Materials Included In Kit

Additional Materials Required

Prelab Preparation

• To prepare the developing solution, place all 100 g of sodium metasilicate in a 5 L beaker or Erlenmeyer flask. Then slowly add 2 L of distilled water and stir the solution until all the solid has dissolved.
• To prepare the etching solution, place all 500 g of iron(III) chloride in a 2 L beaker or Erlenmeyer flask. Then slowly add 1 L of distilled water and stir the solution until all the solid has dissolved. If you intend to store the solution, add about 5 mL of concentrated hydrochloric acid to help prevent the hydrolysis of the iron.
• The teacher handout has four different images printed on it. These should be printed onto the included transparencies and cut out before the lab. Alternatively you can have students make their own designs and print them on the transparencies. In order for the images to transfer onto the resist, they need to be in black and white (no gray) and printed using a laser printer at a resolution of 600 dpi or higher. Ink jet printers will not work.

Safety Precautions

The sodium metasilicate solution used to develop the board is a corrosive alkali solution. It can cause chemical burns. Gloves and goggles must be worn at all times. Iron(III) chloride etching solution is corrosive and acidic. It causes skin irritation, serious eye damage and corrodes metals. It also stains a wide range of materials. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult with your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Both the developing and etching solution can be reused until such time as they take longer than 5 and 60 minutes to work respectively. Developing solution may be rinsed down the drain with exces water according to Flinn Suggested Disposal Method #26b. Used etching solution contains both a mixture of both iron and copper ions and may require neutralization and filtration before disposal. Once this solution is clear and neutral, it may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The glass panes for holding down the transparency need to be basic glass and not conservation or museum glass. The latter two block UV light, which will interfere with the exposure. Large glass beakers and evaporating dishes can also be used to hold the transparency down.
• Developing trays can be made from either plastic or glass. Since the etching solution will be heated, it is highly recommended that these are glass.
• The amount of liquid in the developing trays only needs to be enough to cover the board.
• Make sure students do not contaminate the etching solution with the developer. The developer is basic and will cause iron(III) hydroxide to precipitate from the etchant.
• Exposure time not only depends on the wattage of the light, but also on distance from the board. The exposure time of 8 minutes is for a 15 watt fluorescent light 5 cm above the board. Doubling the distance to the light approximately triples the exposure time.
• If the plate is over exposed, an overly concentrated developing solution is used, or the plate is left for too long in the developing solution, all of the photoresist may come off. If this occurs, have students draw on the exposed copper with a permanent marker. When placed in the iron(III) chloride solution, only the areas that have not been drawn on will be removed. This can also be used to make minor repairs or corrections to the images following development.
• A better result is achieved when the side of the transparency that has been printed on is against the board. For this reason, images should be mirrored before printing. Failing to do so can result in some fine detail being lost.
• Iron(III) chloride reacts with most metal instruments commonly found in the lab. It is for this reason that plastic forceps are provided in the kit. It is also advisable to use either a glass thermometer or an infrared thermometer when checking the temperature of the etching solution, as the solution can corrode metal temperature probes.
• The iron(III) chloride etching solution can be reused multiple times. Adding a small quantity of hydrochloric acid to the solution will help to improve is lifespan. Be aware that as the solutions get older they take longer to etch the copper boards.
• Additional presensitized copper boards are available from Flinn (Catalog No. AP9770).

Teacher Tips

• Some students may wonder why a solution of iron(III) chloride is acidic. This can be an opportunity to introduce the concept of a Lewis acid, or alternatively you can explain by introducing the concept of the complex ion. Equation 3 demonstrates the acidity of the iron(III) hexaaqua complex.
  {12366_Tips_Equation_3}
• This kit can be used as part of a wider activity relating to electricity and circuits. Students can use the materials in the kit to design and print their own circuit board.

Teacher Handouts

12366_Teacher1.pdf

Introduction

Electrify your redox lessons with printed circuit boards! Students will use photoengraving and chemical etching to generate their own printed circuit board while learning valuable chemistry concepts. Students can even design and print their own works of art!

Concepts

• Oxidation–reduction
• Solubility
• Photochemistry

Background

Printed circuit boards (PCBs) are commonly found in everyday pieces of technology. They are inside calculators, computer mice and radios; in fact, almost all but the simplest of electrical products contain a PCB. In this laboratory activity, you will prepare your own printed circuit board using a presensitized copper clad board. This board consists of a sheet of plastic with a thin layer of copper on top. The copper layer is coated with a positive photoresist.

A positive photoresist is a material that becomes more soluble following exposure to light. In order to pattern the board, a transparent plastic sheet with the desired image drawn on it is placed over the board. Following irradiation, the board is placed in a developer solution that will remove the exposed sections. Since the unremoved sections are the same as the image on the transparency, it is considered to be a positive image. Negative photoresists become less soluble following irradiation and require the transparency to be printed with a negative image of the pattern to be developed.

After the irradiated board is developed, areas of exposed copper can be seen. The exposed copper is then removed through an oxidation–reduction (redox) reaction. Redox reactions involve the transfer of electrons from one element to another. There are two key parts present in every redox reaction—an element that is oxidized and an element that is reduced. Reduction of an element occurs when an element gains electrons and becomes more negative. As a result, the charge on a reduced element is decreased during a chemical reaction. Oxidation of an element occurs when the element loses electrons and becomes more positive. The net result is that the charge on an oxidized substance is increased during the chemical reaction.

An etchant is a corrosive chemical used to remove a layer of material. The most common etchant used to remove the exposed copper during the manufacture of PCBs is iron(III) chloride. The removal of the copper takes place over two separate redox reactions. In the first reaction (Equation 1), the iron(III) is reduced to iron(II), and the copper is oxidized from its elemental form to copper(I).

Safety Precautions

The sodium metasilicate solution used to develop the board is a corrosive alkali solution. It can cause chemical burns. Iron(III) chloride etching solution is corrosive and acidic. It causes skin irritation, serious eye damage and corrodes metals. It also stains a wide range of materials. Gloves and goggles must be worn at all times. UV light can cause eye damage; do not look directly at any UV sources.

Procedure

1. Obtain one of the transparencies with the image you wish to develop on your board.
2. Make sure you are aware which side the image is printed on.
3. Darken the room.
4. Open the packet containing the copper board.
5. Peel off the protective material from the board (some boards are in black plastic sleeves).
6. Place the board under the exposure source with the copper side up.
7. Place the artwork on top of the board so that the printed side is against the photoresist.
8. Place the glass on top of the transparency to ensure it does not move during exposure.
9. Turn on the exposure source. For a UV source, exposure time is 60 to 90 seconds. For a 15 W fluorescent lamp, it is eight to ten minutes. Do not overexpose your board.
10. Turn off the exposure source and remove the glass.
11. Transfer the board to the developing tray containing the sodium metasilicate solution (this should be done as soon as possible following exposure).
12. Gently brush the surface of the board with the foam brush to aid removal of the resist layer.
13. As soon as the remaining resist looks exactly like your artwork (this should take no more than two to three minutes), remove the board from the solution and rinse with the distilled water in your rinsing tray.
14. Check the temperature of the developing tray containing iron(III) chloride etching solution. The ideal temperature should be between 50 °C and 60 °C.
15. Immerse the board in the iron(III) chloride solution and gently agitate the solution. This can be done either with a clean brush or by stirring the solution with a glass rod. Be very careful not to splash or spill the etching solution as it is toxic, corrosive and can stain.
16. As soon as the etching process is complete (15 to 60 minutes), remove the board using the plastic tongs and rinse with fresh distilled water in your rinsing tray.
17. The remaining photoresist can be removed by using a cotton swab or cotton ball soaked in acetone or nail polish remover. This step is optional and electrical components can be soldered onto the board through the resist.