How to choose the right heat sink?

• Heat sink functions and features
• How to choose a heat sink?
• Heat sink types and design
• Select a heat sink from TME’s offer
• ATS heat sinks at TME

The TME heat sink catalogue includes hundreds of products in various designs and for many different applications. Among them, ATS heat sinks constitute a large group. The offer of this American manufacturer is very wide and varied in terms of materials, shapes, and heat sink attachment method.

Heat sinks functions and properties

As you know, heat sinks are used to dissipate excess heat energy. They are usually made of metals such as aluminium or copper (due to its increasing price, this material is less and less common) and are part of a combined passive cooling system, which means combining them with heat pipes or or Peltier cells. However, sometimes you will find hybrid solutions where passive cooling heat sinks are combined with active cooling systems, such as fans. Such solutions are introduced when the cooling performance of the heat sink is insufficient – e.g., because the temperature difference between the heat sink and the surrounding air is too low- and requires the use of forced air circulation.

Typically, heat sink fins create a large heat dissipation surface. The thermal energy is transferred from the cooled component through the contact surface, which is why this surface should be sanded as fine as possible. For better adhesion, silicone pastes are often used, which improve thermal conductivity and, if necessary, act as an insulating element for the heat sink. Spacers made of silicone, ceramic, or mica (additionally lubricated with silicone paste) are also used.

Finned heat sink

As heat sinks benefit from convection, many are placed in customised enclosures with air intakes at the bottom and top, positioned vertically to make the best use of convection. This greatly increases airflow and often eliminates the need for a fan, because the hotter the heat sink gets, the faster air will pass through it to dissipate heat. However, it should be remembered that the cooling performance depends largely on the temperature difference between the heat sink and the environment (the greater the difference the better) and if it is not sufficient, then even the largest possible fan may not solve the problem. For this reason, additional ventilation of the entire space in which the heat sink operates is often required.

An additional note regarding the thermal emissivity of aluminium is necessary at this point. Although this metal is a good conductor of heat, it does not perform so well in the field of emissivity. Therefore, very often the surface of aluminium heat sinks is covered with an anodised coating (in various colours, most often black or blue), which significantly increases thermal emissivity of a heat sink – even by 100%.

Heat sinks have many advantages but are not ideal. The advantages include silent operation, no need for electricity and no moving parts that could wear out. All this means almost 100% failure-free operation of these components. On the other hand, the biggest disadvantage is often insufficient cooling power in the case of delicate, highly heated elements. This is quite a common situation, which is attempted to be improved by forcing an adequate air flow through the heat sink – which in itself can be considered a disadvantage, as it implies the introduction of fans, devices with a much higher degree of unreliability. A common obstacle is the size of the heat sink, which has to be large and therefore heavy to fulfil its task, which can hardly be considered an advantage. Admittedly, the use of a fan makes it possible to reach for a smaller heat sink, but this is not always a satisfactory solution. However, it should be remembered that practice shows that many problems are not caused by the heat sinks themselves, but by incorrect selection, which is discussed in the next section.

How to choose a heat sink?

CHOOSING THE RIGHT HEAT SINK requires a few calculations according to simple formulas derived from Ohm’s Law. To do this, you need the following data:

• The power that will be lost,
• Junction-to-case thermal resistance of the cooled component (should be provided by the manufacturer of the element),
• Case-to-heat sink thermal resistance (estimated taking into account the type of contact, number of insulating spacers, etc.),
• Heat sink-to-ambient thermal resistance (provided by the heat sink manufacturer – usually it is the best for blackened and copper heat sinks),
• The lowest possible temperature difference between the component and the environment.

Of course, the power of the natural convection heat sink, i.e., with unforced air flow,  is calculated differently  than in the case of heat sink fitted with a fan or cooling tubes that carry a liquid coolant. Regardless of the situation, when selecting heat sinks, one should try to reduce the thermal resistance to which one has access, with the resistance between the heat sink and the environment at the forefront. Ensure:

• as much heat sink surface area as possible,
• the fastest possible flow of the coolant (air, water in the pipes),
• as much coolant heat capacity as possible. Compared to air, water has a much higher heat capacity, which allows a greater temperature difference between the heat sink and the coolant, and therefore ensures faster heat transfer from the heat sink.

When choosing a heat sink, do not ignore basic parameters including:

• type of mounting socket – the heat sink must fit it,
• heat sink dimensions – it must fit in a fixed space,
• generated noise level – the lower the better.

Heat sink types and design

The basic classification of heat sinks has already been outlined. According to it, we can distinguish:

• Passive heat sinks– perforated, made of aluminium, including straight fin, pin fin, or slant fin heat sinks. They rely solely on passive dissipation of heat;
• Active heat sinks with a fan– These are the above-described designs, equipped with a fan forcing intensive air flow through the heat sink structure;
• Active heat sinks with heat pipes– usually made of aluminium fins, inside which heat pipes collect heat from the cooled component and dissipate it to the fins, which then release it into the air (this is often supported by a fan);
• Active heat sinks with water cooling– similar to those described above, but here the heat is dissipated mainly by the pump forced liquid circulation and controlled by an expansion tank. They are optimal for high-power semiconductors, densely arranged on the board.

When it comes to heat sink profiles, we can also distinguish between standard heat sinks – produced in large series using the extrusion method and commonly used in many applications – and non-standard, made to order, heat sinks, machined from a solid block of aluminium.

From the point of view of the shape of heat sinks, you can most often find a simple division into flat heat sinks, “U” (as well as “Y” or “H”) type heat sinks and finned heat sinks, although it is possible to distinguish many more subtypes in each of these three categories. This diversity in shapes results from the need to mount heat sinks on various types of components, such as diodes, relays, transistors, and even entire modules and specific models of commonly used enclosures. Adjusting their shape to the above-mentioned components is aimed both at optimizing the heat dissipation process (thanks to the largest possible contact surface) and simplifying the assembly itself.

Choosing a heat sink from TME’s offer

The TME catalogue currently includes several hundred different heat sinks. The search filters available make it easy to select a model with specific dimensions, base thickness, and application to suit your needs. The search engine allows you, among others, to select a heat sink according to its shape (finned, U-, Y-, or H-shaped), material used for its production, surface finish (anodized or un-anodized), as well as based on the product’s thermal resistance or its installation method (screwed, soldered, intended for SMT, screw or clip-on mounting). Heat sinks equipped with fans are divided according to supply voltage (12 V and 24 V DC, 230 V AC), which further facilitates browsing. One should also not forget about the ability to browse by manufacturer – TME’s offer includes heat sinks manufactured by many companies, including those from the renowned American company Advanced Thermal Solutions. ATS offers a broad array of heat sink families for cooling various devices. These are BGA, LGA, ASIC, and LED cooling solutions, offered in a variety of fin types, including straight fin, pin fin, and the patented maxiFLOW™ design. The latter is the ATS’ maxiFLOW™ spread fin heat sink design that maximizes convection (air) cooling. All the above-mentioned heat sink families come in multiple attachments options, including the ATS’ maxiGRIP™ and superGRIP™ (provide constant, even pressure from the heat sink to the element, no need to drill holes in the PCB). Customers can choose between high-performance thermal adhesives, push pins, or Z-clips.

ATS heat sinks at TME

The TME catalogue currently includes several hundred different heat sinks manufactured by ATS and grouped according to several criteria, which are used as filters in the above mentioned TME search engine. A brief analysis of the most important of them makes it much easier to navigate through this large group of ATS products.

• Material: Regarding the material used for the heat sink, TME has distinguished aluminium heat sinks (more than 400 models), copper heat sinks, and heat sinks made of the patented Saint-Gobain C1100For a replacement material (the manufacturer withdrew the C1100F from production a few years ago).
• Material finish: anodized or tin plated.
• Heat sink shape: based on this filter, customers receive three product groups – Finned heat sinks (96% of the ATS range), a dozen or so “U-type” heat sinks and a few flat ones.
• Heat sink type: there are three product groups, of which the most numerous  (83% of the offer) are extruded heat sinks. The others are mostly attachable heat sinksor heat sinks with case.

• Dimensions: TME’s ATS heat sink range includes products with lengths from 12.7 to 330.0 mm, widths from 12.8 to 172.0 mm, and heights from 2.0 to 26.0 mm.
• Colour: ATS heat sinks are available in black, blue, red, silver, grey, green, and gold.
• Thermal resistance: from 0.5°C through 5.0°C, up to 29.5°C.
• Another important filter in the TME search engine is the heat sink application. Customers most often indicate standard application (BGA), but also have the option to choose from several TO (Transistor Outline 126/202/220/262) or LED heat sinks.
• The manufacturer’s series: this is the last most important filter, which allows you to navigate through the range grouped according to the manufacturer’s series of heat sinks mentioned earlier. These include series such as maxiFLOW (dominant in numbers), maxiGRIP, StraightFin, SuperGrip and several smaller ones.

Text prepared by Transfer Multisort Elektronik Sp. z o.o.

The original source of text:  tme.eu