Chemical Analysis

DO

• Wear an approved face shield. Because good vision is essential, take great care to keep your face shield clean and free from scratches.
• Wear chemically resistant gloves that protect the hands and forearms.
• Wear a lab coat that protects the arms, torso, upper legs and clothing.
• Keep all analysis tools and chemicals in a clean, safe place.
• Keep all tools clean and in good working order.
• Read the Product Safety Data sheets for all of the chemicals that you use in your shop and the warning labels affixed to the sides of the reagent containers found in the chemical analysis kit.
• Keep the name and phone number of an emergency physician and your local emergency poison response center in a convenient location next to the shop phone.

DON'T

• Handle food or smoking materials with the protective gloves. Actually, you should NEVER eat or drink when operating PCB fabrication equipment. Dissolved copper is a toxin and should be handled with the same caution and respect as sulfuric acid or any other hazardous material.
• Handle the plating or analysis materials in a negligent or careless manner.
• Operate any part of your system until you have thoroughly read and understood all of the instructions that came with it.
• Handle un-rinsed etched or plated objects without gloves.
• Allow untrained personnel access to the shop.

Analysis tools

Automatic Pipettor

Test samples for the techniques presented in this chapter are most easily measured with a 100 uL (0.10 ml) fixed volume "automatic" pipettor. To use this device, place a disposable tip firmly on the end of the pipettor. Depress the plunger until the first stop is felt. Immerse the tip into the liquid to be sampled and slowly release the plunger. To dispense the sample, press the plunger all of the way in (past the first stop) to clear the tip. The disposable tip can be re-used several times, especially if the pipettor is used to consistently measure the same bath.

Digital titrator

Using a digital titrator is far superior to pipette titration in terms of convenience, repeatability, and absolute accuracy. The operating principal behind the titrator is usually well described in the manual from the manufacturer and will not be repeated here. One point that is usually left out however, is that the delivery tubes included with the device can be reused indefinitely if they are rinsed out with forced deionized water and shaken like a thermometer to remove any residual liquid. They may also be blown out if care is taken to use compressed air that has been filtered and dried. Always re-use a delivery tube for the same titrant that it was originally used for. Failure to observe this precaution may cause contamination of the titrant syringes and lead to erroneous results.

Always record the results of these analyses in your dated log book for future reference! This data will allow you to order replenishment chemicals in a timely manner and avoid a possible loss of use of your system.

During any chemical analysis, care should be taken to thoroughly mix all of the ingredients used by swirling the beaker in a vigorous manner. Do not spill any portion of the sample as this could affect your results.
 

Acid Copper Plating Bath Analysis and Maintenance

Electrolytic copper plating, as practiced in the electronics industry , is a fairly well understood and stable process. The past 20 years has seen almost constant improvement in throwing power, plating uniformity, and plated copper ductility. Although the baths from most vendors offer relatively trouble-free operation, in order to maintain consistent, high quality plating, copper based electrolytes must be analyzed for four major components:
 

Copper Concentration Sulfuric Acid Chloride Ion Organic Additives

Tin/Lead (Solder) Plating Bath Analysis and Maintenance

Solder (60/40 Tin/Lead) electroplating has evolved into a highly reliable means for co-depositing tin and lead onto the surface of a PCB in a fixed ratio that can be adjusted by varying the operating points of the bath. With the introduction of plating baths based on organic acids (as opposed to flouroboric acid), solder plating has become far more environmentally friendly. For reliable deposition, there are four components that need to be accurately monitored to insure optimum bath performance.
 

SolderOn acid concentration Tin (as stannous tin) concentration Lead (as lead sulfate) concentration Additive and carrier levels

Acid Tin Plating Bath Analysis and Maintenance

Acid tin electroplating is the most popular means of establishing a metallic resist pattern on a PCB just prior to etching . Possibly the most environmentally friendly of the plating solutions, acid tin has enjoyed widespread acceptance from virtually all sectors of the printed circuit fabrication industry.. For reliable deposition, there are three components that need to be accurately monitored to insure optimum bath performance.

1. Stannous sulfate
2. Sulfuric acid
3. Tin starter

Etchant Analysis and Maintenance

Peroxy Sulfuric etchant is a strong solution of hydrogen peroxide, sulfuric acid, dissolved copper, and various proprietary stabilizers/accelerators. The etching of copper is accomplished via a two stage process. The surface of the copper is oxidized by hydrogen peroxide. The copper oxide that is formed, being very soluble in sulfuric acid, is quickly stripped off to reveal the underlying copper, which is then oxidized by the peroxide. As long as fresh etchant is available, this process continues until all unprotected copper is consumed.

Because of the critical part they play in the etching process, all of the components of the etchant must be maintained at their optimum levels.

Copper Concentration Sulfuric Acid Hydrogen Peroxide Organic Stabilizer

Tips for experienced users

The etching process will consume known quantities of hydrogen peroxide, sulfuric acid, and catalyst depending on the catalyst system). If you are anticipating a day of heavy use, it is recommended that you bring the etcher up to optimum operating conditions as described above and to make incremental addition as you use the bath. These additions can be calculated as follows:

1. Multiply the length of the board (in inches) by the height (also in inches). If the board is double sided, multiply this figure by 2. The result will be the total area of copper to be etched.
2. One ounce copperclad has one ounce (28.4 grams) of copper foil per side per square foot (144 sq.in.). So each square inch of copper foil weighs .197 grams.
   Multiply the total surface area (found in (1)) by .197 grams to get the total weight of one ounce copper foil on the prototype.
3. Multiply the result by .8 to approximate the area of the copper not covered by resist (or by the actual fraction if your CAD package has a copper area calculator).

The amount of hydrogen peroxide (in Liters) that should be added to compensate for etching the board in question is found by multiplying the result of (4) by 0.00128. i.e.

Add catalyst as prescribed by your vendor.

The amount of sulfuric acid (in Liters) that should added is found by multiplying the result of (4) by 0.00084. i.e.

Rinse tanks

A primary tenant of the Green CirKit discipline is the near total elimination of all toxic effluents through the recycling of primary process chemicals and rinse baths. After the first use, the rinse tank (which started out filled with high-purity deionized water) contains valuable dissolved copper and sulfuric acid due to carry-over from the process tank. This same carry-over, along with evaporative losses serve to lower the level of the plating and baths. If proper care is taken not to introduce outside contaminants into the rinse tank, the rinse water provides a handy source of replenishment to compensate for these losses. Experience has shown that 100% of the rinse water can be recycled in this manner.

In the event that rinse water does become contaminated, it should be disposed of using the following procedure.

Rinse water pH determination

The acidity of the rinse water (resulting from the carry-over of sulfuric acid) should not be lower than pH 5.0 for disposal into municipal sewer systems (check local regulations). To determine both the initial pH and the corrective measures needed, use the following procedure:

1. Drain 500 mL of rinse bath into a graduated 1 liter (or 1 quart) container.
2. Using the pH 0-13 Insta-Chek litmus paper determine the acidity of the water.
3. If necessary, use sodium hydroxide (NaOH) pellets to raise the pH to between 6.0 and 7.0. Be careful to note how much NaOH is necessary to neutralize the 500 mL sample. Divide the total volume of your rinse tank by 500 mL and multiply the result by the amount of NaOH needed to neutralize the sample volume.
4. Neutralize the rinse tank by adding the necessary NaOH and stirring until it is totally dissolved.

CAUTION
 BE CAREFUL TO ADD THE SODIUM HYDROXIDE TO THE RINSE TANK SLOWLY. IF THE BATH IS HIGHLY ACIDIC, NEUTRALIZING WITH HYDROXIDE MAY GENERATE CONSIDERABLE HEAT AND CAUSE THE BATH TO FOAM.

ONLY ADD ENOUGH HYDROXIDE TO NEUTRALIZE THE EFFLUENT.

CAUTION
SODIUM HYDROXIDE in concentrated form is capable of causing SEVERE CAUSTIC BURNS and BLINDNESS. The pellets should be handled with great care. Always use the proper attire as outlined above in the SAFETY section. If contact should occur, flush affected area with cool tap water for 15 minutes. CONTACT A PHYSICIAN IMMEDIATELY

Copper concentration in the rinse water

After neutralizing and removing from the rinse tank, the rinse water should be analyzed for dissolved copper content. State and federal laws prohibit the dumping of effluents containing more than 3 PPM of dissolved copper (cupric and/or cuprous ion). Analysis at this level is most easily carried out colorimetrically using one of the "color cubes" manufactured by the Hach Chemical Co. (available through Think & Tinker, Ltd.). Waste water found to exceed 3 PPM dissolved copper should be disposed in compliance with local regulations.

See? It is a lot easier to be careful in the first place and get rid of your rinse water through in-system recycling! Thoroughly rinse the tank before refilling with deionized water.

Tin and lead Concentration in the rinse water

After neutralizing and removing from the rinse tank, the rinse water should be analyzed for dissolved metals content.  State and federal laws prohibit the dumping of effluents containing lead, even with very small concentrations  Analysis at this level is most easily carried out colorimetrically using one of the “color cubes” manufactured by the Hach Chemical Co. (available through Think & Tinker, Ltd.).  Waste water found to exceed local and/or federal limits on dissolved lead should be disposed of in compliance with local regulations.

See?  It is a lot easier to be careful in the first place and get rid of your rinse water through in-system recycling!