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Trouble in Your Tank: Electrolytic Copper Plating, Part 5

In a previous column on the fundamentals of electrolytic copper plating of printed circuit boards, the subject matter covered the general function of organic addition agents and the role of these additives on the grain structure, plating uniformity, and physical properties of the subsequent copper deposit.

Here, I will explore the many other critical plating parameters that influence throwing power, copper deposit thickness, and metallurgical properties. In a future column, I will explore the additives and their functionality in more detail.

Key Plating Parameters
It is important to recognize and understand that these parameters can be controlled. First and foremost, the process engineering team must have a control plan in place to ensure these parameters remain in a tight operating window. These key parameters are listed in Table 1.

It is not only about the organic addition agents. Many other factors play significant roles. All the factors shown in Table 1 can be controlled or manipulated by the fabricator with the exception of the overall circuit board design (the circuit pattern) and the aspect ratio. With respect to the former, the fabricator can work with the circuit board designer to alter the design, if plausible, to enhance the overall manufacturability of the circuit board (also known as DFM). Aspect ratio (the thickness of the circuit board divided by the diameter of the via) is not as simple. The aspect ratio is determined by how many layers the board will have, along with the dielectric thickness of each layer as designed, as well as the diameter of the via (Figure 1). The higher the aspect ratio, the more difficult it will be to achieve optimal throwing power. Throwing power is defined as the thickness of the plated copper on the surface divided by the plating thickness in the center of the hole.

Why does aspect ratio and overall board thickness matter? As shown in Figure 2, the thicker the panel, the larger the current density difference between the surface of the board and the hole. This is particularly critical in achieving throwing power (TP), which is defined by the thickness of the plated copper in the center of the via divided by thickness of the plated copper on the surface.

While there are numerous factors that affect throwing power, including cathodic current density, solution conductivity (sulfuric acid to copper ion concentration), and other additives such as the chloride ion, the aspect ratio is a significant contributor as well. This includes the overall board thickness. As with any circuit board to be electrolytically plated, there is resistance to current flow into the hole vs. current flow on the surface. That is, there is a voltage drop as current flows from the surface through the hole. See an illustration of this in Figure 3.

It is important to note that the board thickness effect is not a simple linear influence. In Figure 3, one can see that board thickness is shown as a squared number, which adds to the degree of throwing power difficulty. The following examples compare two different board designs. For illustration, assume board one has thickness of 0.063" and a via diameter of 0.006", for an aspect ratio of approximately 10:1. Board two is 0.125" thick with a via diameter of 0.012—approximately the same aspect ratio of the thinner board. However, it has been shown that the two will not perform equally. Not all aspect ratios are equal due to the L2 factor in the Ohmic resistance model (Figure 3). The thicker board will be more difficult to achieve optimum throwing power vs. the thinner board of the same aspect ratio.

To understand this a bit better, Figure 4 depicts a high aspect ratio via. Note the tapering of the plated copper from the surface to the center of the via.

The condition shown in Figure 4 is known as “dog boning” for the tapered shape of the plated copper in the via. As the copper plating becomes thinner in the via, the opportunity for copper cracking becomes much greater. Mitigating the “dog boning” is critical to ensuring long term reliability. More about how to optimize throwing power in a future column.

This column originally appeared in the December 2023 issue of PCB007 Magazine.