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

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