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Producing a PCB

8 SAFE AND ECONOMICAL STEPS TO PRODUCING A 'PCB'

The following is a guide to making simple circuits using mega UV units, tanks and consumables.

1. PREPARING THE ARTWORK

Firstly design the required circuit. The best option is PCB design software and print or plot a 1:1 artwork onto a translucent or transparent film. Use a Laser printer onto LaserStar Film 100-062), or on an Inkjet Printer with JetStar Premium Film (100-075) with the software or printer driver mirror image of the circuit as this will ensure the black areas of the artwork will be in contact to the PCB's Photoresist during UV exposure.

2. EXPOSING THE MICROTRAK PCB

Select a suitable piece of our Microtrak Board. If necessary cut a piece using a shear from the range that we offer, (700-400) 8 (700-500) 12 (27-18032) 12 or (700-274) 25 SHEAR.

Remove the protective blue plastic from the Microtrak to reveal the photoresist.

It is very important to ensure that the board is fully exposed. Typically we recommend exposure times of 2 - 3 minutes in our (300-001) LV202E 2 tube exposure unit and 1-2 minutes in all our other exposure units using our inkjet film. However if you are using a more opaque film such as tracing paper or even ordinary paper (not recommended) exposure times will need to be longer. This would have to be determined by trial and error. If the resist is not exposed for long enough it will be difficult to develop it all away.

Note: Always ensure the exposure unit has been warmed up before using it order to ensure the tubes are giving their maximum output of UV light.

3. DEVELOPING THE IMAGE

Mix the developer as follows:

4007 Photoresist universal powder developer part no: 600-007
Add 50 grams of the powder to 1 litre of hot tap water. Stir the powder until it has all dissolved in the water. Allow the developer to cool down to room temperature and use between 18 and 25C.

4006 liquid photoresist developer concentrate part no: 600-010
Add 80 ml of developer concentrate to 920ml of water to make 1 litre of working solution. This is an 11.5:1 mix. Use between 18 and 25C. Although a tray can be used we recommended a Mega heated process tank such as our PA107 or the developing tanks of a PA310 Tri-Tank

Please bear in mind that when exposing and developing a board the quality of the artwork and UV unit are the most critical parts of PCB production. It is very important to ensure the board is not over-developed. The developed image can look good, but over-developing can cause a reduction in the thickness of the resist, which could then break down at the etching stage.

After exposing the board place it in the developer and leave for 30 to 90 seconds depending on the temperature of the developer; the higher the temperature the quicker the development. Agitate the board in the developer every few seconds to ensure even development.

When developing, remove and wash after 15 - 30 seconds. If the board needs a little more development, then immerse into the developer for a few more seconds and then wash immediately again.

4. SPRAY WASHING

Use the Spray Wash Tank of your Rota - Spray Plus or PA Tank, immediately wash the board after it is removed from the Developer. Apart from the PA103 all have a solenoid operated valve to turn the Spray Wash water on. Note: The same instructions apply to immediately wash after removing from the developer if you are processing the board in a tray/sink.

5. ETCHING

The developed and washed board is then placed into the Etching Tank. If you are using a PA series Bubble Etch Tank the board is placed in the lid / basket holder and immersed into the etchant, with the lid on the Air Pump is turned on and at normal etching temperature of 45 C for Ferric etching should take approximately 5 minutes.

For very fine detail, the board should be turned upside-down halfway through the etching cycle (always spray wash the board before handling). Never lift the lid off the etching tank with the air pump still turned on.

If you are using a Rota-Spray follow the instructions supplied with the machine; etching with ferric will take about 90 seconds in a Rota-Spray.

When etching is completed, spray wash as before.

Your etched board will now consist of the circuit covered in photoresist. Your PCB is perfectly functional at this stage and leaving the photoresist on the copper stops it getting oxidised and it also acts as a reasonable solder flux too. Alternatively you can strip the resist and tin plate as below.

6. RESIST STRIPPING

The PCB will now consist of an etched circuit still covered in the photoresist. This can be stripped off using our (600-019) PC155 stripper or solvent such as our (600-024 or 600-025) Universal solvent or acetone. If using the (600-019) PC155 resist strip solution with a PA107 Universal Tank use at 45C - 50C. As before the lid/board holder is raised at any time to examine the board. The photoresist should be stripped off in approximately 2-3 minutes. Once the photoresist has been stripped from the circuit the board holder and board are then spray washed.

7. SCRUB CLEANING

The board is then thoroughly dried and mechanically scrubbed with our (900-009) PC182 Scrub Block This will ensure that the copper circuit is perfectly clean by removing any oxidisation, passivation or chlorides left by the water as well as any other impurities. After scrubbing, any small particles left by the scrub block should be wiped off with a clean dry tissue. Alternatively a Scotchbrite Pad can be used.

8. TIN PLATING

Before the clean board has had a chance to oxidise again it should be immersed in a PA107 Universal Tank or plastic tray containing (600-021) PC168 immerse tin solution for 7-15 minutes. Once immersed in the solution this unique tin which operates at low temperature will begin to work immediately and plate a very smooth surface of small molecules of tin onto the copper circuit. This will stop the copper from oxidising and will also act as a solder flux. For best results when you remove the board from the tin solution immerse immediately in cold water then rinse thoroughly in hot and wipe dry with a paper towel.

TECHNICAL TIPS

CIRCUIT DESIGN

  • Align all your components with the design to make sure they fit.
  • Ensure your tracks are wide enough to carry the required current.
  • Design your circuits, so that all your common components e.g. resistors / capacitors are next to each other. This will greatly speed assembly time
  • Always make sure your circuit is as small as is practically possible.

ARTWORK PREPARATION

With Microtrak Pre-Sensitive PCB material it is very important to ensure that you have a good opaque artwork which blocks out UV light. This can be achieved with our inkjet A4 and A3 Printer and Jetstar premium film.

Alternatively, if you wish to use a laser printer then we would recommend our LaserStar

  • Check to see if the artwork is dense. The black areas should appear black and opaque, not grey when it is held up to the light. If not; touch up the artwork with UV opaque pens
  • Finally, always remember quality artworks will give quality boards.

PCB LAMINATE SELECTION

  • Always use pre-sensitised laminates, not etch resist pens / transfers on uncoated boards. This provides false economies as it is time consuming to do, modifications are difficult, only simple circuits can be made and a mistake found after etching means the whole layout has to be done again.
  • For accurate and safe cutting of laminates, we recommend use of one of our purpose-built shears such as the MEGA (700-400) 8 (700-500) 12 (27-18032) 12 or (700-274) 25 SHEAR. Hacksaws, band-saws etc. produce harmful dust and splinters and accuracy is difficult. Buying larger sheets and cutting them yourself is more economical and you get to keep the off-cut!

EXPOSURE

  • Only remove the blue protective film when ready to expose the board.
  • Typically we recommend exposure times of 2 - 3 minutes in our LV202-E 2 tube exposure unit and 1-2 minutes in all our other exposure units using our clear inkjet film. However if you are using a more opaque film such as tracing paper or even ordinary paper (not recommended) exposure times will need to be longer. This would have to be determined by trial and error
  • Generally under-exposure makes it difficult to develop off the unwanted areas or photoresist and over-exposure causes undercutting and thinning of tracks and pads, as will poor contact between artwork and photoresist layer.

DEVELOPING THE IMAGE

  • For Microtrak board use one of our temperature controlled tanks with our sodium hydroxide free developers (600-007) 4007S005 Seno Universal Developer (powder) or for best results (600-010) 4006V05 Seno Liquid Developer (liquid concentrate). Both developers have a good tank life..
  • If resist is not totally removed during the developing, observe the following: (a) Your exposure time is too short - increase time by 25%, if still poor- check (b) Your developer is getting exhausted -Increase developing time or replenish developer. (c) Developer is too cold - check temperature - it should be 18C minimum.
  • Resist is totally removed by the developer: (a) The boards have been grossly over-exposed or artwork was not opaque enough. (b) Developer is too strong - dilute to suit. (c) Developer is too hot - do not exceed 25C.
  • N.B. If the board has been developed properly, you will find that if you put it in the etchant for 10 seconds, then remove and spray wash it, the copper to be etched away will be a dull pink. If however there are any shiny spots or areas other than that of your image then these will be areas of photoresist which have not developed away. If this is the case, put the board back in the developer for 30 seconds, remove and spray wash it then return board to the etchant. Repeat if necessary.

ETCHING THE BOARD

Ferric Chloride is the most widely used etchant, because it is inexpensive, has long tank life and etches quickly and efficiently. The one drawback is that it stains, however if you wear protective clothing and ensure any spillage is wiped away immediately with a damp cloth you should have no problems. MEGAs Ferric Chloride Stain Remover (600-039) can be used for removing stains from clothes, tanks, etc.

  • The alternative to Ferric is our (600-014) fine etch crystals, which are as safe as ferric and when in solution are very clean. The problem with Fine Etch is: - (a) Longer etching times (b) Short tank life (2 - 3 weeks), which can make it expensive if used infrequently. (c) Higher etching temperature, 50C
  • If the resist breaks down in the etchant: - (a) The board has been over-developed - Reduce developing time (b) Etchant is too hot - check temperature Optimum 40 - 45C ( maximum 55C) for Ferric
  • If the pads and tracks are badly defined and etching time is long: (a) Etchant needs changing - Replace when time exceeds 20 minutes (b) Resist has not been totally removed during developing - Return board to developer for 30 seconds then wash and return to etchant. If results are still bad, board is a failure and you must refer to Exposure and Developing tips for your next board.

IMMERSION TINNING

  • You could just leave resist on, but tinning gives following advantages: (a) You will give a professional finish to your board. (b) The tinning will greatly improve the solderability of the board. (c) The tin will protect the copper from oxidisation.
  • For good tinning (a) Make sure the copper surface is thoroughly clean (b) Avoid solution contamination (c) Carefully follow rinsing instructions (d) Tin looks very thin - change solution.

DRILLING

  • Never use a normal pillar drill - it is inaccurate, tedious and too slow.
  • Always use a precision high speed (15,000-20,000 minimum r.p.m.) drill e.g. our (700-014) DM403 VARIABLE SPEED DREMEL DRILL
  • Use our purpose built (700-310) PCB Drill Stand. A PCB drill stand will improve accuracy and prevent drill wander and breakages.
  • Always use tungsten carbide drill bits. At approximately one third of the price MM reground tungsten carbide drill bits offer excellent savings.

HEALTH & SAFETY

Even though our range of PCB Chemistry is very low in hazardous contents, but their nature all PCB chemicals will be hazardous to some extent - e.g. corrosive; irritant etc.

This means you should ALWAYS wear protective clothing, gloves and goggles and ALWAYS refer to full Health and Safety data sheets before mixing or using.

It is recommended that when using machinery, a R.C.D. Adaptor is fitted to your power supply.

If in doubt consult MEGA.

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