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Trouble in Your Tank: Understanding the Reactions of Resin Systems with Alkaline Permanganate Processes, Part 1
January 10, 2013 |Estimated reading time: 4 minutes
Editor's Note: This article originally appeared in the August 2012 issue of The PCB Magazine.
Introduction
If at first you don’t succeed, then skydiving isn’t for you! Sure sounds a bit confounding. And with the plethora of new laminate materials on the market today, fabricators find themselves in a quandary as to how they can be successful in processing these materials after drilling and prior to metallization. With these higher-performance materials sporting different cross-linking agents, resin materials and fillers, it is no wonder that the PCB fabricator is encountering difficulty ensuring drill-smear removal and enhancing the adhesion and uniformity of the copper deposit. Understandably, if one quotes the phrase “It’s not your father’s FR-4 anymore,” the fabricator would clearly understand. Materials have changed and so must the expectations of the desmear process and subsequent processing steps that change along with it.
It would be appropriate at this time to review the various laminate materials in order to discern some of their most important differences.
Figure 1: Laminate and laminate materials. (Source: Happy Holden)
Laminates
All told, there are many types of laminate materials available. During the last 10 years or so, several laminate manufacturers have begun doing business out of Asia. The laminate materials themselves are made up of a polymer dielectric resin matrix. The resin is most likely (but not always) reinforced with some type of glass fabric or fibers. To enhance electrical performance, ceramic-filled materials are used. In addition, the resin materials are clad with copper foil on one or both sides. Of course, the copper foil is available in various thicknesses.
Figure 2: The material supply chain.
Various Laminate Materials
There are many types of dielectrics. Some are laminates (copper-resin-reinforcement), some films, some liquids, some resin coated foil. Each type of dielectric has a specific advantage. The types of laminates are designed for a particular type of product, depending on performance characteristics. The most common type is the glass fiber reinforcement (FR-4) with various resin systems, and the most common resin is epoxy. Certainly, don’t forget about the fluorocarbon polymeric materials (PTFE), which may also be filled with ceramic materials. (More on PTFE materials in a future column.) Suffice to say, there are many variations of the resin systems, including fillers, cross-linking agents, hardeners and other materials. We will begin with an overview of critical laminate materials attributes.
Laminate Attributes
Dimensional Stability
How stable is the material in question? Does it move during processing? What about shrinkage and expansion? What issues are to be considered with respect to scaling? These are things to keep in mind when selecting materials.
Bondability
Basically, one is asking about the interlaminar bond strength. How strong a bond is there between the resin and the copper circuitry? Certainly there are a number of factors involved including the lamination cycle, interlayer treatment process type and parameters, and the resin material itself. The higher-performance materials tend to possess a higher modulus, thus can be more brittle than standard FR-4. In turn, interlaminar bond strength is reduced.
Desmear and Metallization
This is a considerable challenge for the PCB fabricator. Higher-performance materials, including the polyimide, FR-5, PPE, PPO, tetrafunctional materials are more resistant to chemical processing. This, in turn, impacts the degree of drill smear removal as well as the texture of the resin surface. It is well documented that a fairly smooth resin texture places a strain on the metallization process in use, which in turn can lead to plating voids and lack of adhesion of the plated copper to the resin surface. More on this topic later.
Drilling
As with any of these newer materials, hole quality is dependent on the hardness of the resin materials, glass type, filler materials, and board thickness. Ceramic-filled materials are particularly troublesome with respect to drillability. Ceramic materials increase the wear on the drill bits, significantly reducing the useful life of those bits.
Moisture absorption
Different resin systems vary in their ability to absorb moisture. Be cognizant of these different materials’ ability to absorb moisture. Take special precautions with respect to material storage and lamination procedures. Moisture is a plasticizer and will increase resin flow during lamination. Retained moisture in the resin materials can lead to excessive Z-axis expansion, material movement and delamination.
FR-4
For many years, the PCB industry relied on good old FR-4 epoxy resin systems with little variation in performance. However, as circuit board designers and end-users began to demand higher performance from the materials (electrical, chemical resistance, reliability), variations in the king resin, FR-4, began to gain acceptance.Of course, whenever changes like this are made, there are unintended consequences. Consider the ramifications in Figure 3.
Figure 3: Performance trends in PCB resin systems. (Source: Isola Laminate Systems—Jim Paulos)
While it may be self-explanatory, a few points are worth mentioning and some of these unintended consequences will be explored in future Trouble in Your Tank columns. For now, as just one example, consider that as the Tg increases, there is a greater resistance to chemicals. While a good feature overall, it does mean that these materials are more difficult to desmear properly. As one may have experienced, higher Tg materials do not texture as easily as the standard FR-4 materials. In turn, this can lead to over desmearing, causing a wedge void or overall spotty adhesion of the electroless copper.
Michael Carano is with OMG Electronic Chemicals (formerly Electrochemicals), a developer and provider of processes and materials for the electronics industry supply chain. He has been involved in the PWB, general metal finishing photovoltaic industries for nearly 30 years. Carano holds nine U.S. patents in topics including plating, metallization processes and PWB fabrication techniques.