Category Archives: heat sink

Technical Discussion of ATS Telecom PCB solution

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 ats-hq@qats.com.

Attaching Heat Sinks with Push Pins

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

ATS’ Standard Board Level Heat Sinks for PCB

We’ve just released our new line of standard board level heat sinks. These stamped heat sinks are ideal for PCB application, especially where TO-220 packages are used. Available now through Digi-Key Electronics​ or at this link from ATS http://www.qats.com/eShop.aspx?produc…

 

To play MIT’s Space Invaders Remix, click here https://scratch.mit.edu/projects/1979…

Can The Surface Treatment of a Heat Sink Help Improve it Performance?

Radiation heat transfer is often neglected in thermal design due to its complicated nature and misperceptions about its impact in electronics cooling. However, it turns out that heat radiation can have a significant positive effect on natural convection and especially in low air flow applications. Our new white paper explains how this works and how engineers can take advantage of it. Get the white paper for no cost and no registration at this link here on qats.com

How to Enhance Radiation Heat Transfer of Heat Sinks

Blow Torch, ATS Heat Sink Attachments

This video shows a flame test of the ATS heat sink clips superGRIP and maxiGRIP. These are used in the ATS clipKIT. It shows that the clips do not ignite, a key requirement in Telecomm applications and for general safety.

For more on ATS’s clipKITS, visit http://www.qats.com/Heat-Sink/Attachments/clipKIT