Innerlayer Etching and Resist Stripping
Aim of Process
The aim of this process is to etch the copper not coated with the dry film etch resist and to then remove the dry film etch resist. Providing well defined track.
The cores are placed onto the horizontal conveyorised processor, which sprays a chemical at the copper which chemically dissolves the copper, but does not attack the dry film etch resist, which protects the copper tracks underneath. For enviormental reasons we then spray etch solution at the core which does not contain copper, the core is then transported into the next sections which are water rinsing stages to remove any traces of the etchant chemicals, the cores are then transported into the resist stripping section, which chemically swells the resist of the copper surface, to a size which is then automatically filtered out of solution. The cores are then transported into the water rinsing and drying sections.
When we now see the BGA devive on the pcb it looks like the followiong
The objective of resist stripping is to remove the resist from the copper panel (including fine lines and spaces), while ensuring a non-oxidised surface.
Most alkaline type dry film resists do not dissolve in the stripping solution but are detached from the copper surface in small flakes. This extends the working life of the stripping solution.
Stripping solutions are normally alkaline metal hydroxides, such as sodium or potassium hydroxide, or based on amines such as mono or tri ethanolamine and tetra methyl ammonium hydroxide.
The stripping solution should break the polymer chain at the cross-linking point of the three dimensional structure, which is formed during the polymerisation of the resist and before the bond between the resist and the copper surface is broken. The stripping mechanism depends not only on the cross-link density of the resist but also the number of carboxylic acid groups
on the polymer chain. Therefore, the type and concentration of stripping solution should be optimised for each resist and these must be set to allow the stripping solution to have time to penetrate the resist and break the polymer chain before the resist-to-copper bond is broken.
If stripping trials are conducted in the laboratory prior to production it must be noted that the resist characteristics do change during the electrolytic deposition of copper and tin-lead or tin. This means that any stripping trials should be conducted on panels that have been through the deposition cycle used on production.
The trials are performed not only to optimise the stripping time and flake size, but also to set the concentration of stripping solution that minimises the swelling of the resist in the solution. To obtain complete stripping of the resist from within fine lines and spaces the resist must be removed before it swells and is trapped by mechanical forces within these fine traces.
The stripped flake size for any resist depends on four major factors:
· The type of stripping solution
· The concentration of the stripping solution
· The temperature of the stripping solution
· Design of the stripping equipment
When using any stripping solution the stripping time and stripped flake size is a balance of concentration and temperature. The higher the concentration results in a faster stripping time but with a larger flake size. Conversely, a lower concentration will give smaller flake size but the stripping time is much longer. Increasing the temperature will reduce both the stripped flake size and the stripping time.
Stripping is a diffusion-controlled mechanism. A high volume and spray pressure through each spray nozzle is required. Typically, a minimum flow of 4 litres per minute at a pressure in excess of 1,5 Bar is recommended. When alkaline metal hydroxide solutions are used for stripping electroplated boards, a pressure as high as 6-10 Bar is often used.
An ideal equipment configuration is for the spray jets in each stripping module to be angled at 30 degrees in the four axes of the board. This will ensure that the resist is stripped between fine tracks. Where there is a possibility of overhang of plated metals onto the resist surface, particularly in high current density areas, it is beneficial to have a flooded stripping cell as the final stripping section. It is helpful if the plating overhang is less than 7-8 microns onto the surface of the resist.
Antifoam may be necessary to prevent foaming of the solution. Any antifoam used must be in a minimum concentration and compatible with the stripping solution being used. The antifoam used in the development solution will probably not be suitable for the stripping solution. The total alkalinity of the stripping solution determines which type of antifoam is suitable.
To prevent the dry film from being dissolved in the stripping solution, it is necessary to filter the solution to remove the stripped flake of resist. This filtration will extend the working lifetime of the stripping solution. There are many different types of filtration methods, including external or internal belt filters or angled screen types.