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FIGURE 29.11
Tafel plots: direct current.
The system fits readily into existing in-house equipment and in-house know-how. Figure 29.12 shows an example of a blind via that is 5.0 mil in diameter and 3.0 mil deep, plated with this type of bath at 20 ASF for 90 min.
Pulse Plating (Electrically Mediated Process) In this plating system, the bath is engineered to respond to a periodically pulsed reverse current. The required Tafel slope shift is accomplished through rectification (see Fig. 29.13). The rectifier produces a pulsed wave with a forward cathodic current that is perturbed by short anodic pulses. The forward current at 1X (e.g., 30 ASF) is maintained for 10 ms and the reversed at 3X (e.g., 90 ASF) for 0.5 ms, for example. The duty cycle may vary (e.g., 20 ms forward with 1.0 ms reverse).
FIGURE 29.12 Example of a blind via that is 5.0 mil in diameter and 3.0 mil deep, plated with chemically mediated bath.
PRINTED CIRCUITS HANDBOOK
Log I
Standard Acid Copper System with DC Current
Sppr2
Suitable Chemistry with Pulse Periodic Reverse Technology
Increasing Current Density
Sdc Sppr1
S PPR1 ~ S >>> S PPR2
RpPPR2 >>>
RpPPR1 ~ RpDC
Increasing Potential
FIGURE 29.13 The pulse periodic reverse effect.
There is also room to optimize the forward-to-reverse current ratio; 1:3 is only one example. The shape of the wave is very important here, also. A square wave with minimum rise time gives the best results (see Fig. 29.14).
1000 500 0 -500 -1000 -1500 -2000 -2500
10 or 20
Amps
0.5 or 1
FIGURE 29.14
Periodic pulse reverse rectifier output wave.
Chemical suppliers have designed specific additive packages that ensure maximum response to the periodic pulse reverse wave. During the reverse cycle, the additive is preferentially desorbed from the high-current-density areas. This results in less plating acceleration due to the additive, and more suppression due to the carrier. Since the low-current-density areas of the panel will also receive a lower reverse pulse, the acceleration is reduced to a lesser degree than in the high-current-density areas. This leads to greatly improved plating thickness distribution.
ELECTROPLATING
Often, the difference in plating thickness between isolated surface features and ground planes will be no greater than 0.5 mil.This effect is so powerful that throwing power greater than 100 percent is commonplace. Figure 29.15 shows the plating in a hole that is 0.013 in. in diameter in a 0.100 in. thick board (8:1 aspect ratio). The board was plated using Copper Gleam PPR (a proprietary additive) under the following conditions: forward current density, 30 ASF forward-to-reverse (F:R) CD ratio, 1:2.67 duty cycle, 20:1 ms plating time, and 60 min. The results are an average thickness on the surface of 1.1 mil and an average thickness in the hole of 1.4 mil. Reverse pulse plating gives dramatic improvements in copper thickness distribution beyond the capabilities of the chemically mediated process. It is clearly the wave of the future as the HDI boards become more and more complex. However, it involves capital investment for the pulse rectifier, which may be five to six times the cost of an equivalent DC rectifier. It also involves a learning curve for matching the new parameters (F:R ratio, duty cycle, ASF, and the shape of the wave) to meet the specifications of the part number being plated. 29.5.10 Key Factors for Uniform Plating
Magnification 5x
FIGURE 29.15 One hundred mil board with 13 mil diameter hole plated with Copper Gleam PPR for 60 min. Surfaceto-hole ratio is 0.75.
To have day-to-day control and achieve ductile, strong deposits, and uniform copper thickness, you need the following controls: 1. Maintain equipment following best practices, such as uniform air agitation or e-ductors (also known as ser-ducters ) in the tank, equal anode/cathode distances, rectifier connection on both ends of tank, and low resistance between rack and cathode. 2. Maintain narrow-range control of all chemical constituents, including organic additives and contaminants. 3. Conduct batch carbon treatment as needed. 4. Control temperature at 70 to 85 F, or as specified by supplier. 5. Eliminate contaminants in the tank from preplate cleaners, microetchants, and impure chemicals. The plating thickness distribution of your plating bath can be readily evaluated. Use noncopper-clad or bare epoxy laminate. First, plate 80 to 100 min of electroless copper, then rack the panels and plate at the specified ASF to plate 80 to 100 min of plated copper (usually 1/10 of the plating cycle time). Place the rack in a microetch solution until 60 to 80 percent of the copper is etched away. Remove and examine. The remaining copper is where the highcurrent density is. This is a useful tool in optimizing bath geometry, especially anode placement and panel placement on the rack. It is also useful in designing thieving and shielding for the best thickness distribution. The plated-copper thickness distribution for a specific pattern can also be studied the same way.After plating the electroless copper on non-copper-clad bare laminate, image the pattern. Plate for a limited time to get approximately 80 to 100 min, or 0.2 to 0.3 mm. Strip the resist off the panel and place in microetch until 80 percent of all copper is removed. The remaining copper is where the high-current density is.
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