Estimated reading time: 7 minutes

Happy’s Tech Talk #33: Wet Process Management and Control

My August column introduced sensors that are useful in measuring critical components of the various wet processes in printed circuit fabrication. Now, I will close the loop and discuss building automatic controllers. Table 1 shows the three phases of process control. Hopefully, you have conducted some type of wet process audit using the worksheet in my last column and now have some idea of where improved process control will improve performance.

The Specific Gravity Controller
Figure 1 shows levels of specific gravity (SG) sensors you can buy from Amazon for less than $10. You can also buy relays that can be used with these level sensors. Figure 2 shows the calibration/design chart for the SG sensor I illustrated last month¹.

The operating setpoint for SG that Don Ball of Chemcut recommends for cupric chloride or alkaline etchants is indicated by the red star on the chart. Below the chart is the plastic calibration rod that needs to be fabricated, threaded, and then to have the center drilled out based on the Figure 2 calibration for operating point and adjustment range. With a setpoint of 1.27 SG and a total range of 0.1 SG, the intersection on the chart has a drill depth of 2.30 inches and a nut diameter of 1.55 inches. With 50 full threads for the plastic adjustment nuts (two nuts for locking), the specific gravity setpoint will be from 1.21 to 1.32 ±0.002 SG units (but it can be designed to be wider or narrower in range).

Note: This chart in Figure 2 is set up for the GEM level switch and the CPVC adjustment rod seen below the chart. If the level switch, nuts, or rod have any other geometries or materials, the chart will not be accurate. You will need to change the basic program’s variables (Figure 3) for the displacement of the switch, nuts, and rod as well as the weight of the rod and nuts, as these variables determine the volume displaced (buoyancy force) and the moment from gravity. Remember, this sensor is entirely submerged in the etchant.

Conductivity
Because PCB wet processes are aqueous, conductivity is a very useful and common sensor, especially for rinse water control. A lot of water is used for rinsing in PCB fabrication, and having a conductivity probe for the rinse means that it will only run when it is being used and will stop when the rinse is clean again. The probe can also be used for the processes themselves, but the ranges will be different, and the probes need to be compatible with the chemistries involved. I give more details on these probes in my book on automation and advanced procedures². Because of the popularity of aquaculture and hydroponics (including fish aquariums), there are many available conductivity controllers (Figure 4) and schematics to help build your own.

Units
The units of conductivity are siemens per cm (S/cm). Derived units are µS/cm (one-millionth of a S/cm) and µS/cm (one-thousandth of a S/cm). S/cm is the same as the older unit mho/cm. Certain high-purity water industries, primarily semiconductor and pharmaceutical, use resistivity instead of conductivity. Resistivity is the reciprocal of conductivity. The units are MΩ-cm. Here is the conductivity of various waters:

• Ultra-pure DI water: 0.01 µS/cm
• Good quality distilled water: 1 µS/cm
• Excellent quality tap water: 50 µS/cm
• 0.05% NaCl: 1 mS/cm
• Sea water: 50 mS/cm
• 30% H2SO4 (plating): 1 S/cm

Measurement of Conductivity
There are two types of conductivity measurement: contacting and inductive. The choice of which to use depends on the amount of conductivity, the corrosiveness of the liquid, and the amount of suspended solids. Generally, the inductive method is better when the conductivity is high, the liquid is corrosive, or suspended solids are present. Follow this reference to learn more about the measurement of conductivity³.

If you purchased a portable conductivity or combination probe¹, then you can always cannibalize the probe’s electronics.

Amphere-hour Totalizer
The amphere-hour totalizer is very useful for monitoring electroplating. This is because the components in plating are consumed by the actual current used for the deposition. On newer, more complex plating processes, analytical measurements are still required for some components, using titration and electrochemical techniques like CVS.

A 50 milli-volt or 100 mV current shunt (resistor) rated for the maximum current is used in a series with the power supply and the cathode frame. This current is totalized over time as the plating is conducted and then replenishment can be calculated from the reading.

Figure 5 shows the schematic for such an amphere-hour totalizer. Its simple design is a voltage amplifier for the 50 mV DC signal and a voltage-to-frequency converter to drive the totalizer. They are also available for purchase from the major plating supply vendors, ranging from $450 to $800 based on features; those can include replenishment pumps. Amp-hour totalizers for pulse-plating power supplies are more complex and thus, more expensive.

Spectrophotometers
Spectrophotometers, like conductivity, are very popular in middle and high school science classes, especially for experiments involving pollution. As such, there are several low-cost spectrophotometers that are in the public domain for schools to use. The most used is the LEGO spectrophotometer I previously talked about¹.

Controllers
A controller is constructed by taking the output of the sensor or sensor electronics and comparing it to the setpoint. The simplest controller is just a simple on/off relay, or it can be as complex as a PID controller that is much more expensive. There are controller programs available for all the popular “maker” microcomputers on the market, as well as simply writing your own. Below, I provide some information on microcomputers and programming that will allow for some effective DIY controllers.

Microcomputers and PLCs
Figure 6 illustrates four of the more popular maker or hobby microcomputers. The Arduino Uno, QuePython, Micro:bit and Raspberry Pi are all 32-bit computers with built-in networking features, WiFi, and analog/digital inputs that have various prgramming languages and abundant internet tutorials and software, all for less than $78.⁴

Programming
The Micro:bit from the UK’s BBC is utilized in the UK’s middle and high schools to teach computer programming to students. These are provided free to UK middle students and nearly four million have been sent out. I purchased one for my grandson for $16. As such, there are extensive lesson plans, software, examples, and instructions available on the internet to learn microcomputer languages such as LINUX, Python, and C.

Replenishment
Replenishment of chemicals is accomplished with various valves, be they proportional or just on/off or with pumps, the replenishment-proportional pump being the most popular. You can fabricate systems with your own designs. Examples are shown in Figure 7.

Examples
The electroless copper controller was a particular focus while I was a PCB process engineer. After visiting Photocircuits in Long Island, New York, and seeing the additive electroless copper controller in action, we (HP) decided to design and build one of our own. Figure 8 illustrates the process schematic for that controller (and a close-up photo of it in operation) using the spectrophotometer and pH probes. Also illustrated is the laboratory R&D copper electroplating set-up that utilized six of the sensors that I’ve talked about: Specific gravity, pH, conductivity, ampere-hour, electrochemical CVS, and spectrophotometric.

Summary
The specific gravity controller with a micro:bit or Raspberry Pi and DAC Hat and a DIY Python program is a good place to start, or it can be as simple as a battery-powered with just an LED to show when the SG limit has been hit. All these sensors, except for the SG, are electronic in nature and require an electronic setpoint. The SG sensor is mechanical in nature and quite robust, due to its origin in Archimedes Principles. Because PCB wet processes are water-based, this sensor has applications everywhere and is a good place to start.

Because of the popularity now of aquaculture/hydroponics, home aquariums/swimming pools and hot-tubs, and microbrewing, as well as environmental studies in school science classes, many suitable sensors and controllers can be purchased locally or from the internet, all at reasonable prices.

For additional instructions on sensor technology, visit “Teach Engineering-STEM Learning for K-12”⁵

References

1. Happy’s Tech Talk #32: Three Simple Ways to Manage and Control West Processes,” by Happy Holden, PCB007 Magazine, August 2024.
2. Automation and Advanced PCB Procedures in PCB Fabrication, Chapter 4, by Happy Holden.
3. Theory and Application of Conductivity Application Data Sheet ADS 43-018/rev. D, Rosemount Analytical Inc. 2010.
4. Amazon.com.
5. Browse K-12 STEM Curriculum, teachengineering.org.

Happy Holden has worked in printed circuit technology since 1970 with Hewlett-Packard, NanYa Westwood, Merix, Foxconn, and Gentex. He is currently a contributing technical editor with I-Connect007, and the author of Automation and Advanced Procedures in PCB Fabrication, and 24 Essential Skills for Engineers. 

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