How is the inner layer of the PCB made
Due to the complex process of PCB manufacturing, in the planning and construction of intelligent manufacturing, it is necessary to consider the related work of process and management, and then carry out automation, information and intelligent layout.
Process classification
According to the number of PCB layers, it is divided into single-sided, double-sided, and multi-layer boards. The three board processes are not the same.
There is no inner layer process for single-sided and double-sided panels, basically cutting-drilling-subsequent processes.
Multilayer boards will have internal processes
1) Single panel process flow
Cutting and edging → drilling → outer layer graphics → (full board gold plating) → etching → inspection → silk screen solder mask → (hot air leveling) → silk screen characters → shape processing → testing → inspection
2) Process flow of double-sided tin spraying board
Cutting edge grinding → drilling → heavy copper thickening → outer layer graphics → tin plating, etching tin removal → secondary drilling → inspection → screen printing solder mask → gold-plated plug → hot air leveling → silk screen characters → shape processing → testing → test
3) Double-sided nickel-gold plating process
Cutting edge grinding → drilling → heavy copper thickening → outer layer graphics → nickel plating, gold removal and etching → secondary drilling → inspection → screen printing solder mask → screen printing characters → shape processing → testing → inspection
4) Multi-layer board tin spraying process flow
Cutting and grinding → drilling positioning holes → inner layer graphics → inner layer etching → inspection → blackening → lamination → drilling → heavy copper thickening → outer layer graphics → tin plating, etching tin removal → secondary drilling → inspection →Silk screen solder mask→Gold-plated plug→Hot air leveling→Silk screen characters→Shape processing→Test→Inspection
5) Process flow of nickel and gold plating on multilayer boards
Cutting and grinding → drilling positioning holes → inner layer graphics → inner layer etching → inspection → blackening → lamination → drilling → heavy copper thickening → outer layer graphics → gold plating, film removal and etching → secondary drilling → inspection → Screen printing solder mask → screen printing characters → shape processing → testing → inspection
6) Process flow of multi-layer plate immersion nickel gold plate
Cutting and grinding → drilling positioning holes → inner layer graphics → inner layer etching → inspection → blackening → lamination → drilling → heavy copper thickening → outer layer graphics → tin plating, etching tin removal → secondary drilling → inspection →Silk screen solder mask→Chemical Immersion Nickel Gold→Silk screen characters→Shape processing→Test→Inspection
Inner layer production (graphic transfer)
Inner layer: cutting board, inner layer pre-processing, laminating, exposure, DES connection
Cutting (Board Cut)
1) Cutting board
Purpose: Cut large materials into the size specified by MI according to the requirements of the order (cut the substrate material to the size required by the work according to the planning requirements of the pre-production design)
Main raw materials: base plate, saw blade
The substrate is made of copper sheet and insulating laminate. There are different thickness specifications according to the requirements. According to the copper thickness, it can be divided into H/H, 1OZ/1OZ, 2OZ/2OZ, etc.
Precautions:
a. To avoid the impact of the board edge barry on the quality, after cutting, the edge will be polished and rounded.
b. Considering the impact of expansion and contraction, the cutting board is baked before being sent to the process
c. Cutting must pay attention to the principle of consistent mechanical direction
Edging/rounding: mechanical polishing is used to remove the glass fibers left by the right angles of the four sides of the board during cutting, so as to reduce scratches/scratches on the board surface in the subsequent production process, causing hidden quality problems
Baking plate: remove water vapor and organic volatiles by baking, release internal stress, promote cross-linking reaction, and increase the dimensional stability, chemical stability and mechanical strength of the plate
Control points:
Sheet material: panel size, thickness, sheet type, copper thickness
Operation: baking time/temperature, stacking height
(2) Production of inner layer after cutting board
Function and principle:
The inner copper plate roughened by the grinding plate is dried by the grinding plate, and after the dry film IW is attached, it is irradiated with UV light (ultraviolet rays), and the exposed dry film becomes hard. It cannot be dissolved in weak alkali, but can be dissolved in strong alkali. The unexposed part can be dissolved in weak alkali, and the inner circuit is to use the characteristics of the material to transfer the graphics to the copper surface, that is, image transfer.
Detail 
The photosensitive initiator in the resist in the exposed area absorbs photons and decomposes into free radicals. The free radicals initiate a cross-linking reaction of the monomers to form a spatial network macromolecular structure that is insoluble in dilute alkali. It is soluble in dilute alkali after reaction.
Use the two to have different solubility properties in the same solution to transfer the pattern designed on the negative to the substrate to complete the image transfer).
The circuit pattern requires high temperature and humidity conditions, generally requiring a temperature of 22+/-3℃ and a humidity of 55+/-10% to prevent the film from deforming. The dust in the air is required to be high. As the density of the lines increases and the lines become smaller, the dust content is less than or equal to 10,000 or more.
Material introduction:
Dry film: Dry film photoresist for short is a water-soluble resist film. The thickness is generally 1.2mil, 1.5mil and 2mil. It is divided into three layers: polyester protective film, polyethylene diaphragm and photosensitive film. The role of the polyethylene diaphragm is to prevent the soft film barrier agent from sticking to the surface of the polyethylene protective film during the transportation and storage time of the rolled dry film. The protective film can prevent the oxygen from penetrating into the barrier layer and accidentally reacting with free radicals in it to cause photopolymerization. The dry film that has not been polymerized is easily washed away by the sodium carbonate solution.
Wet film: Wet film is a one-component liquid photosensitive film, mainly composed of high-sensitivity resin, sensitizer, pigment, filler and a small amount of solvent. The production viscosity is 10-15dpa.s, and it has corrosion resistance and electroplating resistance. , Wet film coating methods include screen printing and spraying.
Process introduction:
Dry film imaging method, the production process is as follows:
Pre-treatment-lamination-exposure-development-etching-film removal
Pretreate
Purpose: Remove contaminants on the copper surface, such as grease oxide layer and other impurities, and increase the roughness of the copper surface to facilitate the subsequent lamination process
Main raw material: brush wheel
Pre-processing method:
(1) Sandblasting and grinding method
(2) Chemical treatment method
(3) Mechanical grinding method
The basic principle of the chemical treatment method: Use chemical substances such as SPS and other acidic substances to uniformly bite the copper surface to remove impurities such as grease and oxides on the copper surface.
Chemical cleaning:
Use alkaline solution to remove oil stains, fingerprints and other organic dirt on the copper surface, then use acid solution to remove the oxide layer and the protective coating on the original copper substrate that does not prevent copper from being oxidized, and finally perform micro-etching treatment to obtain a dry film Fully roughened surface with excellent adhesion properties.
Control points:
a. Grinding speed (2.5-3.2mm/min)
b. Wear scar width (500# needle brush wear scar width: 8-14mm, 800# non-woven fabric wear scar width: 8-16mm), water mill test, drying temperature (80-90℃)
Lamination
Purpose: Paste an anti-corrosive dry film on the copper surface of the processed substrate through hot pressing.
Main raw materials: dry film, solution imaging type, semi-aqueous imaging type, water-soluble dry film is mainly composed of organic acid radicals, which will react with strong alkali to make it organic acid radicals. Melt away.
Principle: Roll dry film (film): first peel off the polyethylene protective film from the dry film, and then paste the dry film resist on the copper clad board under heating and pressure conditions, the resist in the dry film The layer becomes softened by heat and its fluidity increases. The film is completed by the pressure of the hot pressing roller and the action of the adhesive in the resist.
Three elements of reel dry film: pressure, temperature, transmission speed
Control points:
a. Filming speed (1.5+/-0.5m/min), filming pressure (5+/-1kg/cm2), filming temperature (110+/——10℃), exit temperature (40-60℃)
b. Wet film coating: ink viscosity, coating speed, coating thickness, pre-bake time/temperature (5-10 minutes for the first side, 10-20 minutes for the second side)
Exposure
Purpose: Use the light source to transfer the image on the original film to the photosensitive substrate.
Main raw materials: The film used in the inner layer of the film is a negative film, that is, the white light-transmitting part is polymerized, and the black part is opaque and does not react. The film used in the outer layer is a positive film, which is the opposite of the film used in the inner layer.
Principle of dry film exposure: The photosensitive initiator in the resist in the exposed area absorbs photons and decomposes into free radicals. The free radicals initiate cross-linking reaction of monomers to form a spatial network macromolecular structure insoluble in dilute alkali.
Control points: precise alignment, exposure energy, exposure light ruler (6-8 grade cover film), residence time.
Developing
Purpose: Use lye to wash away the part of the dry film that has not undergone chemical reaction.
Main raw material: Na2CO3
The dry film that has not undergone polymerization is washed away, and the dry film that has undergone polymerization is retained on the surface of the board as a resist protection layer during etching.
Development principle: The active groups in the unexposed part of the photosensitive film react with the dilute alkali solution to generate soluble substances and dissolve, thereby dissolving the unexposed part, while the dry film of the exposed part is not dissolved.