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Chemical Process Control
- by Matt Stevenson

I have always been fascinated with chemistry and chemical processes, my first degree was in chemistry and my first job out of college was in the PCB manufacturing shop in the analytical chemistry lab. During my initial tour I was so surprised with just how many chemical processes there were in PCB manufacturing.  Keeping all of these chemical baths, sprays, rinses, and processes operating in a tight window to maintain quality and predictable results is a key operation in PCB manufacturing.

There is such a variety of different types of tests and analyses that occur on a daily and weekly basis within the factory that it could really make your head spin. For those of you that still have scars from high school chemistry class you may want to skip to the bottom, otherwise I am going to try to give you a quick overview of some of the most commonly used tests.
 

Titrations: There are quite a few chemical components that can be measured in a working chemical bath by a titration. Simply put,adding one chemical of a known concentration to a small sample of the bath until a given pH is obtained. There are lots of ways to catch this pH endpoint, indicators change color at a given endpoint, tracking the pH with a meter during the titration, some reactions will create a precipitate (solid) at the desired pH. Knowing how much chemical was added and a simple calculation will tell you the concentration of the component in the bath.
 
Colorimetric tests. Some of the tests are done using an instrument known as a UV/Vis spectrophotometer. A sample of a bath is analyzed at a given wavelength of light, the absorption or transmittance of the light as it passes through the sample will give a reading. When this reading is applied to a calibrated curve of known concentrations it is able to provide you with the bath concentration.
 
Elemental analysis. This is a good way of finding concentrations of various metals in a chemical sample, common ones in PCB manufacturing are copper, gold, nickel, silver, tin and lead. One of the most accurate ways is through Flame AA (atomic absorption) analysis. In this method you burn a sample of the chemical bath in an AA flame and it measures the absorption of light at specific wavelengths for gas phase elements. This absorption value is again compared to a standard curve of known concentrations to determine the composition of the solution.
 
Functional tests: These tests are especially used in electroplating to understand how some components are functioning. One example of a functional test is grain refiner in a copper bath tested using a Hull Cell. A sample of bath is added to a special bench top plating cell named a Hull Cell, an anode and a brass plating coupon are added and current is applied. The test plates for a period of time to insure that there is adequate metallic deposit to analyze. The test covers a variety of plating current densities. Standard additions of grain refiner can be made to the plating cell and compared to the plating bath to understand appropriate adds to achieve the desired copper deposit.
All of these results help to maintain the chemical processes within a given operating range, identify problems with equipment or chemistry quickly and insure that an expected outcome is the result of every chemical process.

Okay I am sure that I have nerded out enough on chemistry for one day. Hope you enjoyed getting a little deeper into the chemical analysis for PCB manufacturing. If you have any questions please feel free to let me know.