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Thermal PCB

RF PCB Thermal Management Fabrication Methods

September 5, 2021

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

 

 


Sheet Film Adhesive

There are two main types of silver-filled conductive films: epoxy and silicone.

     1. Silver-filled conductive epoxy films: These are commercially avail- able, and some examples are Henkel CF3350 and Ablefilm 5025E, and                       Rogers COOLSPAN®. The PCB and metal are bonded using temperature and pressure with a sheet film adhesive.

     2. Silver-filled conductive silicone films: These are patented ASC materials and have some differences from the commercially available materials.               The PCB and metal are bonded using temperature and pressure with a sheet film adhesive.

Figure 4-6 shows how a sheet film bonded assembly is put together.

Critical Design Factors

Critical design factors for sweat solder include the following:

  • Adhesive selection: There are several different options that are avail- able, including ASC’s patented material Electrasil-2. Table 4-3 compares Electrasil-2 to commercially available sheet film adhesives from Rogers (COOLSPAN TECA) and Henkel (CF3350 and Ablefilm 5025E) and sweat solder as a reference

 
  • Bonding fixture design: This is also custom-designed by the PCB fabricator to help ensure that there is good adhesion between the PCB and metal and 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 bonding fixtures. Figure 4-7 shows a typical setup of how a PCB is bonded to metal using a sheet film adhesive

  • Metal selection: Typical choices are Aluminum 6061-T6, Copper C110, or occasionally brass
  • Metal surface finish: Table 4-4 illustrates the various surface finish options
  • PCB surface finish: Gold and silver tend to be preferred surface finishes. HASL or lead-free HASL are not acceptable surface finishes. It is also important to review the datasheets of the sheet film adhesive or discuss with the PCB fabricator any specifics related to surface finish choices
  • Hot vias on ground layers: 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

 

 



Quality Control of Post-Bonded PCBs

A post-bonded PCB is not a typical PCB, so we wanted to discuss quality control methods that we employ to ensure that the customer gets what they need.

Machining

  • 100% mechanical inspection of every feature (utilizing visual and contact inspection techniques)
  • Visual inspections derived from CAD data (directly from .iges and .stp files, while inspection from .dxf and .dwg files is possible with interpretation)

Sweat Solder

  • X-ray inspection of the solder joint (standard): Each part is inspected for proper reflow and wetting, and void volume is evaluated, not quantified
  • Visual inspection (standard): No excess solder
  • Peel testing (non-standard): Empirical testing of adhesion on production parts
  • Convection reflow (non-standard): Simulation of subsequent convection reflow soldering cycle and validation of product robustness
  • Sonoscan (non-standard): Non-destructive test, but we currently outsource. This is a technique that can be used to get an image of the entire part and look at the void volume

Sheet Film Adhesive

  • Resistance measurements
  • Visual inspection that ensures no excess flow of adhesive material

Embedded Copper Coins

Coin technology is quickly becoming a preferred alternative to internal heat sinks to draw heat directly down and away from the heat-generating device to the backside of the PCB. The phrase “press fit coin” gets used quite often when discussing various embedded coin applications. The fact is that most practical applications where coins are employed use a coin that is bonded into the structure during the multilayer lamination process. In this case, the coin is bonded into place and sealed by the flow of the prepreg resin, which is adjacent to the coin at the time of lamination. The result is a securely mounted but electrically isolated coin. Either ground via structures are added through a flange in the coin, or the cap plating on the top and bottom of the coin provides the grounding connection.

We have utilized a few different kinds of coins: a center-flanged coin, a bottom-inserted coin (or a T-coin), a coin that does not go through all the layers, a U-coin, and a serrated coin. It is important to understand that every embedded coin part number tends to be a unique engineering project for a PCB board fabricator. Figure 4-9 displays a micro-section of an ASC PCB with a flange coin.

 

Figure 4-10 shows different types of coins. When designing, it is always preferred that the top and bottom of the coin be in a positive or near- neutral position relative to the top and bottom of the PCB. On the bottom, this ensures that intimate contact can be made with any external heat sinking. Being on the top ensures that the device typically mounted on top of the coin can be adequately solder paste printed and soldered properly to the electrically and thermally conductive structure. When you consider both aspects, it is self-explanatory that a negative condition, one with either the top or bottom of the coin recessed within the circuit board, is less than ideal. Tolerances need to be made to allow for reasonable manufacturing yields. In these cases, a preferred tolerance would be ranging from a slight negative condition of -0.0005” or -0.5 mil to a positive condition approaching +0.002” or +2 mils.

Early engagement and ongoing communication with your fabricator are essential to settle on a specific mass of coin required for the thermal dissipation necessary for device operation within the design temperature requirements. The fabricator will need to adjust the actual coin size slightly to ensure proper fitment and clearance within the pocket in the multilayer. This will also ensure proper prepreg resin fill around the coin as well as ensure that the surface flow of resin is minimized.

Reducing resin flow onto the top surface is important when using thinner copper foils on the top and bottom layers since planarization (sanding or disk grinding) is the most common method for removing excess resin flow. When fine lines and spaces are employed, there is always a tendency to use thinner copper to ease manufacturing. Half-ounce copper will, of course, be more susceptible to damage during planarization than one-ounce copper foil. One-quarter ounce copper foil will be virtually impossible to clean mechanically, so chemical or plasma methods will need to be used—both of which are capable of removing only very slight amounts of resin from the surface.

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