Printed Circuit Board Surface Finishes

The U.S. EPA regards meialhzcrs such as platers, surface finishers, and printed circuit board producers as among the mosi impurtani source polluters for metals Much production of electroplated items lias, however, shifted from the United States to less environmentally stringent countries. 

The single largest use for the phenol-formaldehyde resins is in adhesive applications for the production of plywood, chipboard, and particle board. The resin can comprise as much as one-third of the weight of the board, particularly of particle boards, which contributes to a total demand for phenolics in the U.S.A. of over half a million metric tonnes per year. They are also used as the matrix adhesives for the production of several types of grindstones. In combination with paper, woven cotton, glass fiber, etc., components, phenolics contribute to the production of engineering and decorative laminates in the form of rods, tubes, and sheets. The sheet products Arborite and Formica are familiar as the finished surfaces of furniture, bathroom, and kitchen counter tops and other areas where attractive patterns and water resistance are important characteristics. Molded products from phenolics are also important where heat or electrical resistance is required, such as saucepan and toaster handles, switches, and the printed circuit boards used in computers. Recent phenolics production in the U.S.A. has totaled over 500,000 metric tonnes per year, not including fillers [38]. 

After 3 years of development, including 2 years of practical testing in the industry up to a commercial scale, we can report (text written in 1998) an interesting new application the preparation of the surface finish of printed circuit boards. The places where the diodes, resistors, etc., have to be mounted and connected to copper-based printed circuits have to be solderable for a period of at least 1 year in... 

A new application, which might have the chance to become realized more in short term is in the area where Ormecon has successfully introduced a new nanotechnology on a worldwide basis the printed circuit board industry and here, more specifically, solderable surface finishes. The product is a new all-organic solderable surface nanofinish for printed circuit boards (which can be introduced into the markets within a relatively short time). The new products allow producing a solderable surface in a one-step process. 

The laser has proven to be a versatile tool in many industries. For circuit board assembly, it can be used for marking, single-point soldering, or rework. Lasers can accommodate bonding of the finest or coarsest peripherally leaded surface-mount packages, and its performance and applicabUity are independent of device population density on the board, the thickness of the printed circuit board, or the presence or absence of package heat sinks. It can be used with most any component lead or PWB pad surface finish and any solder, including Pb-free alloys. 

Fig. 8.20 Applications of electroless nickel plating Ni-P deposits for wcar>resistant and corrosion-resistant applications in engineering, (a) A cooling-cail assembly. (Photo graphy courtesy Ionic Surface Treatments plating by Dudley Division.) (b) Printed circuit boards for a wrist watch and calculators. Electrically isolated areas of deposited copper may be built up by electroless nickel plating which also provides a corrosion-resistant and solderablc finish. The deposits are various NIKLAD electroless nickels. (Photograph Courtesy Lea Manufacturing.)...

There has recently been a renewed interest in Sn whiskers because of the worldwide conversion to Pb-free solders and finishes in electronic manufacturing. Finishes are applied to printed circuit boards (PCBs) and to the lead frames used to connect device packages to printed circuit boards. Lead frames are typically made of a copper (Cu) or iron-nickel (FeNi) alloy plated with a Sn-Pb alloy. Fig. 5 is a schematic diagram of a lead frame cross section bonded to a chip-carrier package. The surface finish of the lead-frame leg is designed to provide surface passivation and enhanced solderability. Typical Pb-free surface finishes are eutectic Sn-Cu or pure Sn. Tin(Sn) whiskers readily grow on high-Sn content finishes under certain conditions. 

There are two circuit board templates, the oxygen side template, and the hydrogen side template. When you have finished printing your templates on the transfer film, lay the film emulsion side down on the copper surface of each of the circuit boards. 

Organic-coated copper provides a consistent, flat, solderable metal finish. Exposed copper after printed circuit assembly has been a persistent reUabiUty concern, because it is generally not permitted on HASL boards. While exposed Cu on HASL boards is associated with poor solderabUity, which may be due to contaminants that were not removed before the HASL process, there is little evidence that exposed Cu on a properly processed OCC board causes reliability problems. Surface insulation resistance (SIR) testing shows that OCC boards have comparable or better performance than HASL boards in high-temperature, high-humidity storage tests. 

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