The Chemistry of Circuit Boards

Introduction

Circuit boards make almost all modern electronics possible, which is why I chose to study the chemistry involved with them. They are in everything from stoplights to iPods, and you interact with them everyday. Because of this project, I have gained a greater appreciation for circuit boards, and I hope you will too.

Note: I refer to circuit boards as PCBs, which stands for Print Circuit Board, a term that is widely accepted.

Composition of ...
- Substrate (Si)
- Copper foil (Cu)
- For chemical etching: ammonium persulfate (NH4)2S2O8
- Fiberglass epoxy: FR-4, made of epiclyriden (C3H5ClO) and bisphemol (C15H16O2)
- Photoresist: Poly(methyl methacrylate) (C5O2H8)

Main Chemicals, Compounds, Components

Substrate: The substrate is what makes up most of the support for the circuit board, so everything else on the circuit board depends on its durability. It is also the non-conductive part. While it can be composed of only pure silicon, most PCB substrates have the silicon (Si) mixed with an epoxy. This epoxy is composed of bisphemol (C15H16O2) and epiclyriden (C3H5ClO).

Copper foil: The copper foil is the conductive part of the PCB, so it allows electricity to flow to the components mounted on the board. It is simply composed of Copper (Cu). The Copper is pounded into sheets, adhered to the substrate, and then parts are etched off to make the pattern wanted for the PCB.

Chemistry's Role

Substrate: Glass fiber (Si) is sprayed with an epoxy, made of bisphemol (C15H16O2) and epiclyriden (C3H5ClO). It is then heated and squished between rollers to the desired thickness

Foil: The copper foil process takes advantage of metal’s malleable property. It is simply smashed into sheets.

Final Product: For the PCB, copper foil only needs to cover certain areas of the substrate, and those areas need to be precise. To accomplish this, many methods are used, but I will explain an additive photoengraving process. Additive refers to adding the copper foil to the substrate then engraving, as compared to a subtractive process, where the substrate is removed to expose underlying copper. Photoengraving refers the use of light to remove areas, in this case UV light. To start, a positive photoresist mask, made of Polymethyl methacrylate, is placed on the foil. When a positive photoresist is applied to something, it becomes soluble when exposed to light. Then a pattern is placed over the board. The pattern is translucent where the foil should be, so that the photoresist in those areas becomes soluble when the UV light hits. When the radiation is absorbed by the photoresist, acids (hydrogen cations) and sulfonate anions are produced and diffuse. The acid makes the exposed area soluble to the next step, which is where an alkaline developer is introduced, dissolving the exposed photoresist, leaving copper in the shape of the pattern. That copper is coated with a tin-lead solution to prevent oxidation. The excess photoresist is removed. In the final step, the copper on the surface of the board is washed with an iron chloride acid , to expose the substrate beneath. The copper coated with the tin-lead is protected from the wash, and therefore stays on the board.

Background Research

How it is made: Since the manufacturing process contains a lot of chemistry, it is pretty much all explained in the previous section. However, a few more steps need to be completed before the PCB can become a functioning electronic. After the pattern is all set and made, the board needs holes drilled for the leads of the components. Take for example, an LED. The two prongs on the bottom an LED are called the leads, and those would be placed and soldered into the holes. More components like switches, buttons, motors, and ICs are added by a machine called a chip shooter. The chip shooter is incredibly accurate and fast, plus an amazing example of applied science.

Where it is made: Since PCB production requires precision and advanced machinery, they are made in a manufacturing plant. There are ways to do it at home, but those methods do not include the chemical processes described in this project

Resources

https://en.wikipedia.org/wiki/Printed_circuit_board

PCB production process: chemical etching w/ ferric chloride or ammonium persulfate

http://www.madehow.com/Volume-2/Printed-Circuit-Board.html

Materials in PCBs: fiberglass epoxy, copper foil, copper coated with tin-lead

https://en.wikipedia.org/wiki/Photoresist

Photoresist polymers

Poly(methyl methacrylate)

https://en.wikipedia.org/wiki/Poly(methyl_methacrylate)