Multilayer printed circuit boards

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. 

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... 

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. 

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... 

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. 

A.Takahashi, et al., High Density Multilayer Printed Circuit Board for HITAC M-880, op. cit., pp. 

FIGURE 57.6 Effects of substrate Tg and PTH diameter on mean number of cycles to failure. The thermal cycle was 2-h cycle with extremes at -62 and +125 °C. Multilayer printed circuit board thickness 0.10 in (2.5 mm) Cu in unfilled PTH is 1.2 mil (30 pm) thick. Results from Ret. 33. 

M. W. Gray, Inner Layer or Post Cracking on Multilayer Printed Circuit Boards, Circuit World, vol. 15, no. 2, pp. 22-29,1989. 

Methane sulfonic acid (MSA) is a relatively expensive acid used to strip (remove) solder (tin/lead) and copper metals. This acid is also used to condition metal surfaces prior to fluo-borate free-solder electroplating when applying a final etch resist of solder onto multilayer printed circuit boards. By continuously recirculating the acid bath through a diffusion dialysis unit, a 95 percent reduction of chemical purchases (required by the process steps indicated in the first paragraph of this section) may result, producing less than a six-month return on the initial capital investment (ROI).Typically, 80 to 90 percent of the acid is recovered with 70 to 90 percent of the metals removed in an equal volume of dilute acid. 

---