we need or, if we have several components in a row, we can calculate what the rough air temperature rise is going to be along that chain of parts. So, there’s a lot we can do when we get the basic information from the customer just on pen and paper.

JP: What’s the next step beyond analytical?
GW: Well, we can do some lab testing or a lot of times we also use CFD simulations and, if our customer has a model they can supply us, we can plug that into the CFD simulations and we can come up with an initial heat sink design and we can put that into the simulations as well and then we set those up and run them.

The great thing, having done these analytical analyses beforehand, we know what to expect from CFD simulations. So that way, if the simulations don’t run quite right, we already have an understanding of the problem, we know what to expect, because CFD is not 100 percent reliable.

I mean, you can go and plug all this stuff in there but you really have to understand the problem to know if the CFD is giving you a good result. So, oftentimes that’s the next stage of the process and from there we can actually produce low-volume prototypes right here in Norwood (Mass.), in our factory. We have CNC machines and manual milling machines, lathes, all that kind of stuff, and we can produce the prototypes and test them out here in our labs.

JP: How much of a benefit is it to be able to create a prototype and to be able to turn one around quickly like that?
GW: Oh, it’s great. I mean, if we had to wait to get parts from China it will take weeks to get. We can turn them around here in a few days and the great thing about that is we can test them in our labs and, you know, when it comes to getting results nothing beats the testing.

I mean, you can do analytical analysis, you can do CFD simulations, but when you actually test the part in a situation that is similar to what the actual thing is going to be that’s where the real meat comes down.

Heat Sink Design

ATS engineers take customer data and using analytical modeling and CFD simulations can design the right cooling solution to meet the customer’s specific thermal needs. (Advanced Thermal Solutions, Inc.)

JP: So, we test the prototypes before sending them out to the customer? We do the testing here or do we send it to them first?
GW: It all depends on what the customer requires. Sometimes the customer has a chassis that we really can’t simulate in our labs, so we might send the prototype heat sinks to the customer and the customer will actually put them into their system to test them out.

Other times, a customer might have a concept and they don’t actually have a product yet, so we’ll mock something up in our labs and we’ll test it and it all just depends what the customer needs and also how complex the problem is.

If it’s a simple heat sink and pretty simple airflow, we might not need to test that because we understand that pretty well, but the more complex the chassis is and how the airflow bends and stuff like that, the greater benefits we get out of lab testing.

JP: Well, I appreciate it Greg. Thank you for taking us through the process of making a heat sink and solving thermal problems for our customers.
GW: Sure Josh. We love seeing new thermal challenges and coming up with ways of keeping stuff cool.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or [email protected].

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Posted in CFD, Chassis, Consulting, Engineering, heat sink, heat sink design, Heat Sinks, PCB, Thermal Design

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How Do Heat Sink Materials Impact Performance

Posted on April 19, 2017 by | 3 comments

By Michael Haskell, Thermal Engineer
and Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication produced by Advanced Thermal Solutions, Inc. (ATS) dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

Heat Sink Materials

This article examines the difference in thermal performance between copper, aluminum, and graphite foam heat sinks. (Advanced Thermal Solutions, Inc.)

Introduction

As thermal solutions for today’s electronics grow more challenging, demand rises for novel cooling ideas or materials. As a result, the proven methods of analytical calculations, modeling and laboratory testing are sometimes bypassed for a quick “cure-all” solution. Evolutionary progress is required of the thermal industry, of course. But, despite the urgency to introduce new ideas and materials, thorough testing should be performed in determining the thermal performance of a solution before it is implemented.

This article addresses the impact of material choice on heat sink performance. First, an evaluation of different materials is made in a laboratory setting, using mechanical samples and a research quality wind tunnel. This testing compares a constant heat sink geometry made from copper, aluminum, and graphite foam. Next, an application-specific heat sink study is presented using computational fluid dynamics (CFD) software.

In this study, a heat sink was designed in 3D CAD to cool a dual core host processor. The performance of both an aluminum and copper design was then evaluated using CFD.

Laboratory Tests of Copper, Aluminum, and Graphite Foam

The stated thermal properties of engineered graphite foams have enhanced their consideration as heat sink materials. Yet, the literature is void of a true comparison of these materials with copper and aluminum. To evaluate graphite foam as a viable material for heat sinks, a series of tests were conducted to compare the thermal performance of geometrically identical heat sinks made of copper, aluminum, and graphite foam respectively.

Testing was conducted in a research quality laboratory wind tunnel where the unducted air flow was consistent with typical applications.

(The results for ducted and jet impingement flows, though similar to the unducted case, will be presented in a future article along with a secondary graphite foam material.)

Test Procedure

Earlier foam experiments by Coursey et al. [1] used solder brazing to affix a foam heat sink to a heated component. The solder method reduced the problematic interfacial resistance when using foams, due to their porous nature. Directly bonding the heat sink to a component has two potential drawbacks. First, the high temperatures common in brazing could damage the electrical component itself.

The other issue concerns the complicated replacement or rework of the component. Due to the low tensile strength of foam (Table 1) a greater potential for heat sink damage occurs than with aluminum or copper [2]. If the heat sink is damaged or the attached component needs to be serviced, direct bonding increases the cost of rework.

Table 1. Thermal and Mechanical Properties of the Heat Sink Materials. (Advanced Thermal Solutions, Inc.)

To avoid these problems, the foam heat sink can be soldered to an aluminum or copper carrier plate. This foam-and-plate assembly can then be mounted to a component in a standard fashion. The carrier plate allows sufficient pressure to be applied to the interface material, ensuring low contact resistance.

In this study, the heat sinks were clamped directly to the test component without a carrier plate as a baseline for all three materials. Shin-Etsu X23 thermal grease was used as an interface material to fill the porous surface of the foam and reduce interfacial resistance. Five J-type thermocouples were placed in the following locations: upstream of the heat sink to record ambient air temperatures, in the heater block, in the center of the heat sink base, at the edge of the heat sink base, and in the tip of the outermost fin.

Heat Sink Material

Figure 1. Test Heat Sink Drawing. (Advanced Thermal Solutions, Inc.)

A thin film heater was set at 10 watts during all testing, and the heat source area was 25 mm x 25 mm, or one quarter of the overall sink base area, as shown in Figure 1. Both cardboard and FR-4 board were used to insulate the bottom of the heater, The estimated value of Ψjb is 62.5°C/W. Throughout testing, the value of Ψjb was 36–92 times greater than that of Ψja.

Results

As expected, the traditional copper and aluminum heat sinks performed similarly. The main difference was due to the higher thermal conductivity of copper, which reduced spreading resistance. During slow velocity flow conditions, the lower heat transfer rate means that convection thermal resistance makes up a large portion of the overall Θja.

Heat Sink Materials

Table 2. Test Heat Sink Geometry. (Advanced Thermal Solutions, Inc.)

Heat Sink Materials

Figure 2. Experimental Heater and Measurement Setup. (Advanced Thermal Solutions, Inc.)

As flow speed increases, the convection resistance decreases, and the internal heat sink conduction resistance is more of a factor in the overall Θja value. This behavior is evident in the table below, and when comparing the different heat sink materials. The graphite heat sink’s thermal performance was only 12% lower than aluminum at low flow rates. However, the performance difference increased to 25-30% as the flow rate increased (Table 3).

Heat Sink Materials

Table 3. Specific Thermal Test Results. (Advanced Thermal Solutions, Inc.)

Due to the lack of a solder joint, the foam heat sink experienced a larger interfacial resistance when compared to the solid heat sinks. This difference can be seen when comparing ΨHEATER-BASE in Table 3. To decouple the effect of interfacial resistance ΨBASE-AIR can be calculated. When ignoring interfacial resistance in this manner foam performs within 1% of aluminum at 1.5 m/s, and within 15% at 3.5 m/s.

Heat Sink Materials

Figure 3. Heat Sink Thermal Resistance as a Function of Velocity. (Advanced Thermal Solutions, Inc.)

Graphite foam-derived heat sinks show promise in specific applications, but exhibit several drawbacks in mainstream electronics cooling. Due to the frail nature of graphite foam, unique precautions must be taken during the handling and use of these heat sinks. When coupled to a copper base plate, graphite foam can perform with acceptably small spreading resistances.

However, the foam’s lower thermal conductivity reduces thermal performance at high flow velocities compared to a traditional copper heat sink.

The mechanical attachment needed to ensure acceptable thermal interface performance without soldering or brazing also hinders foam-based heat sinks from being explored in mainstream applications. Despite these challenges, the thermal performance-to-weight ratio of foam is very attractive and well-suited to the aerospace and military industries, where cost and ease of use come second to weight and performance.

Thermal Software Comparison of Aluminum and Copper Heat Sinks

A challenging thermal application was considered. This involved the use of a dual core host processor on a board with limited footprint area for a heat sink of sufficient size. A heat sink with a stepped base was designed to clear onboard components. It provided sufficient surface area to dissipate heat (Figure 4).

Due to the complexity of the heat sink, machining a test sample from each material was not practical. Instead, CFD was used to predict the performance difference between the two materials and determine if the additional cost of copper was warranted.

Heat Sink Materials

Figure 4. Stepped Base maxiFLOW™ Heat Sink (ATS). (Advanced Thermal Solutions, Inc.)

Because of the stepped base and a long heat conduction path, spreading resistance was a major factor in the overall thermal resistance. The effect of copper in place of aluminum due to its higher thermal conductivity (400 and 180 W/m*K respectively) is shown in Table 4. The CFD software predicted a 21% improvement using copper in place of aluminum. More importantly, it reduced the processor case temperature below the required goal of 95°C.

The performance improvement with copper is due to the reduced spreading resistance from the processor die to the heat sink fins. This effect is shown in Figure 5, where the base temperatures of both heat sinks are obtained from the CFD analysis and plotted together. The aluminum heat sink shows a hotter center base temperature and a more pronounced drop off in temperature along the outer fins. The copper heat sink spreads the heat to all fins in a more even fashion, increasing the overall efficiency of the design. This temperature distribution can be seen in Figures 6 and 7, which were created using CFDesign software.

Heat Sink Materials

Figure 5. Effect of Heat Sink Material on Temperature Distribution. (Advanced Thermal Solutions, Inc.)

Heat Sink Materials

Figure 6. Aluminum Stepped Base maxiFLOW™ Heat Sink Simulation. (Advanced Thermal Solutions, Inc.)

Heat Sink Materials

Figure 7. Copper Stepped Base maxiFLOW™ Heat Sink Simulation. (Advanced Thermal Solutions, Inc.)

Conclusion

Design engineers have many materials at their disposal to meet the challenging thermal needs of modern components. Classic materials such as aluminum and copper are joined by new technologies that bring improvements in cost, weight, or conductivity. The choice between a metallic, foam or plastic heat sink can be difficult because thermal conductivity provides the only available information to predict performance.

The first method for determining material selection is a classic thermodynamics problem: what effect does conductivity have on the overall thermal resistance in my system? Only once this is answered can the benefits of cost, weight, and manufacture be addressed.

References

1. Coursey, J., Jungho, K., and Boudreaux, P. Performance of Graphite Foam Evaporator for Use in Thermal Management, Journal of Electronics Packaging, June 2005.
2. Klett, J., High Conductivity Graphitic Foams, Oak Ridge National Laboratory, 2003.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or [email protected].

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Posted in Analysis, heat sink, Heat Sink Material, Heat Sinks, Thermal Analysis

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Technical Discussion of ATS Telecom PCB solution

Posted on October 28, 2016 by | 1 comment

Last year, Advanced Thermal Solutions, Inc. (ATS) was brought in to assist a customer with finding a thermal solution for a PCB that was included in a data center rack being used in the telecommunications industry. The engineers needed to keep in consideration that the board’s two power-dissipating components were on opposite ends and the airflow across the board could be from either side.

Telecom PCB

The PCB layout that ATS engineer Vineet Barot was asked to design a thermal solution for included two components on opposite ends and airflow that could be coming from either direction. (Advanced Thermal Solutions, Inc.)

The original solution had been to use heat sinks embedded with heat pipes, but the client was looking for a more cost-effective and a more reliable solution. The client approached ATS and Field Application Engineer Vineet Barot examined the problem to find the best answer. Using analytical and CFD modeling, he was able to determine that ATS’ patented maxiFLOW™ heat sinks would provide the solution.

Vineet sat down with Marketing Director John O’Day and Marketing Communications Specialist Josh Perry to discuss the challenges that he faced in this project and the importance of using analytical modeling to back up the results of the CFD (computational fluid dynamics).

JP: Thanks for sitting down with us Vineet. How was the project presented to you by the client?
VB: They had a board that was unique – where it would be inserted into a rack, but it could be inserted in either direction. So, we faced a unique challenge because airflow could be from either side of the board. There were two components on either side of the board, so if airflow was coming from one side then component ‘A’ would get hot and from the other side then component ‘B’ would get hot. The other thing was that the customer, who is a very smart thermal engineer, had already set up everything and he was planning on using these heat sinks that had heat pipes embedded in them. The goal was to try and come up with a heat sink that would do the same thing, hopefully without requiring the heat pipes.

JO: Can we talk for a second about the application? You mentioned that airflow was from either side, the board was going to be used in a data center or a telecom node?
VB: It was for a telecom company.

JP: Was there a reason he didn’t want to use a heat pipe?
VB: I think probably cost and reliability. We use heat pipes embedded in the heat sinks too, so it’s not a something we never want to use, but the client wanted to throw that at us and see if we had alternatives.

JP: Can you take us through the board and the challenges that you saw?
VB: As you can see from this slide, there are four main components and two of the hottest ones are on the edge. Airflow can be from right to left or left to right, so which one would be the worst-case scenario?

Telecom PCB

JO: From right to left, I think?
VB: Correct. This one is a straightforward one to figure out because not only is the component smaller but the power is also higher. Even though [air] can go both ways, there’s a worst-case scenario.

This was the customer’s idea – a straight-fin heat sink with a heat pipe and he put one block of heat pipe in there instead of two or three heat pipes that would normally be embedded in there. You can clearly see what the goal was. You have a small component in here, you want to put a large heat sink over the top and you want to spread the heat throughout the base of the heat sink. All the other components are also occupied by straight-fin heat sinks.

JO: Okay, at this point in the analysis, this is the rough estimate of the problem that you face?
VB: This is a straightforward project in terms of problem definition, which can be a big issue sometimes. This time problem definition was clear because the customer had defined the exact heat sink that they wanted to use. It’s not a bad heat sink they just wanted an improvement, cost-wise, reliability-wise.

This is the G600, which is the air going from left to right. The two main components are represented here and we want to make sure that the junction temperatures that the CFD calculated is lower than the maximum junction temperatures allowed, which they were. These heat sinks work. As we always like to do at ATS, we like to have two, independent solutions to verify any problem. That was the CFD result but we also did the analytical modeling to see what these heat sinks are capable of and the percent difference from CFD was less than 10 percent. Twenty percent is the goal typically. If it’s less than 20 percent then you know you’re in the ballpark.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JO: Do you use a spreadsheet to do these analytical modeling?
VB: HSM, which is our heat sink modeling tool, and then for determining what velocity you have through the fins, the correct way of doing this is to come up with the flow pattern on your own. You go through all the formulas in the book and determine what the flow will be everywhere or figure out what CFD is giving you for the fan curve and check it with analytical modeling. You can look at pressure drop in there, look at the fan curve and see if you’re in the ballpark. You can also check other things in CFD, for example flow balance. Input the flow data into HSM and it will spit out what the thermal performance is for any given heat sink. HSM calculations are based on its internal formulas.

JO: We effectively have a proprietary internal tool. We’ve made a conscious decision to use it.
VB: To actually use it is unique. Not everybody would use it. A lot of people would skip this step and go straight to CFD. We use CFD too but we want to make sure that it’s on the right path.

JP: What do you see as the benefit of doing both analytical and CFD modeling?
VB: CFD, because it’s so easy to use, can be a tool that will lead you astray if you don’t check it because it’s very easy to use and the software can’t tell you if your results are accurate. If you do any calculation, you use a calculator. The calculator is never going to give you a wrong answer but just because you’re using a calculator doesn’t mean that you’re doing the math right. You want to have a secondary answer to verify that what you did is correct.

JP: What was the solution that you came up with for this particular challenge?
VB: We replaced these heat sinks with the heat pipe with maxiFLOW™, no heat pipe needed. One of the little tricks that I used was to off-set the heat sinks a little bit so that these fins are out here and so the airflow here would be kind of unobstructed. And I set this one a little lower so it would have some fins over here, not much, that would be unobstructed. The G600 configurations worked out with the junction temperatures being below what the maximum requirement was without having to use any heat pipes for the main components. There is also a note showing that one of the ancillary components was also below the max. Analytical modeling of that was within 10-11 percent.

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

As you noted, this was the worst-case scenario, going from right to left and you can see because it’s the worst-case scenario this tiny little component here that’s 14 watts that’s having all this pre-heated air going into it, it’s junction temperature was exactly at the maximum allowed. That’s not entirely great. We want to build in a little bit of margin but it was below what was needed.

The conclusion here was that maxiFLOW™ was able to provide enough cooling without needing to use the heat pipes and analytical calculation agreed to less than 20 percent. We would need to explore some alternate designs and strategies if we want to reduce the junction temperature even further because that close to the maximum temperature is uncomfortable. The other idea that we had was to switch the remaining heat sinks, the ones in the middle, which are straight fin, also to maxiFLOW™ to reduce pressure drop and to get more flow through this final component.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JP: If you have an idea like that, is it something that you broach with the customer?
VB: They really liked the result. If this was a project where the customer said, ‘Yep, we need this,’ then we would have said here’s the initial result and we have an additional strategy. At that point the customer would have said, ‘Yeah this is making us uncomfortable and we need to explore further’ or they would have said, ‘You know what? Fourteen watts is a max and I don’t know if we’ll ever go to 14 watts or the ambient we’re saying is 50°C but we don’t know that it will ever get to 50°C so the fact that you’re at max junction temperature at the worst-case scenario is okay by us.’

JP: Do you always test for the worst-case scenario?
VB: It’s always at the worst-case scenario. It’s always at the max power and maximum ambient temperature.

JP: Was this the first option that we came up with, using maxiFLOW™? Were there other options that we explored?
VB: Pretty much. The way that I approached it was doing the analytical first. You can generate 50 results from analytical modeling in an hour whereas it takes a day and a half for every CFD model – or longer. These numbers here were arrived at with analytical modeling; the height, the width, the top width, were all from analytical modeling, base thickness to measure spreading resistance, all of that was done on HSM and spreadsheets to say this will work.

JP: Do you find that people outside ATS aren’t doing analytical?
VB: No one is doing it, which is really bad because it’s very useful. It gives you a quick idea if it’s acceptable, if this solution is feasible.

To learn more about Advanced Thermal Solutions, Inc. (ATS) consulting services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].

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Posted in Analysis, CFD, Consulting, heat sink, PCB, Thermal Analysis

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Attaching Heat Sinks with Push Pins

Posted on June 1, 2016 by | Leave a comment

Heat Sink with push pin attachment and maxiFLOW fins

In certain conditions, lightweight heat sinks can be mounted to hot components with thermally conductive adhesive tape.

But, many heat sinks need a mechanical attachment system for optimum thermal performance and security. These systems typically feature metal and/or plastic hardware, along with a high performance TIM (thermal interface material).

Several attachment systems are available, and one way to categorize them is by whether or not the circuit board becomes part of the solution. For example, will holes be drilled into the board for mounting pins or anchors to help clamp down the heat sink?

If such holes can be safely added around a component, the most versatile heat sink attachment method is push pins. These are now used with many commonly available heat sinks. The sinks have integral holes that align with standard PCB locations. Each pin has a pointed barb end that attaches permanently through the drilled hole. A wire spring on the pin adds a continuous compressive force.

Push pin type heat sinks provide many options for a wide variety of conditions under which electronics are deployed.  They come in a range of material and lengths, as well as choices of springs.

Common push pin material options include:

  • Plastic push pin
  • Brass push pin
  • Stainless Steel PEM

Plastic Push Pins are useful for applications where the push pin heat sink attachment should not conduct heat or electricity. They are a good choice when weight is a critical design factor.  Plastic is also a good option when water or high humidity conditions can occur. Corrosion and chemical resistance are two key advantages of plastics. As with any plastic fastener, the plastic itself has to be particularly robust in order to handle the strain of fastener insertion and subsequent high stress around the pin.

plastic push pins to attach a heat sink to a PCB

Thought should be given to the material type of the pin and the plating used in the PCB through hole that will sheath the fastener when you attach the heat sink to the PCB.  Depending on what material is used, that material will have a CTE (co-efficient of thermal expansion) that needs to be matched to the attachment being specified.

Brass push pins are useful for applications that are corrosive, high heat, and require a strong, durable, material for attachment.  Brass can also be used in situations where it is important that sparks not be struck, as in fittings and tools around explosive gases. Brass attachment should not be used in environments that include ammonia or that release ammonia as this compound can cause stress corrosion cracking in brass.

Brass can often be cheaper than the same attachment in stainless steel since brass costs much less to machine.  Brass is a reasonably good conductor of heat as well (109 W/(m KM)), increasing the overall thermal management of an application where it used to secure a heat sink.

brass push pin attachment for heat sinks being mounted to a PCB

And, push pin fasteners cost less than metal PEMs, which can be similarly used to mount heat sinks via PCB holes.

Screwed in PEM fasteners are perfect for applications where there is only a plain, round hole. They provide high push-out and torque-out resistance. The holes for these fasteners do not need to be specially prepared by deburring or chamfering.  PEMs are also good for meeting DFMA requirements because there are few parts to handle and few assembly steps. Because many of the PEMs used in heat sink applications are made from stainless steel, they have good corrosion resistance, strength and fabrication characteristics.  Like brass, stainless steel is excellent for use in corrosive environments.  But stainless steel’s low thermal conductivity (16 W/(m KM) means that in applications where the heat conduction of the heat sink attachment must be as low as possible, while still providing corrosion resistance and strength, stainless steel can be a reasonable choice.

push pin attachment schematic showing length

brass and plastic push pins side by side comparison

The right length for a push pin is determined by the combined thickness of the heat sink base, the hot component, thermal interface material (TIM) and the thickness of the board.

The other variable is the choice of compression springs, an essential feature on push pin fasteners. Springs add the force needed to hold the assembly together. They’re sized for the length of the pin. Here, length refers to the space between the bottom of the heat sink and the top of the PCB. Overall height refers to the length of the pin, from is barbed tip to the top of its flat head. For ATS brass push pins, overall heights for brass push pin sizes range from 9 to 20mm. Plastic push pins are a standard 7.3 mm in length.

stainless steel springs for push pin heat sink attachment

Spring Choices

Wire compression springs come in choices of size (diameter and length) and material type. The pin length dictates the free length of the spring, but its solid length – when fully compressed, varies by the spring’s diameter and its material. The basic material choices are music wire, a commonly used carbon steel alloy, and stainless steel 302 wire. The music wire has a standard zinc plated finish, and the stainless steel wire has a passivated finish per ASTM A967.

The compressive force for achieving the solid length is determined by the combination of the spring’s free length, wire diameter and its inside and outside coil diameters. For ATS push pin springs, compression requirements range from 0.211 up to 3.543 lbs/mm. The final spring choice should provide a force that meets the performance needs of the TIM, and does not cause undo upward force on the component or on the PCB itself. Too great an insertion force can result in the die cracking and consequent component failure.

Installing Push Pins

All push pins feature flexible barbs that lock securely into PCB holes. The location of the holes in the heat sink will determine where holes must be drilled into the board. Industry standards for these locations are readily available for board designers or from ATS. The required hole diameter for all ATS push pins is 3.175 mm

Each push pin has a flexible barb at its install end that engages with the bottom of the hole in the PCB; once installed, the barb securely retains the pin. The compression spring holds the assembly together and maintains contact between the heat sink and component.

Pre-Load Advantages

Push pin springs add a pre-load pressure on the TIM in the completed assembly. Pre-load is the force holding the sink/TIM/component assembly together before the component is operating. Once the component heats up, a phase-change TIM will turn liquid (from a waxy solid) to increase thermal transfer. The push pins’ permanent pre-load pressure helps optimize the TIM’s thermal transfer performance with every power up and resulting TIM phase change.

Attachment Using PEMs

Push pin fasteners cost less than metal PEMs, which can be similarly used to mount heat sinks via PCB holes. However, PEMs have some advantages.

PEMS for mounting heat sinks to a PCB Board

Screwed in PEM fasteners are perfect for applications where there is only a plain, round hole. They provide high push-out and torque-out resistance. The holes for these fasteners do not need to be specially prepared by deburring or chamfering.  PEMs are also good for meeting DFMA requirements because there are few parts to handle and few assembly steps. Because many of the PEMs used in heat sink applications are made from stainless steel, they have good corrosion resistance, strength and fabrication characteristics.  Like brass, stainless steel is excellent for use in corrosive environments.  But stainless steel’s low thermal conductivity (16 W/(m KM) means that in applications where the heat conduction of the heat sink attachment must be as low as possible, while still providing corrosion resistance and strength, stainless steel can be a reasonable choice.

References for this post:

  1. Canadian Centre for Occupational Health and Safety, “Non-Sparking Tools”, http://www.ccohs.ca/oshanswers/safety_haz/hand_tools/nonsparking.html
  2. Thermal conductivity of material, Engineering Toolbox  http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
  3. Machine Design, “Comparing Brass and Stainless Steel Inserts”, http://machinedesign.com/materials/comparing-brass-versus-stainless-steel-threaded-inserts
  4. ECN Magazine, “The Art of Using Plastic Instead of Metal”, https://www.ecnmag.com/article/2005/04/art-using-plastic-instead-metal
  5. Mechanical Design, “Joining Plastic”, http://machinedesign.com/fasteners/joining-plastic
  6. PEM, The Self Clinching Fastner Handbook, http://www.pemnet.com/fastening_products/pdf/Handbook.pdf
  7. Angelica Spring, “Stainless Steel Music Wire”, http://angelicaspringcompany.com/index.php?Stainless%20Steel%20Music%20Wire – See more at: http://www.coolingzone.com/index.php?read=539&onmag=true&type=press#sthash.DtkLI2ig.dpuf
  8. Design Guidelines for the Selction and and Use of Stainless Steel  https://www.nickelinstitute.org/~/Media/Files/TechnicalLiterature/DesignGuidelinesfortheSelectionandUseofStainlessSteels_9014_.pdf

Brass, Plastic, and PEM Push Pin Heat Sink Attachments Offer the Right Solution for Almost Any Environment and Application http://www.coolingzone.com/index.php?read=539&onmag=true&type=press

Temperature Cycling Fatigue Electronics  (plated through hole fatigue)
http://www.dfrsolutions.com/white-papers/temperature-cycling-fatigue-electronics/

Optimizing thermal and mechanical performance in PCBs: http://www.smtnet.com/library/files/upload/712mangroli

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Posted in heat sink, Heat Sink Attach