Materials for High Density Interconnects
In this article we will discuss the materials used to manufacture HDI circuits. Several good resources exist on the subject of materials for PCBs (such as the Printed Circuit Handbook edited by Holden & Coombs) so we will concentrate on those materials that are specific to HDI.
TOPICS IN THIS SOLUTION
In this chapter we will discuss the materials used to manufacture HDI circuits. Several good resources exist on the subject of materials for PCBs (such as the Printed Circuit Handbook edited by Holden & Coombs) so we will concentrate on those materials that are specific to HDI.
The current HDI materials market worldwide was estimated by BPA Consulting Ltd. to be 83 million square meters. The breakdown by BPA Consulting of the eleven (11) HDI materials used, in order of usage:
- Laser-Drillable Prepregs-40.4%
- Conventional Prepregs-17.2%
- Photo Dry film-0.1%
- Photo Liquid-~0.0%
The major material components of PCBs are the polymer resin (dielectric) with or without fillers, reinforcement, and metal foil. A typical construction is shown in Figure 1. To form a PCB, alternate layers of dielectric, with or without reinforcement, are stacked in between the metal foil layers.
The majority of the materials are epoxy, but some are BT, PPE, cyanate ester, and modified acrylates. The newest materials are the growing number of laser-drillable prepregs.
FIGURE 1. Construction of a PWB laminate [Source: PC Handbook,7th Ed]
The backbone resin of the industry has been the epoxy resin. Epoxy has been a staple due to its relatively low cost, excellent adhesion (both to the metal foils and to itself), and good thermal, mechanical, and electrical properties. As demands for better electrical performance, ability to withstand lead-free solder temperatures (see Table 1), and environmental compliance have entered the picture, the basic epoxy chemistry has been dramatically changed over the years.
Epoxies are thermosetting resins and use hardeners and catalysts to facilitate the cross-linking reactions that lead to the final cured product. Epoxies are also inherently flammable, so flame retardants are incorporated into the resin to greatly reduce the flammability. Traditionally, the main curing agent was Dicy, but now various phenolic compounds are used. The traditional bromine compounds (i.e., TBBA) used as flame retardants are being substituted with other compounds such as those containing phosphorous because of concerns about bromine getting into the environment when the PCBs are disposed of. Many companies have gone to a “Halogen-Free” requirement in anticipation of an eventual ban or for the appearance of being “green.”
FIGURE 2. Some STII values of common laminates.
Other resins that are in common use are typically selected to address specific shortcomings of epoxy-resin systems. BT-Epoxy is common for organic chip packages due to its thermal stability, while polyimide and cyanate ester resins are used for better electrical properties (lower Dk and Df) as well as improved thermal stability. Sometimes they will be blended with epoxy to keep costs down and improve mechanical properties. An important thermal property for lead-free assembly is the STII and some laminates values are seen in Figure 2.
Besides thermosetting resins, thermoplastic resins are utilized including polyimide and polytetrafluoroethylene (PTFE). Unlike the thermoplastic version of polyimide which is relatively brittle, the thermosetting version is flexible and is supplied in film form. It is typically used to make flexible circuits as well as the combination circuits called rigid-flex. It is also more expensive than epoxy and is only used as needed.
FIGURE 3. Laminate substitution chart for many PWB laminates
Laser-Drillable and Conventional Fiberglass
Most of the dielectric materials that are used to make printed circuit boards incorporate reinforcement into the resin system. Reinforcement usually takes the form of woven fiberglass. Woven fiberglass is just like any other cloth, made up of individual filaments that are woven together on a loom. By using different diameter filaments and different weave patterns, different styles of glass cloth are created.
Fiberglass adds both mechanical and thermal durability to the dielectric, but it does present some problems when used in HDI constructions. Figure 5 shows that glass fabric is woven, and the table shows the styles, yarns, and thicknesses of those yarns. When lasers are used to create the vias, the difference in ablation rates between the fiberglass and the surrounding resin can cause poor hole quality. Also, since the fiberglass cloth is not uniform due to having areas with no glass, areas with one strand, and the intersections of strands (also known as knuckles), it is difficult to set up drilling parameters for all these regions. Usually the drilling is set up for the hardest to drill region which is the knuckle area.
The fiberglass manufacturers have created so-called laser-drillable dielectrics by spreading the yarns in both directions and making the fabric more uniform which minimizes the areas with no fiberglass as well as the knuckle area. Figure 4 shows the 12 currently available LDPs and their properties. It still takes more energy to penetrate the fiberglass than the resin, but now the drilling parameters can be optimized to get consistent results throughout the panel.
FIGURE 4. Table of cloth specifications for laser-drillable fiberglass.
Resin Coated Copper (RCC) Foil
The limitations of fiberglass-reinforced dielectrics prompted companies to look at alternative dielectric solutions. In addition to the problems with laser drilling (poor hole quality and long drilling times), the thickness of woven fiberglass limited how thin the PCBs could be. To overcome these issues the copper foil was utilized as a carrier for the dielectric so it could then be incorporated into the PCB. These materials are called “Resin Coated Copper’ or RCC. RCC foil is manufactured using a roll to roll process.
FIGURE 5. Photos of standard and laser-drillable fiberglass cloths.
The copper goes through a coating head and the resin is deposited on the treated side of the copper. It then goes through drying ovens and is partially cured or “B” staged which will allow it to flow and fill in the areas around the internal circuitry and bond to the core. The resin systems are usually modified with a flow restrictor to prevent excessive squeeze-out during the lamination process.
Most of the RCC foil is manufactured this way, but additional types exist. One of these types is a two-stage product (Figure 6 ). After the first resin layer is coated, it is put through the coater again to add a second layer. During the second coating the first layer is fully cured, while the second layer is “B” staged. The benefit of this process is that the first stage acts like a hard stop and guarantees a minimum thickness between layers. The disadvantage is that the product is more expensive than the single coated version.
For all the benefits of RCC foil, there are concerns over the lack of reinforcement in terms of dimensional stability and thickness control. A new material was developed to address these concerns. MHCG from Mitsui Mining and Smelting incorporates an ultra-thin fiberglass (either 1015 or 1027) during the resin-coating process. The fiberglass is so thin that it cannot be made into a prepreg since it cannot go through a treater tower like traditional fiberglass. There is also a polyimide / epoxy RCC available.
The fiberglass does not impact laser drilling significantly, yet it provides dimensional stability equal to or better than standard prepreg. Dielectric layers as thin as 25 microns are now available allowing for very thin multilayer products.
Cost is another aspect of RCC foil that is of concern. RCC foils almost always cost more than the equivalent prepreg/copper foil combination. However, the RCC foil can actually result in a less expensive product when laser-drilling time is taken into consideration. As the number of holes and size of the area increase, the improved throughput of the laser drills more than offsets the increased cost of the RCC foil.
FIGURE 6. Four available styles of resin coated copper (foil).
Optimized liquid epoxy can provide the lowest cost of any of the dielectrics for HDI. It is also the easiest to apply in thin layers for fine-line wiring. It can be coated by screen printing, vertical or horizontal roller-coating, meniscus coating, or curtain-coating. The Taiyo Ink brand is the most used but Tamura, Tokyo Ohka Kogyo, and Asahi Denka Kogyo also have products.
Polyphenyl Ethers/Polyphenylene Oxide: M.P > 288° C are thermoplastics of Polyphenyl Ethers (PPE) or Polyphenylene Oxide (PPO) with melting points well over 288°-316° C. PPO/Epoxy blends have a Tg >180° C with higher decomposition temperatures. Their popularity is their excellent electrical performance due to having lower dielectric constants and loss tangents than many of the thermosets like epoxy and BT with low water absorption. Their high melting points and chemical resistance make desmearing a critical process.
Figure 7 displays the dielectric constants (Dk) and dissipation factors (Dj) of popular dielectrics, including those suitable for very high-speed logic. Table 2 lists other electrical characteristics related to high-speed performance for HDI design.
TABLE 2. Other important electrical performance considerations when designing high-speed circuits.
For very high-speed logic, the signals travel on the surface of the conductor (the Skin Effect). Smooth copper foils enable the fabrication of very fine traces and spaces with less copper losses. (See Figure 8) in Figure 9, ultra-fine traces are capable with the 5 micron and 3 micron copper foils, or with a mSAP process.
FIGURE 8. Foil treatment for adhesion comes in four profiles and is important for copper losses (skin effect).
FIGURE 9. Very thin and smooth copper foil can permit very fine traces and spaces
Materials for High Density Interconnects is a serious subject for PCB designers and Electrical Engineers. Several good resources exist on the subject of materials for PCBs and the focus here has been HDI Materials to help the engineer design printed circuit boards.