Multilayer printing

Printed circuit boards manufacture is aided by the use of KMnO. Alkaline permanganate solution is used to remove resin smeared on the interior hole wall of multilayered printed circuit boards. Additionally the hole wall is etched, resulting in a surface with excellent adhesion characteristics, for electrodeless copper (250). The alkaline permanganate etchback system containing >60 g/L KMnO and 40-80 g/L NaOH at 70—80°C, is effective for difunctional, tetrafiinctional, and polyimide resin substrates, where the level of etchback is direcdy proportional to the immersion time (10—20 min) (251). 

Fig. 2. Multilayer printed circuit board composite. Constmction is multiple layers of epoxy—glass and foil copper. Foil copper outermost layer and drilled through-holes are sequentially plated with electroless copper, electrolytic copper, electroless nickel, and electroless gold.

The controller electronics are located on a single multilayer printed circuit board. The circuit board is shown in Fig. 5.46. 

Step coverage — From the process flow schematics shown previously, it is apparent that printed transistors inherently have substantial topology within their cross-sectional structure. As a consequence, step coverage becomes an important parameter in process optimization. Given the large steps (typically several tens of nm or more) and the use of relatively thin subsequent layers, it is important that the layers cover each other adequately liquids must be able to coat the vertical sidewaUs of steps during a multilayer print process. This places constraints on fluid viscosity, evaporation rate, wetting, etc. 

Wallig, L.R., Dry Film Photoresists in The Multilayer Printed Circuit Board Handbook, edited by J.A. Scarlett, pp. 111-153, Electrochemical Publications, Ayr, Scotland, United Kingdom... 

T. Kessler and R. Alkire, Copper Plating of Multilayer Printed Wiring Boards, Plat. Surf. Finish., 63, No. 22, 22-27 (1980). 

Ordered polymer films made from poly benzthiazole (PBZT) and poly benzoxazole (PBO) can be used as substrates for multilayer printed circuit boards and advanced interconnects to fill the current need for high speed, high density packaging. Foster-Miller, Inc. has made thin substrates (0.002 in.) using biaxially oriented liquid crystal polymer films processed from nematic solutions. PBZT films were processed and laminated to make a substrate with dielectric constant of 2.8 at 1 MHz, and a controllable CTE of 3 to 7 ppm/°C. The films were evaluated for use in multilayer boards (MLBs) which require thin interconnect substrates with uniform controllable coefficient of thermal expansion (CTE), excellent dielectric properties, low moisture absorption, high temperature capability, and simple reliable processing methods. We found that ordered polymer films surpass the limitations of fiber reinforced resins and meet the requirements of future chip-to-chip interconnection. 

Heikkla, K., R. Williams, and B. Bohnen. Selection and control of plating chemistry for multilayer printed wiring boards. Electronics, 31... 

A Model for Copper Electroplating of Multilayer Printed Wiring Boards". 

Fig. 6. The Diene-Siloxane resins have low dielectric constants allowing labrication of multilayer printed circuit boards for use in computer applications (courtesy R. LiebfKed, Hercules).

L. S. Rubin, K. Jayaraj, and J. M. Burnett, Liquid crystalline polymer multilayer printed circuit boards for controlled coefficient of thermal expansion. Proceedings lEPS, September (1991). 

Bismaleimide resins are extensively used in multilayer printed circuit boards. They can also be used in insulation fibres for protective clothing. Due to their ability to adhere and retain adhesive strength at high temperature, bismaleimides are widely used as a base material for flexible printed circuit boards in the electrical and electronics industries. Bismaleimides are often linked with rubber chains for use as flexible linking molecules to reinforce the rubber compound for tyre applications. The double bond of bismaleimides readily reacts with all hydroxy or thiol groups found on the matrix to form a stable carbon-sulfur, carbon-nitrogen or carbon-carbon bond. 

Lee, L.C. et al. 1984. Micromechanics of multilayer printed circuit board. IBM Journal of Research and... 

Hirogaki T, Aoyama E, Inoue H, Ogawa K, Maeda S and Katayama T (2001) Laser drilling of hhnd via holes in aramid and glass/epoxy composites for multilayer printed wiring boards, Compos Part A 32 963-968. 

Polyimides are used in the construction of multilayer printed circuit boards because of their low coefficient of thermal expansion, high bond strength/peel strength of copper foil at elevated temperature, and no resin smear. 

Screen printing is expected to replace wet powder spraying in cathode production in the long run. The same procedure can also be applied for electrolyte production. The development target is multilayer printing, i.e., all layers will be printed on top of an anode substrate to form the SOFC cell. This would result in a reduction of sintering steps and therefore in considerable effort and cost reduction. 

Other innovations include the smart surface technology process which Bayer has developed in conjunction with Lumitec of Switzerland and which involves the production of flat light sources. These electroluminescent elements, whose smart outer skin is a printed Makrofol/Bayfol (PC/PC + PBT blend), do not dissipate any heat and are maintenance free. The light is produced in the multilayer printed system beneath the skin and shines through it. The system itself is an electrode, Baytron P for example, with a counter-electrode underneath The two electrodes are separated by a non-conductor. The mouldings used are produced by a single step process and require minimal assembly work. Bayer has recently invested 24.5 million in a start-up company to produce electroluminescent films. 

A noteworthy enhancement in the mechanical and electrical conductivity of CNT-filled epoxy composites lead to the development of conductive materials for electronics, automotive shielding, electrostatic dissipation, conductive coating, multilayer printed circuits, and electromagnetic inference. 

The most commonly referenced specification for base materials is IPC-4101, Specification for Base Materials for Rigid and Multilayer Printed Boards. Historically, the National Electrical Manufacturers Association, NEMA, has also been used in specifying base materials. 

PC-4101 Specification for Base Materials for Rigid and Multilayer Printed Boards... 

The growth in multilayer printed circuits has resulted in HTE becoming the most commonly used foil, since its excellent ductility at elevated temperatures helps prevent inner-layer copper foil cracking. As a printed circuit experiences a thermal cycle, the base materials will expand. The z-axis expansion applies stress to the connection of the inner-layer foil and the plated hole. With HTE foil, the reliability of this connection is improved.This property is particularly important in thicker circuits and high resin content constructions where increased z-axis expansion occurs. 

Kelley, Edward and Christofferson, Owen, Multilayer Printed Circuits with Exacting Registration Requirements, Achieving Next Generation Design Capabilities, IPC Printed Circuits Expo, May 1998. 

FIGURE 13.9 Cross section of 14-Iayer multilayer printed wiring board, showing a typical inner layer and prepreg material relationship. In this case, to reduce z-axis expansion, the innerlayers are polyimide, while the prepreg material is semicured polyimide. Typical signal, power, and ground layers are also indicated, as well as the thickness of the copper foil for each layer. 

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