Editor's Note: This article is from Chapter 4 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.

Earlier chapters primarily focused on the different methods of using metal to enable improvements in thermal management. It is also important to understand that apart from the thermal management function, the metal can also act as a grounding layer and, therefore, a thermal and electrical connection between the circuitry and the metal. At a high level, there are two ways to achieve thermal management of PCBs utilizing metal: the first is pre-bonded, and the second is post-bonded (Figure 4-1).

In a pre-bonded circuit board, the PCB supplier buys the laminate material pre-bonded to the metal. Most of the available high-frequency laminate materials can also be bought in a pre-bonded configuration. The two laminate suppliers that have most of this market are Rogers and Taconic (now AGC Nelco). The PCB manufacturer is then tasked with processing this material and making circuits and machining the metal. In a post-bonded circuit, the PCB supplier manufactures the PCB and the metal separately and then bonds the two together using a variety of methods.

There are a number of pros and cons to each of these two methods. Pre-bonded PCBs are typically used in high-reliability, military, aviation, and telecom applications since they offer precise dielectric constant (Dk) control, no risk of delamination, and high reliability. The two disadvantages of this methodology are that it is restricted to a single layer of circuitry and, in general, costs tend to be significantly higher with pre-bonded vs. post- bonded. The reasons that the costs are higher is because the laminate materials are significantly more expensive, processing is more challenging, and any yield issues result in very expensive scrap. There are cases where pre-bonded PCBs can be converted to post-bonded PCBs for cost reduc- tion. Any multilayer applications that require metal for thermal manage- ment will need to utilize post-bonding.

Pre-Bonded Laminates

There are several design parameters that need to be considered. The first step is to determine the dielectric material, dielectric thickness, and the copper foil weight that is required. The next step is to determine the thick- ness and type of metal (Figure 4-2). The typical metals used are aluminum 6061-T6 and copper C110, but we have also occasionally seen brass used. Aluminum is usually lighter and cheaper than copper, and each has their own set of properties (Table 4-1). The pre-bonded metal thickness should be at least three times the dielectric thickness to minimize warpage.

In pre-bonded applications, PCB processing is more difficult with aluminum vs. copper. The laminate manufacturer uses lamination temperatures that are hot enough to anneal the aluminum. The 6061-T6 aluminum permanently softens and is more difficult to machine. Once you determine laminate selection, you really should check with the PCB supplier or the laminate manufacturer that your selection is available as you have it configured. Depending on machining requirements, one may have to start with a metal thickness that is higher than the finished thickness.

Post-Bonding

In a post-bonded application, a double-sided PCB or a multilayer PCB is first manufactured. Typically, the bottom side is mainly a ground layer that may have a few circuits integrated. While the PCB is being manufactured, the metal can be simultaneously machined on a CNC machining center. There is a lot more flexibility in terms of the shape and features in a post-bonded application, as it is processed separately from the PCB. The metal can then be plated.

Once the PCB and metal are completely manufactured, they can be bonded together (i.e., post-bonded). This is typically done in piece form (a single PCB bonded to a single fabricated metal). Interestingly, we have seen several applications where multiple PCBs may be bonded to the same piece of metal. These different PCBs may or may not be built with the same dielec- tric materials or thickness.

Some of the advantages of post-bonding include:

• Ease of utilizing multilayer structures
• Simplified processing (PCBs manufactured conventionally)
• Metalwork milled and plated conventionally
• Concurrent processing (boards made separately, as well as carriers/ pallets/heat sinks manufactured separately)
• Yield (good board joined to a good carrier)
• Cost (reduced through standard/simplified processing)

The PCBs are post-bonded using sweat solder or sheet film adhesive. In both techniques, some sort of a bonding fixture is required to ensure that there is good registration between the PCB and the metal carrier and that the correct amount of pressure is being applied in the bonding process. Table 4-2 shows the requirements for bonding registration holes. Figure 4-3 shows two different examples of how the registration holes can be managed and the impact on overall registration.

Most often, the bottom layer is primarily a ground layer and has very little or no solder mask. Sweat solder and sheet film adhesive are two of the bonding techniques that will be explored in the next section.

Sweat Solder

High-temperature solder is used to bond the PCB to the metal. The solder and temperature used are such that there is no risk of de-bonding in subse- quent assembly operations. One of the biggest discussions associated with this technique is void volume. Since the solder paste is a mixture of flux plus solder, the flux creates air gaps as it volatilizes. Typically, we screen the solder paste, but in certain high-volume applications where the void volume control is essential, we have also used solder preforms—which is a more expensive method—to better control the void volume. Figure 4-4 shows how a sweat solder board utilizing solder paste is assembled.

Critical Design Factors

Critical design factors for sweat solder include the following:

• Solder/paste selection: It is important to ensure that the solder utilized does not de-bond in subsequent component assembly operations. Some of the options that we have successfully used on sweat soldering applications include:
  • Eutectic tin/lead
    • Sn63/Pb37
    • Melting point of 183°C (361°F)
  • SAC305 (ROHS-compliant)
    • Sn96.5/Cu3.0/Ag0.5
    • Liquidus temperature of 220°C (428°F)
    • Solidus temperature of 217°C (422°F)
  • Tin antimony (ROHS-compliant)
    • Sn95/Sb5
    • Liquidus temperature of 240°C (464°F)
    • Solidus temperature of 235°C (450°F)

• Sweat solder stencil design: The stencil to screen solder on the PCB or metal must be custom-designed to minimize voids, especially in the critical areas. The PCB fabricator will design this. It is preferred to screen paste on the metal; however, depending on the features of the metal, this may not be practical. In those cases, the solder is screened on the PCB

• Sweat solder fixture design: This is also custom-designed by the PCB fabricator to help minimize solder voids and ensure good registration between the PCB and the metal. It is important to work with a fabricator that has a lot of experience with custom designing both stencils and fixtures

• Metal choice: Typical choices are aluminum, copper, and occasionally brass

• Metal surface finish: If aluminum is the metal that is being used, it needs to be plated at least on the side that is being soldered since it is not possible to solder to bare aluminum

• PCB surface finish: The following surface finishes are preferred—Ag, Au, Sn, and bare copper

• Hot vias on the ground layer: If there are either hot vias or circuit lines on the bottom layer, they should be covered with solder mask. In addition, the metal should either be partially milled out or cut out completely. This helps prevent opportunities for shorts