2. Wet film coating and drying : Before introducing the process, wet film components need to be explained first.
2.1 Chemical composition and function of photosensitive wet film: Photosensitive wet film should only be applied as a resist resist on the large copper surface of the non-porous inner layer. Because the wet film used in the inner layer board is very thin and coverless film, the light energy must be strongly sensitive and fast to avoid interference from oxygen . In addition, the wet film needs to be automated and its environmental requirements are stricter than dry film. Classification of
2.2Binder (plastic adhesive) and acid-containing water-soluble image: Plastic adhesive is the backbone of film formation, which can be distinguished into two types: (1) non-photosensitive type and (2) photosensitive type. The unpolymerized part can be washed and hydrolyzed by the imaged weak alkali sodium salt and dissolved.
2.3Acrylates (photosensitive acrylic monomer) has the effect of the amount of the monomer: the large amount of the monomer is used, and the photosensitive is fast, but the hardness is insufficient after polymerization. The small amount is high, the hardness is high, and the brittleness is high, which leads to difficulties in image disassembly and film stripping. The formula must be adjusted closely with the user. There are two sources of adhesion of the photosensitive wet film, one is the mechanical linkage of slightly etching and coarsing of the copper surface, and the other is that some functional groups of Binder can form a miscombination reaction with the copper surface (complexing, requiring sufficient standstill time).
above is an introduction to the wet film components and functions related content. Next, we will mainly explain the wet film coating process.
Wet film coating: The coating of photosensitive wet film is mainly based on the roller coating method .
Failure analysis: One of the factors influencing the Roller Coater is the abnormal ink amount of rubber grooves on the surface of the roller. As shown in the figure below, when the ink amount is abnormal, it is easy to cause insufficient ink on the plate surface, causing air to enter the ink body when it is coated. For the control of the ink volume of rollers, it can be improved through the adjustment of Dial and the timing of ink addition.
wet film drying
wet film after hot drying, most of its solvent will escape film thickness will be significantly thinned.
3.wet film exposure
exposure area. The photosensitive agent in the film material first absorbs energy to generate radicals , and is quickly transferred to the photoretent, and PI itself will absorb light energy and can also cleave it into free radicals. Such activated PI will cause photopolymerization and crosslinking reaction of monomers or oligos to appear curable film . The exposure amount should be based on the NO.5-8 semi-residual grid of Stouffer 21 Step exposure stage table.
Supplementary Note : Photo-Initiator and Photosensitiser
Since the effective energy peak of the current UV light source's spectrum is mostly concentrated at 365nm, it is necessary to use the specialties of photosensitizer to absorb light energy first and first form free radicals, and then transfer it to PI. The PI itself can also absorb light energy to make active PI and then promote the polymerization of monomers. The Japanese industry claims that the 405nm photoresist has better solution to thin lines than 365nm, but due to the difficulty of matching the current exposure machine with production lines, it is not yet popular.
Negative-Working Photoresist Photoresist Process
Mechanism of photoresist reaction
•The photosensitive agent in the photoresist film first absorbs UV energy and splits into free radicals and passes it to the photoresist
•The photoresist (PI) itself will absorb UV energy and then splits into free radicals (Free Radical)
•The active PI will attack the monomer and oligos and produces polymerization reaction, but it takes time to complete, so the hold time after exposure must be more than 15 minutes, but it cannot exceed 12 hours to reduce the residual resistance of the resist.
•The monomers in the reaction that do not have the photosensitive area will diffuse to the photosensitive area, so it is easy to cause the trouble of residual inhibitor.
The polymerization reaction occurs after photosensitive to minimize the interference of oxygen
In order to make it easy to distinguish whether the production line has been exposed, two color changes of "photofugitive" and "phototropic" are deliberately designed to prevent stupidity and avoid errors.
close-connected exposure of the inner wet film should be parallel optical machine
Usually the exposure thickness of the wet film is about 10-15μm, and the required energy is as high as 80-120mj/cm2 twice as high as 45-60mj/cm2 of the dry film with a thickness of 25-35μm. In order to reduce the various dirty points caused by the dust on parallel light , the wet film environment of the inner layer must be cleaned to level 1000 to reduce the loss of thin lines.
Supplementary Note: Small hole camera principle
4. Development (Development)
development action is to use weak alkali to rinse it off the membrane that has not undergone polymerization, leaving only the polymerized and cured membrane on the copper surface as a resistor (Resist).
5. Acid etching
Since the resist formed is insoluble in acid, acidic etching of copper chloride or iron chloride can be used to form lines. Copper chloride tank liquid is easy to be automated and can be self-sufficient as long as hydrochloric acid is provided, but the etching factor is poor. As for ferric chloride, automated management (Aqua) equipment is more expensive and tank liquid needs to be purchased from outside.
conventional large inner layer plates without through holes will first produce a pool effect on the top surface during horizontal erosion, and when the dense line area is half eroded, the more annoying ditch effect will appear. Intermittent spraying (Impact) method can also be used to improve
redox potential ORP: Regardless of whether the photoresist is a wet film or a dry film, the direct etching (Print and Etch) can only use acid tank liquid to prevent the resistor from alkaline dissolution. There are only two types of acidic copper etching: FeCl3 and CuCl2. The former has good etching factor, but it cannot be recycled and the amount of copper etching is not high. Therefore, operators often use CuCl2 for direct etching. The blue Cu++ in the tank liquid acts as an oxidizing agent. With the assistance of Cl--, Cu0 on the plate surface can be oxidized to Cu+ and then dissolved into the liquid. However, Cu0 in this reversible reaction in will also reduce Cu++ to Cu+, which hinders etching. Therefore, the Oxidation Reduction Potential (ORP) of Cu++/Cu+ needs to be controlled, and must exceed 500mv (550±50mv), so that its etching rate can meet the requirements of mass production. When ORP is high, its Cu++ concentration will also be high. If the ORP is too low, it will not only etch slowly, but it may also produce toxic chlorine Cl2. The general management points of
copper chloride etching tank solution are: (1) ORP>500mv, (2) Temp>40℃, (3) The acid concentration is 1.5-3.0N, and the etching rate can reach more than 25μm/min under these conditions.
If you use CuCl2 tank liquid etching, you must use the multi-function controller of Japanese Aqua to automatically manage the specific gravity, ORP, acid concentration (conductive liquid), temperature, Cu++ concentration and other parameters.
6. Stripping : The film stripping of the wet film uses strong alkali (NaOH, KOH; 3%) to attack both Binder and Acrylic, and degrade it into sodium salt during the hydrolysis reaction. The strong alkali area can also attack the membrane body at the same time to cause swelling and rupture, and then form fragmented peeling to obtain clear copper wire.
or above is an introduction to the content of wet film photoresist. If you have any questions, please feel free to exchange and discuss.
Finally, there is some expansion of exposure knowledge:
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-