The Electronic Services Printed Circuit Facility

* Introduction
* Equipment
* Technique
* Boards
* Hole drilling
* Production Procedure
* Preparation
* Exposure
* Developing
* Preparing the etch solution
* Etching
* Surface finish
* Before you go ...

[ Workshop index]

Introduction

The Electronic Services Unit in the department of Electrical Engineering, room 4 AB 21, has a facilty for the production of simple printed circuit boards (PCBs). These will be of glass fibre, single or double sided and up to 22 by 25cm in size. It is not possible to include plated-through holes.

This guide is intended for those wishing to operate part or all of the process themselves.

Equipment

Consumables -

Technique

The standard photo-lithographic method is used. The copper-clad board is coated with a 'photo-resist' layer sensitive to ultra-violet (UV) light and exposed to UV through a mask containing the pattern of circuit lines. The board is then passed through a developer to wash away the exposed parts of resist. Next, the board is immersed in an etchant solution to remove the parts of the copper surface no longer covered by the resist layer. Finaly, the remaining photo resist is removed with solvent to produce a surface to which solder can adhere.

Artwork

You need high quality artwork to obtain satisfactory circuit boards. Many software packages are available to design board layouts. Which one you use is less important than the method of printing the design.

A positive image of the circuit tracks must be obtained on transparent or translucent plastic film (i.e. the parts destined to become copper tracks must be opaque). Several different methods are used to achieve this but with each it is preferable if the image exists on the side of the film physically touching the board during exposure. If this is not possible expect a loss in line width comparable to the thickness of the film. This is due to seepage of UV under the mask. If only a negative image is available a positive image can be produced from it using a photo-sensitive reversing film.

The highest quality results will be from films produced by companies specialising in the conversion of data generated by computerised PCB layout design machines. Though this will cost at least £40, even for slow 'turn-around', the method is almost essential when laying out boards with complex digital logic.

The circuit may be output directly to a laser printer on special plastic film ( Rapid Electronics 39-0776). If the printer has a contrast adjustment then this needs to be set to maximum to obtain opaque traces. The HP Laserjet 5 in the first year laboratory is adequate.

Layouts existing on plain paper may be transfered to transparent film by direct Xerographic photo-copy. Unfortunately, the copy will not be high contrast and the quality of boards produced in this way will depend upon the amount of effort expended in manual re-touching of the image.

The amount of etchant used will be reduced if the layout is arranged to leave as much as possible of the board unetched. Large areas of 'ground-plane' will usualy improve circuit performance as well.

Boards

The quickest and best results are obtained using ready-coated positive glass fibre boards (e.g. RS397-0097). This can be supplied by Electronic Services.

Hole drilling

A light-duty PCB drilling machine is available to users with small (<20) numbers of holes to drill, together with tungsten-carbide drill bits of 0.8, 1.0 and 1.3mm diameter. For larger jobs users are encouraged to use the machine in the Mechanical Services Unit.

Production Procedure

The following procedure should be followed by those operating the PCB fabrication equipment. It should be read through in entirety before begining work. If any detail is unclear please seek assitance from a member of the work-shop staff.

Preparation

Boards that are to be dip-coated should be cut to size with a border of about 5mm; trimming to the exact size may be carried out after etching. For all boards, a hole of about 1.7mm should be drilled in one corner.

The raw guilotined edge of the board will have a large number of loose fibres and debri along it. It is ESSENTIAL that as much of this as possible is prevented from contaminating the resist dip-tank. Although PCB fabrication does not require dust-free conditions as stringent as those employed for micro-electronic work, the quality of results depends upon the degree of cleanliness employed. Use a file or emery cloth to smooth the edges of the board. The copper surface should be prepared using a rubber block containing abrasive. This is followed by wiping with a pad soaked in iso-propyl alchohol, taking care to clean the edges. After this stage, hold the board only by the edges to avoid finger marks.

Fan ventilation for the fabrication room, 7 AB 21, is operated by a switch to the left of the door. IRRITANT and CORROSIVE fumes are involved; please ensure the ventilation is in operation. Protective clothing is available to avoid stains from splashes of resist, developer or etchant. The latter is particularly difficult to remove.

A key to the fabrication room can be obtained from the work-shop. Please return it IMMEDIATELY after the door has been unlocked.

Exposure

For double-sided boards, the simplest technique is to expose both sides simultaneously using the large Packman exposure unit. The two artwork masks may be taped together along three edges to ensure registration of the images, then the board inserted between them to form a 'sandwich' arrangement. Pull out the drawer from the front of the exposure unit and lift the top sheet of plastic. Place the board and masks on the drawer as far towards the back as possible and lower the top sheet back down over them. Holding the gray time control knob down, rotate it anti-clockwise to the '0' position.

Switch on the mains and the vacuum pump. Air should then be evacuated from between the two plastic sheets, thus esuring good contact between the photo-masks and the board. Close the drawer. Push and rotate the time control knob to the 3.5 minute setting or whatever time is indicated by the table of exposure times displayed above the unit. Using the toggle switches, select both banks of lights and operation by timer. Press the red reset button to begin exposure. When complete, switch off the pump and open the drawer to remove the board. A faint image of the tracks can usualy be seen at this stage.

For single-sided small boards use the Tecam exposure unit, making sure the mask is the correct way up and positioned between the glass and the board which should have the copper side facing downwards.

Whichever unit is used, the exact exposure time is best determined by trial and error. It is influenced by a great many variables such as the viscocity of the photo-resist, its age, the contrast ratio of the artwork and the age of the UV fluorescent tubes.

At all times avoid looking directly at any source of UV since it is harmful to the eyes.

Developing

Having exposed the board to UV, it must now be 'developed' by imersion in a solution of sodium hydroxide (NaOH). This will disolve those portions of photo-resist which were exposed to UV. The solution is made up by adding 150 grams of sodium hydroxide powder to a full tank (10 litres) of water. When development times become excessive then further lots of NaOH may be added to replenish the solution.

You have a choice of two methods for suspending the board in the developer:

  1. Place the board in the basket supplied with the tank. This works for larger boards.
  2. Pass a length of PVC coated wire or cord through a hole drilled in the corner. This is best for small boards which would otherwise be at risk being dislodged from the basket by tank agitation.

An image of the tracks should be quite clear after one minute and development complete within about 5. Again, however, development time is influenced by several factors and the board should be inspected every 2 minutes until all unwanted areas of photo-resist have been removed.

If required, development can be halted by washing the board in the spray tank. The stopcock for this is located under the sink by the tanks. If it is considered that development is incomplete the board may be returned to the sodium hydroxide bath. If after about 15 minutes the board has not finished developing, it is probable either that it was not exposed for sufficiently long or else there might be a fault with the resist or developer.

The last traces of resist may have to be removed by gentle wiping with a damp cloth. Avoid skin contact with the developer, which is corrosive. If contact occurs wash with cold water.

If, due to an artwork fault or exposure error, part of a track has lost its resist coating this may be manualy restored using an 'etch resist pen' to ink in the line.

Preparing the etch solution

An aqueous solution of ferric chloride (FeCl) is used. FeCl hexahydrate in crystalline form is available from RS Components Ltd., stock No. 551-277. It is available in the more convenient granular form from Mega Electronics Ltd., stock No. 600-013. Do not use FeCl in its anhydrous form.

Fill the etch tank to capacity (10 litres) with hot tap water, switch on the heaters and bubbler and slowly add 5kg of crystals.

The use of eye protection is recomended during use of chemical solutions.

Etching

Make sure all developer has been rinsed off the board. As a final check that development has taken place correctly, dip the board (suspended from a length of plastic cord) into the ferric chloride etchant for 5 seconds. Remove and wash in the water spray tank.

The portions of the board to be etched away should have turned a dull pink colour whilst the tracks remain the same in appearance. If the result of this test is unsatisfactory the board may be cleaned, re-coated and re-exposed. If, however, it seems that only a small area of copper that should remain is going to be lost then this may be prevented by manual re-touching with an etch resist pen.

If all is well at this stage, top up the level of etchant with water until it is within about 3cm from the top of the tank. Switch on the tank heater. To reduce evaporation losses do not switch on the air supply until the tank has reached working temperature (about 40 minutes from cold) and keep the lid of the tank on.

Suspend the board in the etchant, checking its condition every 2 minutes or so until all unwanted copper areas have been removed. This point is readily apparent due to the difference in colour of the underlying glass fibre. As a rough guide, etching will take as little as 5 minutes with a hot, fresh etch solution using bubble agitation but up to an hour with cold, exhausted solution without agitation.

To avoid splashing, keep the lid on the tank whenever air agitation is used. Use the water spray tank to clean any etchant clinging to the board. Switch off the air bubbler and replace the lid of the tank. Clean any spills of developer or etchant immediately since the latter are extremely hard to remove when dry.

Surface finish

An unprotected copper surface is vulnerable to tarnishing and finger-marks. Whilst it is possible to leave the tracks coated with resist, this makes soldering considerably more difficult. The usual alternative is to remove the resist and spray the entire board with a thin coat of laquer (RS 554-989), although this, too, inhibits soldering to some extent.

The photo-resist left on the tracks may be removed by placing the board in the large glass tray, applying a spot of acetone and wiping off with a dry cloth. The surface may be protected by spraying on a thin coat of PCB laquer. Return the board to the drying cabinet for 15 minutes or so for accelerated drying.

Before you go ...

When you have finished with the dark-room facilities please ensure -

If using the work-shop PCB drill take particular care that the chuck is free to rotate before switching on. The chuck is locked (in order to insert the drill bit) by gently rotating the collar anti-clockwise until the green dot is on the right hand side. It may be neccessary to move the spindle slightly to achieve this. After opening the chuck and inserting the bit, rotate the collar back to the front of the drill to release the spindle.


E-mail:R.Clarke@surrey.ac.uk
Last modified: 2000 February 11th.