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Trouble in Your Tank: Things You Can Do for Better Wet Process Control

For 40 years, I have been involved in the printed circuit board, circuit board assembly, and semiconductor technology segments, preaching about minimizing defects and improving yields. This is especially true as technology becomes increasingly complex, and additional focus must be placed on yield improvements. Process management and wet process control must be front and center, so it’s quite interesting and timely to talk about wet process control and management for this month’s issue. This theme fits quite well with today's global events. For this industry, the technical curve has steepened dramatically in the past few years.

The Current State of Affairs
As the level of complexity increases (higher layer counts, HDI and ultra high density designs, and smaller footprint IC substrates, etc.), the process windows for wet chemical processing (etching, developing, and plating) are narrowing. This is because the fabrication process needs to minimize variation in line widths and spaces, circuit trace thicknesses, and overall plating uniformity. These factors influence critical properties, including impedance, reliability, and assembly. To further complicate matters, many fabricators tend to optimize their operation around cost, rather than quality and yield considerations, and if either of those things suffer, the blame game begins.

Things go wrong. Stuff happens. Though true, that is not the attitude to have when yields go south, and a process may be out of control. The first thing to know is what’s out of control? Is it the pH of the resist developer solution, or the specific gravity of the alkaline etchant? Are these special or common causes? A special cause indicates that something in the related processes caused the change. In other words, there is an assignable cause. Where is the process control management plan, and where is it listed for all to see? What if there was no significant change? Then one does not have a specific assignable cause. Engineers should not assume this is a one-off, and the problem has disappeared. There are multiple variables within the printed circuit board manufacturing process. That is a fact. Having a deep understanding of these possible variables and their ultimate effect on quality is the cornerstone of process control and management.

Wet Process Control: Not Only About the Chemistry
Now I will explore some process control ideas that are easy and cost-effective to implement. These include conductivity probes, online real-time analysis of electroplating solutions, and simple specific gravity measurements.

I strongly suggest that operators and engineers monitor all aspects of the processes for each unit operation. This includes walking the line, taking measurements, recognizing what is not in control, and getting it back within range. Quality and yields should come first. SPC charts that dot the walls of the company’s laboratory give the impression that key unit operations in manufacturing are in control. That is not always accurate. For example, several processes may be set up for automated analysis and replenishment of critical chemical additives. However, this often only covers the basic additives in the electroplating or electroless plating processes (copper concentration, acid levels, etc.). What about the organic additives (leveling agents, throwing power enhancers, and stabilizers) that have a profound effect on the overall quality and reliability of the plated deposit? How do you know with any confidence that these critical additives (special additives that influence ductility, grain structure, and resistance to thermal excursions) are in the proper range?

Organic Addition Agent Control in Electroplating
I suggest utilizing a real-time analyzer (Figure 1). This unit provides real-time and accurate analysis of the key organic addition agents in the electrolytic copper plating solutions.

It is understood that the organic addition agents (grain refiners, leveling agents, and high current density suppressing additives) enhance the uniformity, leveling, and reliability of the plated copper deposit. Properties of the deposit, such as ductility (elongation) and tensile strength, must be in tight control to prevent failures in the plated through-hole and blind vias due to thermal and mechanical excursions.

Figure 2 shows the result of low additive concentration in the electrolytic copper plating solution. While many rely on the old-fashioned Hull Cell method to control organic addition agents, this method does not yield quantitative results. With the real-time analyzer, it is possible to measure the organic addition effects accurately and within a short time. This provides data in real-time and allows the operators and engineers to quickly make necessary adjustments in chemistry while maintaining a very tight process control window.

When the key organic addition agents are maintained within a narrow operating window, there will be minimal variation in the physical properties of the copper deposit. This cannot be over-emphasized.

Rinsing
Another often overlooked process is rinsing. PCB fabrication wet processes require good quality rinse water and the implementation of best practices with respect to proper rinsing design. Here again, is the quest to reduce cost by cutting back on rinsing. This is not an ideal situation.

It is a hard fact that processing printed circuit boards consumes large volumes of water, a precious resource. However, there are ways to perform the function of removing contaminants from the printed circuit board and still conserve water.

Rinsing is often defined as the removal of process solution from the panel. This is mostly true, if not absolutely true. Rinsing, in general, is not the complete removal of the contaminants, but rather a dilution of a process solution from the work (panel) down to “manageable” concentrations. With this definition in mind, rinsing systems can be designed to minimize harmful contaminants on a printed circuit board and reduce water consumption. I’m often asked whether there is some “standard” that can be applied to the rinsing process. Are all types of contaminants the same? Is there a hard and fast rule for rinsing? The short answer is, not really. What constitutes a “manageable” concentration is dependent upon three conditions:

• The type of contaminant
• The tolerance of the following process step for the particular contaminant in question
• The effect the residual contaminants have on the work

The bottom line is to use tempered water rinses along with sufficient dwell times required to remove the majority of these harmful contaminants. Counterflow rinses are particularly effective.

When rinsing is ineffective (Figure 3), the stark reality of contaminant drag-in to subsequent process steps is certain.

Consider issues related to the electroless copper metalization process. Poor rinsing after the micro-etch step will lead to copper ion drag-in to the palladium-based catalyst. It is well documented that high copper levels in this process will act as a catalytic poison, leading to possible voids in the copper deposit.

Conductivity Probes
One of the least expensive things you can purchase for process control is a conductivity probe. This little unit (which looks like a ballpoint pen) costs about $200. It is a great way to monitor the rinse water cleanliness and the overall effectiveness of the rinsing operation. These probes are also an excellent method of measuring contaminant build-up in certain chemical processes.

Filtration
The need for filtration must be emphasized, especially when plating through-holes and blind vias. Any void in the plating in the hole caused by small pieces of contaminant reduces the area available to carry the electric current. Rejects can also occur when insoluble debris is co-deposited on the surface or in the hole.

Flow rates are the only means of carrying solids to a filter or bringing fresh solution into contact with the particulate matter. The rate of flow is referred to as the turnover—total gallons pumped per hour in relation to the size of the tank (for example, 200 gallons per hour on a 100-gallon tank is two turnovers per hour). Dirt holding capacity is essential and can be attained with throw-away filter cartridges of different porosities, or filter surfaces coated with a filter aid. Porosities of 100 microns down to less than 1 micron are typical. In practice, the average plating solution is turned over once per hour. The recommended flow rates should provide at least two complete tank volume turnovers per hour. However, to achieve the ultimate in clarity, turnovers of up to 10X per hour may be necessary. Keep in mind that the initial flow rate is not the average flow rate. In other words, if one started at 1,000 gallons per hour, and cleaned or replaced the filter when the flow was reduced to 200 gallons per hour, the actual average flow would probably be about 600 gallons per hour, depending upon the type of filter media used. Ineffective filtration leaves debris in rinse and process tanks (Figure 5).

Temperature Control
All too often, while process engineers and technicians focus on wet analysis, the operating temperature checks are overlooked. Rinsing and developing are typical processes that are more effective with tempered rinses. Too frequently, when rinsing uses incoming water (e.g., during the winter months in very cold climates), the water temperature is quite cold unless heaters are in place.

An increase of 10°K at temperatures not far from room temp will increase the reaction rate by a factor of 2 to 3 (see chart below).

At some point, the reaction rate increase starts to level off. What’s the point? Temperature has a significant influence on chemical reactions and rinsing.

Summary
Certainly, most operators and engineers worry about controlling the wet chemistry of the various processes of the printed circuit board fabrication process. Yet, they are often surprised and disappointed that the quality of the finished product is not meeting stringent requirements and key workmanship standards. Further examination will find that other, often overlooked aspects of the process were not controlled and maintained. It only makes sense that in addition to wet chemistry, other non-chemical factors be monitored and controlled. These include rinsing, filtration, agitation, and temperature controls. Ignore these best practices at your own peril.

This column originally appeared in the August 2024 issue of PCB007 Magazine.