- Use of formaldehyde as reducing agent.
- The process is inherently unstable, requiring stabilizing additives to avoid copper precipitation.
- Environmentally undesirable complexing agents, such as EDTA, are used.
- The large number of process and rinse tanks causes high water consumption.
The electroless copper process consists of four basic operations: cleaning, activation, acceleration, and deposition (Exhibit 3-17). An anti-tarnish bath is common after deposition. Virtually all shops purchase a series of proprietary chemistries from a single vendor that are used as the ingredients for the several process baths in the electroless copper process line. Only the micro-etch, its associated sulfuric dip, and the anti-tarnish baths are likely to be non-proprietary chemistries.
Cleaning. The cleaning segment begins with a cleaner-conditioner designed to remove organics and condition (in this case swell) the hole barrels for the subsequent uptake of catalyst, followed by a microetch step. The cleaner-conditioners are typically proprietary formulations, and mostly consist of common alkaline solutions.
A microetch step can be found on the electroless line, oxide line, pattern plate line and with chemical cleaning if that is the cleaning method used. Three chemistry alternatives are available. Sulfuric acid-hydrogen peroxide (consisting of 5% sulfuric acid and 1% to 3% peroxide) is most common, followed by sulfuric acid-potassium (or sodium) persulfate (5% sulfuric, 8 to 16 ounces/ gallon persulfate) and ammonium persulfate. In each case, the microetch bath is followed by a sulfuric acid dip, which serves to remove any remaining oxidizer. About 40 microinches of copper are etched for the making holes conductive process. Based on a 3-4 ounce copper carrying capacity, approximately 0.0183 gallons of microetch are used per square foot of product run. This figure does not include any solution that may be dragged out when the panels are moved to the next tank. The sulfuric-peroxide alternative has some attractive waste treatment and performance features (ref. 31):
- No spent etchant disposal. The etchant is replenished as it is used, and copper is removed with a recovery unit in the form of copper sulfate crystals. These crystals form when the solution is cooled to room temperature or lower. Smaller shops may use a batch treatment where the solution is pumped to another tank to cool and crystallize. After removing the copper crystals, the solution can be transferred back to the process line and reused.
- Constant etching rate. The etching rate is dependent on temperature and hydrogen peroxide concentration, not the copper concentration.
- Simple waste treatment. No chelators are present in sulfuric-peroxide microetchants.
- A high copper capacity of 3 to 4 ounces/gallon.
- Efficient copper recovery. Copper sulfate recovery is usually 90-95% efficient.
Persulfate microetchants must be treated in-house or shipped to a licensed disposal facility. The etching rate is difficult to control since it declines as panels are processed and copper builds in the solution. Ammonium persulfate is uncommon due to high waste treatment costs.
Activation and Acceleration. Activation, through use of a catalyst, consists of two process tanks. A pre-dip, for the drag-in protection of the expensive activation (also called catalyst) bath, usually contains hydrochloric acid and possibly tin or sodium chloride. The activation bath itself consists of hydrochloric acid, tin chloride, a palladium chloride. The Sn+2 ion reduces the Pd+2 to Pd, which is deposited on the panel. The remaining Sn+2 and Sn+4 are selectively removed from the hole barrels by the accelerator (also called the post-activator). Fluoboric acid is a common accelerator, as is sulfuric acid with hydrazine.
Copper Deposition. Electroless copper baths can be divided into two types: heavy deposition baths (designed to produce 75 to 125 micro-inches of copper) and light deposition baths (20 to 40 micro-inches). Light deposition must be followed immediately by electrolytic copper plating. The more common heavy deposition can survive the outer layer imaging process, and copper electroplating occurs thereafter. The main constituents of the electroless copper chemistry are sodium hydroxide, formaldehyde, EDTA (or other chelater), and a copper salt. In the complex reaction, catalyzed by palladium, formaldehyde reduces the copper ion to metallic copper. Formaldehyde (which is oxidized), sodium hydroxide (which is broken down), and copper (which is deposited) must be replenished frequently.
Most heavy deposition baths have automatic replenishment schemes based on in-tank colorimeters. Light deposition formulations may be controlled by analysis. Formaldehyde is present in light deposition baths in a concentration of 3 to 5 grams/liter and as high as 10 grams/liter in heavy deposition baths.
When light deposition is applied, the next process step must be electrolytic copper plate. This is either a full panel plate (the typical 1 mil is plated in the holes and on the surface) or a "flash" panel plate, designed only to add enough copper to the hole barrels to survive the imaging process. Flash-plated panels return to copper electroplating after imaging to be plated up to the required thickness. This double plating step has made heavy deposition the more common electroless copper process.
Process Waste Streams. The electroless copper line typically contributes a significant percentage of a PWB shop's overall waste volume. Water use is high due to the critical rinsing required between nearly all of the process steps. Copper is introduced into the wastewater stream due to drag-out from the cleaner-conditioner, micro-etch, sulfuric, accelerator, and deposition baths. Much of this copper is complexed with EDTA and requires special waste treatment considerations. Furthermore, waste process fluid generation is high. Micro-etch baths are exhausted when 2 to 4 ounces/gallon of copper is dissolved, and this bath life is usually measured in days. While the electroless copper bath is relatively long-lived (usually several weeks or months), a considerable bailout stream (including formaldehyde) is generated (several gallons of site concentrated bath chemistry per day in production shops). This waste must either be treated in-house or shipped off-site, which adds another cost to using electroless copper.