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8. Polakovic, F., Contaminants and Their Effect on the Electroless Copper Process, IPC Technical Review, October 1984, pp. 12 16. 9. Blurton, K. F., High Quality Copper Deposited from Electroless Copper Baths, Plating and Surface Finishing, Vol. 73, No. 1, 1986, pp. 52 55. 10. Lea, C., The Importance of High Quality Electroless Copper Deposition in the Production of Plated-Through Hole PCBs, Circuit World, Vol. 12, No. 2, 1986, pp. 16 21.
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29.1 INTRODUCTION
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The reality in printed circuit evolution is that the parts are continuing to increase in degree of difficulty and complexity. A new genre of high-density interconnect (HDI) boards is making the transition from leading edge to mainstream. These boards are characterized by a combination of a series of complex features that include buried and blind vias, high-aspect ratio plating, small-hole plating (as low as 6 mil diameter holes), and fine lines and spaces side by side with ground plane areas of different sizes. The industry, however, still has a need to produce simpler products, such as single-sided and double-sided boards; these are still in demand. Multilayer boards with lower layer counts (four to eight) fall in this category also. This chapter focuses on all aspects of electroplating. Emphasis will be on acid copper plating as the main process for providing interconnection. Tin, tin-lead, nickel, and gold electroplating will also be covered as important plated materials. In addition, the latest innovations in technology for meeting the challenges of product complexity, such as pulse plating and horizontal conveyorized plating, are discussed in detail.
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29.2 ELECTROPLATING BASICS
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Electroplating is the production of adherent deposition of conductive surfaces by the passage of electric current through a conductive metal-bearing solution. The rate of plating depends on current and time and is expressed by Faraday s law (Eq. 29.1): W = (ItA)/(nF) where W = metal, g I = current, A t = time, s A = atomic weight of the metal n = number of electrons involved in metal ion reduction F = Faraday s constant (96,485 C/mol) (29.1)
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Plating occurs at the cathode (the negative electrode). Accordingly, deposition thickness is determined by time and by the current that is impressed on the surface being plated; for example, by using the preceding formula, one can readily calculate the weight of the deposited metal. The weight can then be converted to a specific thickness over a known area. The rate of deposition of the most common metals is shown in Table 29.1.
TABLE 29.1 Rate of Deposition of the Most Common Metals Grams Grams deposited per amp.hr 1.186 2.214 3.865 1.095 7.348 Amp. hr. per sq.ft. to deposit 0.001 in 17.8 7.8 6.9 19.0 6.2 Amp. hr. per sq.dm. to deposit 25 mm 1.88 0.82 0.73 2.00 0.65
Metal Copper Tin Lead Nickel Gold
To plate 1.0 mil of copper, one would need 17.8 amperes per square foot (ASF) or 1.88 amperes per decimeter (ASD) for 1 hour (60 min.). Properties of electrodeposits are shown in Table 29.2.
TABLE 29.2 Properties of Electrodeposits Property Melting point, F Hardness, VHN CTE X 10 6/ F Conductivity, %IACS Electrical resistivity, mW/cm Thermal conductivity, CGS C Cu 1980 150 9.4 101 1.67 0.97 Ni 2600 250 8.0 25 6.8 0.25 Au 1945 150 8.2 73 2.19 0.71 SnPb 361 12 12.2 11.9 14.5 0.12 Sn 450 4 12.8 15.6 11.1 0.15
HIGH-ASPECT RATIO HOLE AND MICROVIA PLATING
The combination of increasing layer counts (panel thickness) and decreasing hole size results in higher aspect ratios.
Aspect Ratios Aspect ratios are calculated by dividing panel thickness by the hole size. (See Fig. 29.1 for example of a high-aspect hole with a 15:1 ratio.) Standard direct current (DC) plating bath formulations are not sufficient to plate these geometries. Modifying the concentrations of the electrolyte components is required. In general, lowering the copper concentration and increasing the acid concentration improves through-hole distribution, as indicated in Table 29.3. Proprietary additives play a role maximizing through-hole distribution as well. Solution agitation is an integral part of successfully plating high-aspect ratio through holes. Ninety degree work rod agitation with a 1/2 in. stroke is the most common method of replenishing
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