Editor's Note: This article is from Chapter 2 of the eBook "The Printed Circuit Designer's Guide to...Thermal Management: A Fabricator's Perspective" authored by Anaya Vardya, CEO, American Standard Circuits, and published on iConnect007 here.

Insulated metal PCBs (IMPCB) or metal-clad PCBs (MCPCB) are a thermal management design that utilizes a layer of solid metal to dissipate the heat generated by the various components on the PCBs. When metal is attached to a PCB, the bonding material can either be thermally conductive but electrically isolative (IMPCBs or MCPCBs), or in the case of RF/micro- wave circuits, the bonding material may be both electrically and thermally conductive. The reason that RF designers usually have the bonding material thermally and electrically conductive is that they are using this not only as a heat sink but also as part of the ground layer. The design considerations are quite different for these different applications.

This chapter will focus on the IMPCB design considerations, and Chapter 4 will focus on RF thermal management. We will focus on things designers should be discussing with their PCB supplier to ensure manufacturability and a successful product launch. Since the choices, options, and decisions can be extremely complicated, it is critical to engage early and collaborate with the PCB fabricator about the specific design to ensure the most cost- effective solution.

Some of the more common applications of IMPCBs include:

Power Conversion: Thermal-clad offers a variety of thermal performances, is compatible with mechanical fasteners, and is highly reliable

LEDs: Using thermal-clad PCBs ensures the lowest possible operating temperatures and maximum brightness, color, and life

Photovoltaic Energy: Renewable energy to power telecommunications, military camps, residential and commercial structures, and battery charging stations

Motor Drives: Thermal-clad dielectric choices provide the electrical isolation needed to meet operating parameters and safety agency test requirements

Solid-State Relays: Thermal-clad offers a very thermally efficient and mechanically robust substrate

Automotive: The automotive industry uses thermal-clad boards, as they need long term reliability under high operating temperatures coupled with their requirement of effective space utilization

IMPCB Benefits

• Excellent surface mount cooling
• High electrical isolation, insulation, and thermal dissipation
• Low cost
• Robust thermal performance
• Thermal conductivity of the dielectric in the range of 0.6–8 W/mK
• Manufacturability (integrates with standard PCB processing)

Thermal Properties Explained

A thorough understanding of a number of different thermal properties is needed to be able to design the appropriate IMPCB solution to a thermal condition, including thermal conductivity, thermal impedance, and thermal resistance.

Thermal Conductivity

Measurement of the ability of a substance to conduct heat (W/mK)

A material property, meaning that it does not change when the dimensions of the material change, as long as it is made up uniforml. For example, the thermal conductivity of 1 cm3 of gold is exactly the same as the thermal conductivity of a 100 m3 of gold

Generally obtained in the industry using one of two tests: The D-5470 test, or the E-1461 standard ASTM tests

The D-5470 test measures the thermal impedance in Kcm2/W of the sample and determines the thermal conductivity through the following relation:

Thermal conductivity = Thermal diffusivity * Specific heat capacity * Density

Thermal Impedance

• This is the opposite of thermal conductivity. It is a measurement of the ability of a material to oppose the flow of heat, so from a PCB point of view, we want this value to be low. The lower the thermal impedance, the quicker heat flows through the PCB and to the heat sink where it is dissipated
• Its value depends on the thermal conductivity of the material and its thickness; in other words, this is not a material property, but is an object property, as changing the thickness of the material will change this value. However, changing the area of the material will not change this value (as long as the thickness stays constant)
• For example, the thermal impedance of a sheet of laminate is the same as the thermal impedance of a cut piece of the laminate, say 1 cm2 of it. Whereas the thermal impedance of a sheet of gold of 1-mm thickness is different from the thermal impedance of a sheet of 2-mm thickness 
• This is generally obtained using the D-5470 test mentioned above and relates to the thermal conductivity via the following relation:

Thermal impedance = Thickness / Thermal conductivity

Thermal Resistance

• Thermal resistance (measured in K/W) is basically the same as the thermal impedance. The difference is that it takes into account the area of the sample as well as the thickness and conductivity
• Changing either the thickness, or the area of the material, will change the associated value of the thermal resistance as follows:

Thermal resistance = Thickness / (Thermal conductivity * Sample area)

Single-Sided IMPCBs

In its simplest form, an IMPCB is a piece of copper foil that is bonded to a thermally conductive dielectric and a metal substrate (Figure 2-1). Typi- cally, a PCB supplier can buy the copper foil laminated to the base metal from several different laminate manufacturers. A laminate selector guide is provided in Appendix B.

Some of the key design factors to consider include the following.

Copper Thickness

Typically ranging from 1–6 ounces with 1 and 2 ounces being the most commonly used. The thicker the copper, the more expensive the cost of the PCB.

Thermally Conductive Prepreg

This is one of the most important elements of this construction and what typically differentiates the various suppliers. This is the substance that both electrically isolates the copper circuitry from the main metal and helps with the rapid transfer of heat between the two. It ensures that heat generated by the components is dispersed to the base metal (heat sink) as quickly as possible. The prepreg is typically an organic resin with ceramic fillers to increase thermal conductivity. The filler type, size, shape, and percentage are some of the factors that determine the thermal conductivity performance. The usual ceramic fillers are Al2O3, AlN, BN, etc.

The performance of the various prepregs is measured by the thermal conductivity (W/mK) and thermal impedance (Km2/W). The higher the thermal conductivity, the better the heat transfer, and the lower the thermal impedance, the better the heat transfer. However, it is also important to understand that the better the heat transfer associated with the prepreg, the greater the cost. Therefore, it is critical not to over-design. To put this in perspective, the thermal conductivity of FR-4 is approximately 0.2–.4 W/mK, whereas the thermally conductive prepregs that are available on the market today range from 1–7 W/mK. Apart from thermal conductivity, the thickness of the dielectric can be critical. Typically, the thickness of the dielectric ranges between 2–6 mils, with 3-mil dielectric as the most common.

Base Metal

Aluminum is the most common base metal used. The two most common types are 5052H32 and 6061T6. 5052H32 is typically less expensive and a lot more available than 6061T6. The thickness of the aluminum typically ranges between 40–120 mils, but 40 and 60 mils are the most common thicknesses available.

    Table 2-1: Properties of various base metals.

There are also cases where copper is used as a base metal. Copper is some- times used for better thermal conductivity, mechanical strength, and CTE match to thicker copper foils. In most applications, the thermal advantages of the copper base plate are insignificant because the thermal resistance of the base metal is small relative to the thermal resistance of the dielectric layers and the components. This is a significantly more costly solution, as well as significantly heavier. A brief comparison of the various base metals is illustrated in Table 2-1.

Maximum Operating Temperature

Work with your PCB fabricator and raw material supplier to ensure that the MOT you require is being met by the material selected.

Breakdown Voltage

Ensure that you understand the voltage at which the material dielectric will breakdown and short the circuit. As a general rule, the thinner the dielectric, the lower this value will be.

Panel Utilization

The IMPCB laminate materials are significantly more expensive than FR-materials. As a ballpark, a 0.062” IMPCB material may be three times more expensive than an 0.062” FR-4 material. It is, therefore, extremely important to understand how the board/array designs utilize the production panel. This is another area where early engagement with the PCB supplier is important. The most popular size for a working panel on these materials tends to be 18” x 24.” As many PCBs are processed as arrays, it is critical to ensure that array designs are such that panel utilization is maximized. Many large PCBs may be processed without rails through the assembly operation due to the rigidity of the material. This can vastly help improve panel utilization.

Machining/Fabrication

Scoring is the most common process used for square or rectangular shapes. The advantage of scoring is that it assists in maximizing material utilization since zero spacing is needed between parts to score them. In contrast, routing is the most expensive process since it is slower and requires spacing between parts and will likely reduce material utilization. Make sure that the PCB fabricator has a scoring system that is specifically designed for scoring aluminum. The scoring machine should be equipped with a lubrication system. It is recommended to use diamond-coated scoring blades and router bits when dealing with aluminum base metal.

Solder Mask

There are many single-sided IMPCB designs that are used for LED lighting applications. A majority of these applications require white solder mask. Thus, it is important to address this as all white solder masks are not made equal. A lot of LED customers are looking for consistency in the color of their white solder mask. The marketplace today has several different solder masks that are marketed as LED solder masks.

Figure 2-2: Different colors on two different types of solder mask.