Trade secretsHow to make a printed circuit board

Jonathan Hare explores the chemistry behind yourfavourite gadgets

Printed circuit boards (PCB) are found in almost allelectronic devices including mobile phones, iPods,computer motherboards, TVs, calculators, washingmachines, radios and car electronics. A PCB is a rugged

8 | The Mole | July 2013

1. Cut the printed circuitboard to the correctsize and clean it toremove all the dirt andgrease.

2. Draw on the resistartwork pattern,shown in blue (I wrote'The Mole and RSC'and drew in threepads / connections fora resistor, LED andbattery).

3. Etch it for 1015minutes.

4. Remove the board frometch and wash it.

5. Remove the resist withsolvent to reveal thecopper tracks.

6.Drill and fit thecomponents through theholes.

7. Solder the components'leads to the coppertracks. With the PCBturned over you cansee the simple LEDcircuit on the fibre sideof the board.

and effective way of wiring-up electronic components tocreate electronic devices.The basic PCB consists of a thin laminate or fibreglassboard covered on one side with a thin layer of copper.The copper is etched to form a complex circuit of tracksthat are used to wire-up and interconnect the electroniccomponents attached to the board. Modern PCBs inmobile phones and computers usually consist of anumber of very thin boards sandwiched together tomake a complex multilayer PCB.Etching a connectionThe PCB track pattern is made by etching away theunwanted copper to leave the desired interconnections.Firstly an etch-resistant pattern the resist is drawnon to the copper. This can be painted on using asuitable marker pen or screen print techniques can beused. The PCB is then placed in an etch that removes allthe bare uncovered copper, leaving the covered copper(the tracks) untouched.Various chemicals can be used as an etch to removethe copper. Nitric acid and ammonium persulfate canbe used but iron(III) chloride is standard. The reaction ofcopper with FeCl3 is a two step redox reaction:

FeCl3 + Cu FeCl2 +CuCl

FeCl3 + CuCl FeCl2 + CuCl2

Warming and agitating the etch solution (usingvibrations and air bubbles) can greatly speed up theprocess and produce a cleaner finish. After etching,cleaning and drilling, the PCBs are often dipped in atin(II) chloride (SnCl2) compound to deposit a thin filmof tin on the copper tracks. This provides a surface thatdoes not tarnish as quickly as the pure copper but isstill easy to solder. In addition to the chemical etchingmethods, modern CNC micro-milling machines are alsobeing used to create PCBs. Channels can be cut into thebare copper using a tiny cutting tool to create complextracks and wiring circuitry.www.rsc.org/TheMole