The Printed Circuit Boards ( PCBs) are a junction of electrical traces and tracks that connect the components mounted on PCB with each other. These traces are made of copper tracks and in this article we will discuss about the electroless deposition of copper on PCB. The electroless copper plating is done by means of series of chemical process applied on PCB structure. The PCB panels are mounted with the overhead cranes and these cranes will carry the panels over the chemical and rinsing tubs. These cranes are fully computer automated and controlled.
The IPC-2221A table 4.2 standard says that the minimum copper thickness being applied by electroless copper plating method on the walls of PTH for average copper deposition is 0.79 mil for class 1 and 2 and 0.98 mil of class 3 PCB.
However the thickness of copper plating can be increased to specific thickness on the PTH holes by electroplating method. The chemical deposition (Electroless plating method) can also be used to increase the overall copper thickness of whole PCB. The Plated through Hole (PTH) can be described as the interconnection between the components and tracks and also the interconnection between the layers of Multilayer PCB
Speaking in common way, a PCB is a thick hard board made of fiberglass material laminate epoxy resin, having components soldered in the holes and tracks are printed or etched on the board to make connectivity between different electronic components.
Types of Substrate Materials:
1. Polyamide (PI) resin Base material is base material for PCB in early 1950s. Generic name is Nylon used for thermoplastic polymer. Highly recommended for flexible PCBs. High temperature/heat durability. Light weight and expensive than FR-4.
2. FR-2 has the phenol formaldehyde resin impregnated with Phenolic cotton paper. It is lower standard than FR-4 and 105OC rated.
3.FR-3 structure is cotton paper impregnated with epoxy resin. Better than phenolic cotton paper.
4. FR-4 structure is fiber glass impregnated with epoxy resin. It is very commonly used in most PCBs. This is hard rigid in nature. Can withstand temperature up-to 130O Good insulating properties.
5. Aluminum Insulated metal substrate like commonly used in high current PCBs requiring heat dissipation. Example is LED lights and lamps
The electroplating of copper is done after the chemical plating is done on the walls of the holes on PCB. The large PCB panels act as a cathode and the walls of the holes have already applied/deposited carbon on it. The PCB panel copper surface is washed, cleaned and activated in different ways in different bath tubs and finally it is electroplated.
All the holes bored in the PCB panels are electroplated simultaneously. These holes are interconnect between the components of external layers and as well as interconnect between the inner layers. Like blind and buried vias / holes. This electroplating of copper is responsible for better soldering of components and strong solder joints. This is only possible when the uniform plating is ensured throughout the panel PCB board.
The process of copper plating is automated using computers to maintain accuracy and timing. The panels are remained dip in bath for exact amount of time for better result. For good conductivity, the walls of the holes must be plated to amount of 25 microns. Also the surface tracks should be plated to 30 microns. So if a copper foil 17.5 micron thick is used then the total thickness can reach up-to 40 micron after complete process.
After the copper is plated, a thin layer of tin is also plated. This is known as “Tin plating”. It is important to note that the thickness of copper can also be expressed in terms of weight (ounce) per square foot of copper foil.
Chromic Acid Method:
The PCB plated through holes are punctured by the hexavalent chromium particles that can cause obstruction in coverage of copper foil, restricting the oxidation reduction reaction and destructing the tin-palladium colloid. This issue can be resolved by secondary activation method. But the secondary activation process is too expensive.
The alternative way is to use a “chromic acid” like sodium hydrogen sulfite solution to turn the hexavalent chromium into chromium with valence trivalent. The temperature of sodium hydrogen sulfite is 100 degree Fahrenheit. While the temperature maintained during washing is 120F to 150F after neutralizing. The concentrated sulfuric acid and potassium permanganate can be used to remove stains generated during etching process and plating.
Electroless Copper Plating Detail Explanation:
The electroless plating is also called Plated through Hole (PTH) is an oxidation-reduction reaction that is autocatalytic. In this a hole that is bored has the walls of non-conducting substrate upon which the copper is chemically deposited for subsequent copper electroplating process.
The flow chart of the electroless copper plating is described as under.
Step-1: Alkaline Degreasing:
The first step is the removal of oil from the board surface. It also includes the removal of dust, fingerprints and other debris in the holes of PCB panel board
The charge adjustment means that the resin surface is transformed into weak positive charge from weak negative charge. This is because of the charge factor of resin itself. This is done after the application of correction liquor. However sometimes application of continuous positive charge polar surface can guarantee the effective absorption of activators on the walls of holes in later process. This charge adjustment is also called “Super Impregnation”.
The washing/cleaning of contamination (due to above mentioned process) is the important step in order to carry upcoming process of micro-etching and activation. This cleaning step will ensure the proper and tight bonding between the chemical copper deposited on the wall and substrate plated copper.
The cleaning water temperature and amount of flow rate of cleaning water must be controlled.
The micro-etching is the process of roughening the surface in order to create a strong adhesion between the chemically deposited copper and copper substrate. The main purpose of this step is to create a coarse active copper surface by removing the oxide on the surface. The new active coarse copper will now be able to absorb colloidal palladium well.
The main purpose of rough surface is
1. Surface area and surface energy is greatly increased that provide more contact area between chemical deposited copper and copper at the bottom of substrate
2. During the washing step, if some contamination or surfactant is not cleaned properly than the micro-etching inhibitors can work at the bottom of the substrate to etch way the copper foil resulting in the removal of surfactant/contamination on the substrate surface. Totally relying on these inhibitors for washing is not realistic for larger surface contamination surfactant.
To prolong the palladium tank service life, the pre-impregnation is done. This step is used to avoid the contamination from pretreatment tank into the palladium tank. In pre-impregnation step, the palladium chloride facilitates the activation liquid in the hole walls in the upcoming process/step.
This is the main step in which hole walls that are positively charge in previous step, can efficiently absorb negative charge colloidal palladium particles to make the hole walls smooth, even and compact with continuity and average
The activation is important in order to ensure high quality copper sinking. The factors that need to be controlled for high quality activation are specific time, standard ion concentration of tin (valence II) and chloride ions, temperature, acidity and specific gravity.
In this step, the tin (valence II) ions are removed. The colloidal palladium particles are exposed to catalyze the reaction of chemical copper precipitation. The fluoroboric acid is the good debonding agent to be used.
Step-6: Electroless Plating of Copper:
The copper precipitation reaction will continue to induce electroless copper. This is done by the activation as discuss above. Hydrogen is the by-product of the reaction and it is used as reaction catalyst so that precipitation is continued. As a result of this electroless plating of copper, the chemical copper is deposited on the walls of holes and on surface. The bath is maintained under normal air agitation. It is significant to mention that the quality of end product (PCB) is directly proportional to above mentioned steps. Any mishandling in this procedure can cause the entire batch of PCBs to go wasted. Hence it is recommended to strictly follow the standard work instructions.