Plasma PCB Etching MachinesIn the past, manufacturers used concentrated acid for their etch process and to clean drill holes in printed circuit boards. Many different chemicals were used to clean the holes, but all of these chemicals were harmful to the environment and dangerous to workers.
Etching with ChemicalsChemical etching is the traditional method of creating circuit boards. An etch resist consisting of tin or tin and lead protects certain areas of the copper foil while the rest of the copper is etched away.
This process uses an ammoniacal etching solution to remove the copper. The ammoniacal solution will not attack tin or lead, so the copper under the tin remains as a "wire" or a path for electrons to follow around the completed circuit board.
The quality of chemical etching can be defined by the completeness of the copper removal which is not protected by the etch resist. Quality also refers to the straightness of trace edges and the level of etch undercut.
Etch undercut is caused by the chemical’s non-directional etching, allowing sideways etching once downward etching has taken place. Less undercut is considered to be higher quality. These undercuts are measured and referred to as “etch factor.”
Banking agents are chemicals added to the etching solution to reduce the amount of “etch factor.” Their components, however, are usually proprietary and not shared as their success produces a step up over competing etching facilities.
All of the steps in the etch process are connected and a poor etch quality could be the result of the etching solution or the resist that was used.
Chemical etching uses many harmful chemicals and is not an environmentally friendly etching process.
Etching with PlasmaPlasma etching became popular in the 1980's as an environmentally friendly etching process to remove sludge from drill holes in PCB's.
Plasma is the fourth state of matter, formed in a vacuum by ionizing gas particles using radio frequencies at 13.56 MHz.
Plasma PCB descum techniques also return better quality etching and through-hole contaminant removal.
Plasma PCB etching machines, as the name implies, use a technique of etching in which plasma is created under strict conditions and used to clean the left behind material in the holes drilled in PCB boards.
In order to fully understand the PCB etching process, it is important to understand how a plasma etcher works. The plasma machine consists of two electrodes for the generation of radio frequency and a ground electrode.
There are usually two gas inlets through which oxygen, CF4, or another etching gas enters the system. The gases are usually mixed at a pre-determined ratio, depending on the material being etched.
As the gases enter the system, radio frequency is applied to ionize the gas particles. 13.56 MHz is considered to be the standard plasma forming frequency.
The radio frequency excites the gas electrons and changes their state. The machine generates high speed pulses of plasma in order to etch.
The PCB etching system produces volatile chemical compounds as by-products during the chemical reactions that take place. It takes 10 to 15 minutes for the plasma atoms to clean the holes in the circuit board.
Plasma is also popular for cleaning the leadframes inside the chip packaging. The leadframes carry electrical signals to the outside of the package and must be cleaned of all organics before being incorporated into the package.
Plasma PCB Etching MachinesThere are many types of plasma etching depending on the type of the material to be etched, the nature of the gases used, and the etching type desired.
Temperature and pressure play an important role in the type of plasma etching performed as well. Small variations in the operating temperature or pressure can change the collision frequency of electrons considerably.
Reactive Ion Etching (RIE) uses physical and chemical mechanisms to achieve high levels of surface etching in one direction.
Because the RIE process combines physical and chemical interactions, it’s much faster than plasma etching alone.
The high energy ion collisions strip the plasma of its electrons and allows treatment with positively charged plasma.
Plasma Desmear and Plasma DescumPlasma desmear and plasma descum are generally considered to be the same thing. The desmear or descum process is used to clean the slag residue in a drilled hole on a PCB after the drilling process.
This slag is left behind because multiple layers of the copper are insulated with a polyimide or FR4 epoxy material. When drilled, the insulating material slag prevents a good connection between the layers of copper and must therefore be cleaned away.
Etch back takes plasma desmear to the next level. For this process, not only do we clean the residue left behind on the copper by the polyimide or FR4 epoxy by the drilling process, but we also etch away a small amount of the material.
This allows multi-surface bonding to the solder or other conductive material, resulting in a stronger and electronically superior connection.
The insulating material in between layers of copper is etched away by the plasma.
Etch back is often used in high grade and military applications where a very reliable and consistent plasma etch process is required.
Benefits of a Plasma PCB Etching Machine Over Chemical Etching:
- Plasma is safer and more environmentally friendly than a chemical etch process.
- Plasma technology is also excellent at cleaning. Plasma removes all of the unwanted organic residues from the copper surface.
- Plasma etching improves the physical properties of the etched material.
- Plasma does not change any property of the metal even while it improves chemical and physical properties of most metals.
- Any by-products formed by plasma etching machines are volatile.
A plasma PCB etching machine can effectively improve the quality of integrated circuits while lowering their environmental impact. Additionally, it can be used for treating almost any material including delicate semiconductor wafers and substrates.
Next: Reactive Ion Etching