Newer PCB Materials
If you are hearing about the newer, higher density HDI-PCBs used by Apple called SLP, and their processes like SAP, or uSAP or mSAP (Semi-Additive Processes) recently, these are the printed circuits that are getting down to 30 micron and 25 micron (1 mil) traces and spaces. HDI is now estimated to be over 250 million square meters produced per year-and still growing! But SAP is not NEW! We used it early in the 60’s and 70’s when we were making the epoxy-coated steel boards and when Photocircuits produced full-additive PCBs.
Figure 1 shows a breakdown of the 11 materials currently used for HDI. In order of usage:
- Laser Drillable Prepregs-38.4%
- Conventional Prepregs-19.2%
- ABF Film-5.0%
- Liquid Epoxy-3.3%
- Photo Dry fi lm-0.1%
- Photo Liquid Dielectrics-~0.08%
The major material components of PCBs are the polymer resin (dielectric) with or without fillers, reinforcement, and metal foil. To form a PCB, alternate layers of dielectric, with or without reinforcement, are stacked in between the metal foil layers.
Table 1 shows a current list of HDI materials taken from the Internet. Many 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.
TABLE 1. This survey of HDI materials including thermally cured and photo-cured types contain laser drillable laminates, buildup films, RCF, and liquid dielectrics. Conventional laminates were excluded.
FIGURE 1. Chart of typical PCB dielectrics and their electrical properties.
Besides thermosetting resins, thermoplastic resins are utilized including polyimide and polytetrafluoro-ethylene (PTFE). Unlike the thermosetting version of polyimide, which is relatively brittle, the thermoplastic 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. The polyimide film can be made in several ways and is usually supplied as a completely cured laminate with copper foil on one or both sides. It is also more expensive than epoxy and is only used as needed. Figure 1 shows the range of electric properties of all the common HDI dielectrics. PTFE is valued for its excellent electrical properties and low moisture absorption. It typically has filler added to modify its dielectric constant for specific applications. A typical example is a circuit used in microwave applications where low loss and high Q are needed at very high frequencies. PTFEs are among the most expensive materials, but as frequencies continue to rise and wireless applications grow, their use continues to grow.
The fiberglass manufacturers have been working on ways to minimize drilling problems. They 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 as illustrated in Figure 2.
FIGURE 2. Close-ups of conventional prepregs and laser drillable fiberglass prepregs (Courtesy of NanYa Plastics)
Fabric manufactured with this type of glass is more expensive and only used when necessary. It is also used in conjunction with high-performance resin systems making the resulting dielectric relatively expensive.
Other reinforcements have been used to make PCBs over the years. One product that is no longer available was a chopped aramid fiber paper called Thermount. It was supplied by DuPont and had several good properties. Being a thermoplastic material, its laser ablated more like the resin and since it was a paper, it did not have the issue of knuckles to deal with like woven fabric. It also had a very good dielectric constant which was an advantage for high-speed circuitry. Thermount had some issues with moisture absorption and was relatively expensive. In 2006, DuPont decided to discontinue manufacturing this product. However, Shin-Kobe Electric in Japan still manufactures three different types of aramid laminates and prepreg. Alternatives to this product are still being investigated. In addition to Thermount, other non-woven reinforcements have been used, including a chopped fiberglass paper and an expanded PTFE matrix. The PTFE matrix has been incorporated with a high-performance resin and is available as a product called GorePly. While GorePly has excellent electrical properties, it is expensive and only used when necessary.
Ajinomoto Buildup Film
The Ajinomoto Buildup Film (ABF) is the primary dielectric for SAP. It is a series of very thin film dielectrics made with epoxy/phenol hardener, cyanate ester/epoxy, and cyanate ester with thermosetting olefin (Table 2). The epoxy type is also available halogen-free. The thin film (15 – 100 um thick) is supported by a 38 um PET film and protected by a 16 um OPP cover film. The material is vacuum laminated in special conveyorized machines (Figure 3) using a five-step process:
- Surface preparation of core dielectric and copper
- Dry core (130 C for 30 min)
- ABF auto-cutting, remove cover film, and placement
- ABF vacuum lamination and metal hot-press
- Remove PET film and post-cure (170-190 C for 30 min.)
FIGURE 3. Conveyorized vacuum lamination and hot press for ABF film.
ABF films, like the liquid dielectrics and dry films, must be semi-additively metallized (SAP). The critical steps are desmear, solvent swell, film etch before metallization, etching, and annealing (post-curing). The conditions and steps will determine the resulting copper peel strengths and are covered in ‘Chapter 8—Desmear and Metallization’ of the HDI Handbook.
Photo Dry Film
Photosensitive dry film dielectrics were once thought to be the ultimate HDI dielectric as no external equipment was required to produce the vias. This proved not to be the case, as these were negative acting photosystems. By negative acting, ultraviolet energy was required to cure the dielectric permanently. Any area of the film which is unexposed would develop away. The problem proved to be the cleanliness of the coating and exposure facility. A semiconductor Class-100 Clean Room was the condition required and proved to be too expensive for most fabricators. In Japan, certain OEMs that still have their own facilities continue to use this dielectric. It has worked very well with laser drilling and SAP.
TABLE 2. Ajinomoto Buildup Film characteristics