SOP 12.Standard Operating Procedure for Printed Circuit Boards

I.Statement of Coverage

The intent of this SOP is to demonstrate the utility of this format in creating your own SOP’s for the specific hazards in your own laboratory.

The cleaning, exposing, and etching of copper printed circuit boards using Kodak KPR photoresist and developer, Ammonium Persulfate etchant and Kepro Tin Plating Solution involves potential exposure to corrosive oxidizers and toxic organic and inorganic chemicals.

II.Hazard Assessment

Primary hazards include:

ProcessStep Hazard

Ammonium Persulfate Etchant Strong, corrosive oxidizer

Mercuric Chloride Etch CatalystToxic inorganic

Monomethyl Glycol Ethers Photoresist Solvent Carcinogenic, mutagenic, neurotoxic, bone marrow and immune system depressant

III.Resources

A.Existing Standards

B.Operating Manual Instruction

Kepro BTE-402 Bench Top Etcher Operating Manual

C.Text and Literature References

1.Printed Circuits Handbook, C. F. Coombs, ed., McGraw- Hill, New York, 1979, Chapter 8.17.

D.CHP Appendix III (Section 23.3) Chemical Information Tables

Table 4. Corrosive Chemicals

Table 8. Strong Oxidizers

Table 12. Toxic Metals

IV.Chemical Storage

A.Special Storage

Ammonium Persulfate is a strong oxidizer and should be stored away from easily reduced materials in a cool dry location.

B.Securing Gas Cylinders

V.Personal Protective and Emergency Equipment

A. Eye and Face Protection

Refer to the Eye Protection Policy, Appendix IIB (Section 23.2). At a minimum, safety glasses with permanently attached top and side shields must be worn in the laboratory. These glasses, however, do NOT protect against splash hazards. When performing a hazardous activity, a face shield must be worn in addition to the safety glasses OR switch to chemical splash goggles (with shielded ventilation ports). Face shields are available from the Safety Office (free of charge).

B.Gloves

Butyl rubber gloves should be worn when handling photoresist or developer coated boards and when filling the etcher and other chemical tanks. Gloves should also be worn when cleaning, stripping and tin plating the boards.

C.Protective Clothing

Lab coats should be worn to protect against the oxidizing etchant.

D.Hearing Protection

Not required.

E.Respirators

Not required.

F.Eye Wash

Where the eyes or body of any person may be exposed to hazardous materials suitable facilities for quick drenching or flushing of the eyes and body shall be provided within, or near, the work area for immediate emergency use. Bottle type eyewash stations are not acceptable.

G. Safety Showers

A safety shower should be available and functioning as specified by ANSI Z358.1

H.Fire Extinguishers

All laboratories must contain at least one Carbon Dioxide (Type B-C) or Dry Chemical (Type A-B-C) fire extinguisher. Additional fire extinguishers are located near exits and/or stairwells in each building. Special Class D fire extinguishers (for certain metal fires) are available from the Safety Office.

VI.Controls

A.Designated Areas

Mercury compounds are reproductive toxins.

B.Chemical Fume Hoods

Standard hoods are required for enclosing etchant tank, dip coater, oven, and developer.

C.Glove Boxes

Not required.

D.Safety Shielding

Not required.

E.Special Ventilation

Not Required

F.Vacuum Protection

Not required.

G.Signs and Labels

  1. Doorways: Reproductive Toxin
  2. Containers: All hazardous materials must be clearly labeled with the correct chemical name.

H.Utilities

  1. Photo-safe lighting desirable for handling photo-sensitive materials.
  2. Hot water required for cleaning boards and for completion of tin plating.

I.Fire Protection

VII.Specific Procedures

A.Preparation of Copper Blanks

After cutting board to approximately 1/2 to 1" larger than finished board size and removing rough edges with a coarse file, board blanks are washed with "409" cleaner or Kepro copper cleaner. Boards are scoured with a green scouring pad to produce a clean lightly abraded surface. Scrubbing the whole board right to left, followed by top to bottom is recommended. Boards are either air dried or dried with a lint free towel, the latter is preferred.

B.Coating Copper Blanks

Attach clip to upper edge or corner and suspend from dipping frame. Remove tank cover and start motor to drop boards into tank. Boards should be withdrawn at a slow speed (7 or less) to produce a thin coating. Coated boards are placed in a 90 to 1000 C oven for 10 minutes to dry and harden and then transferred to a dark cupboard to cool. Clips are removed when the board is placed in the oven and washed in the developer tank to remove photoresist. Close the cover on the photoresist tank to reduce evaporation.

C.Exposing Coated Boards

  1. Artwork must be on a transparent or highly translucent carrier. The artwork is placed on Color Key so that the emulsion (duller) side is on the bottom, the shinier backing side is next to the artwork. This sandwich is placed on the rubber mat of the photo-frame with the artwork on top and the cover is closed. The vacuum is turned on to hold everything tight to the glass plate and the UV lights are switched on. Expose for 4 minutes. The Color Key sheet is placed in a shallow dish and enough hand developer is poured in to cover the sheet. Lightly rub with a Kim Wipe or fingers to loosen the unpolymerized coating and aid in the development of the negative. The openings in the negative should be where the copper on the board is to remain. If not, the process should be repeated to reverse the first "negative". The final negative should be marked to indicate top (circuit) side. Filter Hand Developer when returning to bottle to remove pieces of photo-resist.
  2. Attach the developed Color Key negative to the printed circuit board blank with the correct orientation and place on the rubber mat of the photo-frame. Close the cover and start the vacuum pump to draw the cover tight onto the board. Expose for 4 minutes with all lamps lit. Flip double sided boards and repeat on second side.

D.Developing Exposed Boards

Remove negative and attach hemostat to hold board. Immerse in developing tank with occasional agitation for 2 minutes. Rinse board with hot water and check on removal of resist. Re-immerse for another minute if resist remains where copper is to be removed. Repeat until removal is complete.

Troubleshooting.

  1. If photoresist strips too easily either the board was not cleaned and scoured properly, the photoresist has gotten old, the coating was too thick or exposure was too short.
  2. If the photoresist will not strip clean either the exposure was too long or the developer has gotten contaminated.

E.Etching Developed Boards

Turn on and warm up the etcher for at least one hour prior to etching boards. Check fluid levels on the tube in front and wait for the heater light to go out before starting to etch a board. Attach board to carrier with light pressure only and suspend over etching solution. DO NOT OPEN COVER WHEN THE ETCHANT PUMPS ARE OPERATING AND DO NOT TURN ON PUMPS WITH THE COVER OPEN OR YOU WILL BE SPRAYED WITH ETCHANT. USE CAUTION WHEN OPENING COVER, FUMES AND AEROSOLS ARE RESPIRATORY IRRITANTS. Close cover and turn on pump switches. Turn timer to 2 minutes and etch. Rotate board and repeat until copper is removed.

Troubleshooting.

  1. If etching is too slow, undercutting will occur. This may be caused by too low of a temperature of etchant, weak etchant or a lack of catalyst. Temperature should be 90 C for best operation.
  2. If copper turns silver and will not etch, replace etchant solution.
  3. If spots of copper remain, gently scrape and repeat etching or do a post etching mechanical cleaning on the remainders with a sharp knife blade.
  4. If photoresist falls off, see developing troubleshooting comments.

F.Post Etch Preparation

  1. Wash boards in warm water to remove any adhering etchant and dry.
  2. Drill pad holes with high speed drill. Safety glasses are required while drilling. Check oil reservoirs before drilling. Remove drill bits before adjusting height and after drilling to prevent breakage. Only carbide drill bits should be used in the high speed drill.
  3. Cut board to size with the shear and smooth edges and corners with file.
  4. Remove remaining photo-resist with Kepro photo-resist stripper. Board should be dry before applying stripper. Eye protection is required and gloves should be worn to protect hands. Use of a green scrubbing pad to complete removal of the stripper is sometimes useful and should be used if the board is to be tin plated. Rinse boards thoroughly with warm water after cleaning.
  5. Boards to be tin plated should be transferred immediately from the rinse to the Kepro tin plating solution. Scouring the board lightly with a clean, green scrubbing pad will produce a more adherent coating. Boards should be allowed to plate for 1 to 2 minutes or until a bright tin surface is obtained. Complete plating reaction by soaking board in very hot water for 1 to 3 minutes and, then, thoroughly washing with cold water. Filter tin plating solution back into bottle. Dry boards and solder.

Trouble shooting

  1. If there are spots which do not tin plate, clean board with green scrubbing pad.
  2. If tin plating is slow, clean board in solution with scrubbing pad. If this does not resolve the problem, replace the plating solution.
  3. Soldering works best on a freshly plated board. Photoresist can be left on the board to protect from oxidation until the board is needed.

G.Etchant Maintenance

  1. When etchant becomes weak it needs to be replaced with a new solution. Empty the tank with the siphon hose which also serves as solution height indicator on the front of the etcher into a bucket and dispose of etchant as described above. Save approximately 500 ml of old solution for priming new solution.
  2. Check pumps and spray heads and perform any needed maintenance. Return siphon hose to normal position.
  3. Measure out about 2 liters of powdered ammonium persulfate and place in a plastic bucket and carefully add hot water to dissolve. The solution will cool while dissolution is attempted. Carefully pour solution into tank and repeat with additional hot water to dissolve most of the persulfate before adding to tank. Add enough water to bring up to operating level.
  4. Close cover and use pumps to mix and heat up solution. 3 to 4, full timer cycles are usually needed. Add 500 ml of old solution if available and 10 to 12 ml of Mercuric Chloride catalyst (27.15 g/l) and run one more timer cycle with all four pumps to complete mixing.

VIII.Emergency Procedures

A.Notification

Refer to the “Emergency Response” section of the Department Safety Handbook for generic emergency response procedures.

Contact: (University police at 292-2121 or 292-2525, and Division of Environmental Health and Safety at 292-1284 during normal working hours. The Safety Office number is 9-679-1820.

B.Spill Response

Photoresist and developer can be removed from skin or clothing with the Kepro resist stripper, do not use water as this will create an insoluble polymer which is difficult to remove. Contact of any of the processing chemicals with eyes should be followed by examination with a physician.

IX.Decontamination and Waste Disposal

A.Decontamination Procedures

  1. Personnel: See Spill Response.
  2. 2. Area: or
  3. Equipment: Photoresist and solvent should be removed with resist stripper, the area washed down with methanol or iso-propanol and then soap and water.

B.Waste Disposal

Spent etchant will be collected in a 20 gallon bucket and allowed to evaporate to a wet crystal mass before disposal. Etchant contains about 100 ppm of mercuric chloride catalyst in a 3 kg mixture of copper and ammonium, sulfate and persulfate salts.

X.Approvals

XI.SOP Prepared by ___Dr. Dale H. Karweik______Date __7/27/94_____

Reviewed by______Date ______