Plasma refers to light like purple light and neon light, and it also has a fourth phase called copy board material. The plasma phase is a state of disorderly movement of excited electrons and molecules in atoms, so it has a relatively high energy.
(1) Mechanism:
Applying energy (such as electric energy) to the gas molecules inside the vacuum chamber, the collision of accelerated electrons causes the outermost electrons of the molecules and atoms to be excited, and ions or highly reactive free radicals are generated.
The ions and free radicals generated in this way are accelerated by continuous collisions and electric field forces, causing them to collide with the surface of the material, and destroy molecular bonds within a few microns, induce a certain thickness reduction, generate a concave-convex surface, and form a gas component at the same time. The physical and chemical changes of functional groups and other surfaces can improve copper plating adhesion and decontamination.
The above-mentioned plasma processing gases commonly include oxygen, nitrogen, and carbon tetrafluoride gas. The following uses a mixed gas composed of oxygen and carbon tetrafluoride gas to illustrate the mechanism of plasma treatment:
(2) Purpose:
1. Etching/de-hole wall resin contamination;
2. Improve surface wettability (Teflon surface activation treatment);
3. Using laser drilling to process the carbon in the blind hole;
4. Change the surface morphology and wettability of the inner layer to improve the bonding force between the layers;
5. Remove resist and solder mask residues.
(3) Examples:
A. Activation treatment of pure polytetrafluoroethylene material
For the activation treatment of pure PTFE material, a single-step activation through hole process is adopted. Most of the gas used is a combination of hydrogen and nitrogen.
The plate to be processed does not need to be heated, because the PTFE is processed to become active and the wettability is increased. Once the vacuum chamber reaches the operating pressure, the working gas and RF power supply are activated.
The processing of most pure PTFE sheets only takes about 20 minutes. However, due to the recovery performance of the PTFE material (return to a non-wetting surface state), the hole metallization treatment of electroless copper must be completed within 48 hours after the plasma treatment.
B. Activation treatment of filler-containing polytetrafluoroethylene material
For printed circuit boards made of filler-containing polytetrafluoroethylene materials (such as irregular glass microfibers, glass braided reinforced and ceramic-filled polytetrafluoroethylene composites), two-step processing is required.
The first step is to clean and micro-etch the filler. Typical operating gases for this step are carbon tetrafluoride gas, oxygen and nitrogen.
The second step is equivalent to the one-step board copying process used in the surface activation treatment of the aforementioned pure PTFE material.
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