Making PCBs

Manufacturing PCBs at home successfully

Introduction

Previously I have written articles on how to setup and use Eagle CAD to capture schematics and design PCBs. This article is not CAD tool specific, it details the procedure and equipment used to successfully circuit board at home, with some lessons learned.
A UV photo sensitive process is used.

Other articles in the series:
Starting out with Eagle CAD
Practical PCB design tips with Eagle CAD

Creative Commons License
This work is licensed under a Creative Commons Attribution 3.0 Unported License  

Suppliers

All the equipment listed below came from either Rapid Electronics (www.rapidonline.com) or ESR Electronic components (www.esr.co.uk).
The Photo sensitive PCB comes from the aforementioned suppliers, though I occasionally use Farnell (www.farnell.com) or Spiratronics (www.spiratronics.com).

Equipment

  • Fine tipped permanent marker pen, for minor corrections
  • Seno PCB developer or Sodium metabisulphate developer
  • A Mega Electronics LV202 UV light box
  • Mega Electronics Laserstar Artwork film (Rapid P/N 39-0774)
  • Tracing paper, 63 gsm
  • Developer tray (Rapid P/N 50-5488 or ESR P/N 334-190)
  • Ferric chloride etchant
  • Velleman ET20 bubble etch tank
  • Mini drill and stand.
  • PCB drills.
  • PCB Flux spray

Choosing the right PCB material

I only use, pre-coated, photo-sensitive PCB material. In the past I have tried and failed, to successfully coat copper clad board. If the coating did go on evenly, it would not expose properly under UV light. It has been cheaper to buy coated PCBs.

The next choice is the material, SRBP or FR4. SRBP is recommended for hobby projects as it is easy to cut and drill and cheaper. It is not as rigid as FR4 and has a slightly lower glass transition temperature but for all but the finest SMT design will suffice.

FR4 is more rigid, harder wearing and has improved dielectric properties, important for higher speed (1GHz edge rates or greater) designs. It is more resilient to higher temperatures, within reason.

Procedure

The process flow successfully used many times is detailed below:

  1. Warm UV light box up for at least 5 minutes.
  2. If using tracing paper transparency, 2 minute exposure, otherwise a 2.5 minute exposure.
  3. Develop PCB in developer solution or use Seno 120 developer.
  4. Wash board and allow to dry
  5. Heat ferric chloride to 50C
  6. Insert board and turn on bubble etch, wait 5 minutes or until boards have been etched.
  7. Remove and clean PCBs
  8. Allow to dry.
  9. Trim board to size.
  10. Drill it
  11. Clean it, removing photo-resist.
  12. Spray PCB flux.
  13. Assemble PCB.
  14. Inspect PCB.
  15. Test PCB

Most of the points are straightforward but some deserve further explanation.

Exposing and developing artwork

This covers steps 1-3 of the list above.
When printing the artwork, it is important to ensure that the printed side of the transparency is in contact with the PCB. This is to reduce the chance of the UV light undercutting the negative image, thus reducing your trace widths. For a bottom only single sided board, the orientation should be correct, for a top layer, you will need to 'mirror' the artwork. It is easier if you print this out and try it.

The type of transparency used has an impact on the exposure and etching process, this will be shown later on. For best results, on SMT boards, you must use a PCB transparency, like the LaserStar material listed above. For larger geometry boards, for example through hole, you can use tracing paper, which is considerably cheaper.

Transparencies-small
On the left is the 63gsm tracing paper, on the right is the Laserstar transparency. Notice how the blue background is different under each transparency, the Laserstar has greater opacity, hence the longer development time under the UV light.

All transparencies are printed using a laser printer. They are readily available and cheap these days and perfect for PCB work. I personally have not used any inkjet transparencies for PCB work.

Drilling PCBs

If you intend to build a number of PCBs, it is worth investing in some good quality PCB drill bits, drill and stand.

Reduced shank drill bits, shown here are less flexible due to their thicker shaft and are normally tunsten carbide tipped. A standard HSS drill bit will not last long when drilling FR4.
Drilling_part2-small

These drill bits are readily available from Rapid Electronics, ESR and from Ebay. As a minimum, for PCB work, I would recommend 5 sizes, 0.60 mm, 0.70 mm, 0.90 mm, 1.00 mm and 1.20 mm as this will cover the vast majority of through hole components, from small diodes, sockets to power transistors and 7/0.2 (24 AWG) wire.

To protect your fragile drill bits, purchase a suitable mini-drill and stand, I use an old Expo drill and stand.
Drilling_part1-small1
 You will also notice the overhead lamp, important to see where you are drilling!
























Example boards

A couple of example PCBs manufactured using the process shown here.

PCB_Laser_star_fine_detail-smallThis is the prototype micropower Arduino PCB. It uses 0805 SMT discretes, SOT23 ICs and has 0.254 mm track spacing. It was developed using a laserstar transparency. Note the text in the copper.

PCB_tracing_fine_detail-smallThis board was made in the same batch as the one above, the difference was the transparency. You can see the rough edges on some of the 28 pin SPIC package pads and the text on the right hand side has gone.

 

Conclusion

With some patience and trial and error, it is possible to produce detailed, good quality PCBs at home. This article demonstrates what is possible and identifies some of the mistakes made and how to fix them.

The most important points are that once you have established a process that works, stick to it! Before committing to a PCB, ensure that your tracking rules are sensible for your PCB fabrication method. If you fail to follow the tracking rules, you will have short circuits or broken tracks, causing many lost hours.

Updated 08 July 2017