“Due to the complex process flow of PCB manufacturing, in the planning and construction of intelligent manufacturing, it is necessary to consider the related work of technology and management, and then carry out automation, informatization, and intelligent layout.
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1. How is the inner layer of the PCB made?
Due to the complex process flow of PCB manufacturing, in the planning and construction of intelligent manufacturing, it is necessary to consider the related work of technology and management, and then carry out automation, informatization, and intelligent layout.
1. Classification of technological processes
According to the number of PCB layers, it is divided into single-sided board, double-sided board and multi-layer board. The process of these three boards is not the same.
There is no inner layer process for single-sided and double-sided panels, and it is basically a material cutting-drilling-subsequent process.
Multilayer board will have inner layer process
1) Single panel process
Edge grinding→drilling→outer layer graphics→(full board gold plating)→etching→inspection→screen printing solder mask→(hot air leveling)→screen printing characters→shape processing→testing→inspection
2) Process flow of double-sided spray tin plate
Cutting edge grinding → drilling → copper sinking thickening → outer layer graphics → tin plating, etching and tin removal → secondary drilling → inspection → silk screen 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 → copper sinking thickening → outer layer graphics → nickel plating, gold stripping etching → secondary drilling → inspection → silk screen solder mask → silk screen characters → shape processing → testing → inspection
4) Process flow of multi-layer board spray tin board
Slitting and grinding → drilling positioning holes → inner layer graphics → inner layer etching → inspection → blackening → lamination → drilling → copper sinking thickening → outer layer graphics → tin plating, etching and tin removal → secondary drilling → inspection →Silk-screen solder mask→Gold-plated plug→Hot air leveling→Silk-printed characters→Outline processing→Testing→Inspection
5) Process flow of nickel-gold plating on multilayer boards
Edge grinding → drilling positioning holes → inner layer graphics → inner layer etching → inspection → blackening → lamination → drilling → copper sinking thickening → outer layer graphics → gold plating, film removal etching → secondary drilling → inspection → Silk-screen solder mask→screen-printed characters→shape processing→testing→inspection
6) Process flow of multi-layer immersion nickel gold plate
Slitting and grinding → drilling positioning holes → inner layer graphics → inner layer etching → inspection → blackening → lamination → drilling → copper sinking thickening → outer layer graphics → tin plating, etching and tin removal → secondary drilling → inspection →Silk Screen Solder Mask → Electroless Nickel Immersion Gold → Silk Screen Characters → Shape Processing → Test → Inspection
1. Inner layer production (graphic transfer)
Inner layer: cutting board, inner layer pretreatment, lamination, exposure, DES connection
Cutting (Board Cut)
1) Cutting board
Purpose: Cut the large material into the size specified by MI according to the order requirements (according to the planning requirements of the pre-production design, the substrate material is cut into the size required for the work)
Main raw materials: substrate, saw blade
The substrate is made of copper sheets and insulating layers. 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 and other types.
Precautions:
a. To avoid the impact of the edge of the board on the quality, after cutting, carry out edge grinding and rounding treatment
b. Considering the effect of expansion and contraction, the cutting board is baked before being sent to the process
c. Cutting should pay attention to the principle of consistent machine direction
Edge grinding/Rounding: remove the glass fiber left by the right angles of the four sides of the board by mechanical grinding, so as to reduce the scratching/scratching of the board surface in the post-production process, resulting in hidden quality problems
Baking board: remove moisture and organic volatiles by baking, release internal stress, promote cross-linking reaction, and increase sheet dimensional stability, chemical stability and mechanical strength
Control point:
Sheet: Panel size, sheet thickness, sheet type, copper thickness
Actions: bake time/temperature, stack height
(2) inner layer production after panel cutting
Function and principle:
The inner layer copper plate roughened by the grinding board is dried by the grinding board, and then affixed with a dry film IW, and then irradiated with UV light (ultraviolet light). After exposure, the dry film becomes hard. The unexposed part can be dissolved by weak alkali, and the inner layer circuit uses the characteristics of the material to transfer the pattern 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, and the free radicals initiate a cross-linking reaction of the monomer to generate a spatial network macromolecular structure that is insoluble in dilute alkali, while the unexposed part is due to the unexposed part. The reaction is soluble in dilute alkali.
The image transfer is completed by transferring the pattern designed on the negative film to the substrate by using the two with different solubility in the same solution.
The circuit pattern requires high temperature and humidity conditions, generally requiring a temperature of 22+/-3°C and a humidity of 55+/-10% to prevent the deformation of the film. The requirements for dust in the air are high. With the increase of the circuit density and the smaller the circuit, the dust content is less than or equal to 10,000 or more.
Material introduction:
Dry film: Dry film photoresist referred to as dry film (Dry film) is a water-soluble resist film, the thickness is generally 1.2mil, 1.5mil and 2mil, etc., divided into polyester protective film, polyethylene diaphragm and photosensitive film three layers. The function of the polyethylene film is to prevent the soft film blocking agent from sticking to the surface of the polyethylene protective film during the transportation and storage time of the roll dry film. The protective film can prevent oxygen from penetrating into the resist layer and reacting unexpectedly with the free radicals therein to cause photopolymerization. The dry film that has not been polymerized is easily washed off by sodium carbonate solution.
Wet film: Wet film is a one-component liquid photosensitive film, which is mainly composed of high photosensitive resin, photosensitive agent, colorant, filler and a small amount of solvent. The viscosity for production is 10-15dpa.s, and it has corrosion resistance and electroplating resistance. , Wet film coating methods include screen printing, spraying, etc.
Process introduction:
Dry film imaging method, the production process is as follows:
Pretreatment – Lamination – Exposure – Development – Etching – De-filming
Pretreatment (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 grinding method
(2) Chemical treatment method
(3) Mechanical grinding method
The basic principle of chemical treatment method: use chemical substances such as SPS and other acidic substances to uniformly etch the copper surface, and remove impurities such as grease and oxides on the copper surface.
Chemical cleaning:
Use alkaline solution to remove oil stains, fingerprints and other organic stains on the copper surface, then use acidic solution to remove the oxide layer and the protective coating that does not prevent copper from being oxidized on the original copper substrate, 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 wear scar width: 8-16mm), water grinding test, drying temperature (80-90℃)
Lamination
Purpose: To paste the dry resist film on the copper surface of the treated substrate by hot pressing.
Main raw materials: dry film (Dry Film), solution developing type, semi-aqueous solution developing type, water-soluble dry film mainly contains organic acid radicals in its composition, which will react with strong alkali to become organic acid radicals. dissolve.
Principle: reel 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 plate under the condition of heating and pressure, the resist in the dry film The layer is softened by heat and its fluidity is increased, and the film is completed with the help of the pressure of the hot pressing roller and the action of the binder in the resist.
Three elements of reel dry film: pressure, temperature, transfer speed
Control points:
a. Lamination speed (1.5+/-0.5m/min), lamination pressure (5+/-1kg/cm2), lamination 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: to transfer the image on the original film to the photosensitive base by the action of the light source.
Main raw materials: The negative film used in the inner layer of the negative film is a negative film, that is, the white light-transmitting part undergoes a polymerization reaction, and the black part is opaque and does not react.
The principle of dry film exposure: the photosensitive initiator in the resist in the exposed area absorbs photons and decomposes into free radicals, and the free radicals trigger the cross-linking reaction of the monomer to form a spatial network macromolecular structure that is insoluble in dilute alkali.
Control points: precise alignment, exposure energy, exposure light ruler (6-8 grades of cover film), dwell time.
Developing
Purpose: Use lye to wash away the part of dry film that has not undergone chemical reaction.
Main raw material: Na2CO3
The dry film that has not undergone polymerization reaction is washed away, and the dry film that has undergone polymerization reaction remains on the board surface as a resist protection layer during etching.
Development principle: The active groups in the unexposed part of the photosensitive film react with the dilute alkaline solution to form soluble substances and dissolve, thereby dissolving the unexposed part, while the dry film of the exposed part is not dissolved.
Control points:
a. Development speed (1.5-2.2m/min), development temperature (30+/-2℃)
b. Development pressure (1.4-2.0Kg/Cm2), developer concentration (N2CO3 concentration 0.85-1.3%)
Etching
Purpose: Use the chemical solution to etch away the copper exposed after development to form the inner layer circuit pattern.
Main raw materials: etching liquid (CuCl2)
The principle of inner layer etching: In the inner layer pattern transfer process, D/F or ink is used as anti-etching, anti-plating or anti-etching, so most of them choose acid etching (dry film/wet film to cover the surface of the circuit pattern) .
Prevent copper etching: other copper that is not exposed on the substrate will be removed by chemical reaction to form the required circuit pattern.
Common problems: dirty etching, over-etching, thin wire, open circuit, short circuit.
Control points:
a. Etching: speed, temperature (48-52℃), pressure (1.2-2.5Kg/cm2)
b. Stripping: 44-54℃, 8-12%NaOH solution
Stripping (Strip)
Purpose: Use strong alkali to peel off the resist layer protecting the copper surface to expose the circuit pattern.
Main raw material: NaOH
2. Counting the stack structure of the RF board and wiring requirements
1. Laminated structure of RF board
In addition to the impedance of the RF signal line, the stack structure of the RF PCB single board also needs to consider issues such as heat dissipation, current, devices, EMC, structure and skin effect. Usually we are in the layering and stacking of multi-layer printed boards. Follow some basic principles:
A) Each layer of the RF PCB is grounded in a large area, and there is no power plane. Both the upper and lower adjacent layers of the RF wiring layer should be ground planes.
Even if it is a digital-analog hybrid board, the power plane can exist in the digital part, but the RF area still needs to meet the requirements of large-area paving on each layer.
B) For RF double-sided panels, the top layer is the signal layer and the bottom layer is the ground plane.
Four-layer RF single board, the top layer is the signal layer, the second layer and the fourth layer are the ground plane, and the third layer is the power supply and control line. In special cases, some RF signal lines can be routed on the third layer. More layers of RF boards, and so on.
C) For the RF backplane, the upper and lower surface layers are both ground. In order to reduce the impedance discontinuity caused by vias and connectors, the second, third, fourth, and fifth layers carry digital signals.
The other stripline layers on the bottom are the bottom signal layers. Similarly, the two adjacent layers above and below the RF signal layer should be the ground, and each layer should be covered with a large area.
D) For high-power, high-current RF boards, the RF main link should be placed on the top layer and connected with a wider microstrip line.
This is conducive to heat dissipation and reduce energy loss, reducing wire corrosion errors.
E) The power plane of the digital part should be close to the ground plane and arranged below the ground plane.
In this way, the capacitance between the two metal plates can be used as the smoothing capacitance of the power supply, and the ground plane can also shield the radiation current distributed on the power supply plane.
The specific stacking method and plane division requirements can refer to “20050818 Printed Circuit Board Design Specifications – EMC Requirements” issued by the EDA Design Department, and the online standard shall prevail.
2. RF board wiring requirements
2.1 Corner
If the RF signal trace runs at a right angle, the effective line width at the corner will increase, and the impedance will be discontinuous and cause reflection. Therefore, to deal with the corners, there are mainly two methods: chamfering and rounding.
(1) Corner chamfering is suitable for relatively small corners, and the applicable frequency of chamfering can reach 10GHz.
(2) The radius of the arc angle should be large enough. Generally speaking, it should be ensured that: R>3W.
2.2 Microstrip wiring
The top layer of the PCB carries the RF signal, and the plane layer below the RF signal must be a complete ground plane to form a microstrip line structure. To ensure the structural integrity of the microstrip line, the following requirements are required:
(1) The edge on both sides of the microstrip line should be at least 3W wide from the edge of the ground plane below. And within the 3W range, there must be no non-grounded vias.
(2) The distance from the microstrip line to the shielding wall should be kept more than 2W. (Note: W is the line width).
(3) The uncoupled microstrip lines in the same layer should be treated with ground copper skin and ground vias should be added on the ground copper skin. The hole spacing is less than λ/20, and the holes are evenly arranged.
The edge of the ground copper foil should be smooth and flat, and sharp burrs are prohibited. It is recommended that the distance between the edge of the ground copper skin and the edge of the microstrip line is greater than or equal to 1.5W or 3H, and H represents the thickness of the microstrip substrate medium.
(4) Prohibit RF signal traces across the ground plane gap of the second layer.
2.3 Stripline routing
The RF signal sometimes has to pass through the middle layer of the PCB. It is common to go through the third layer. The second and fourth layers must be a complete ground plane, that is, an eccentric stripline structure. The structural integrity of the stripline shall be guaranteed to require:
(1) The edges on both sides of the stripline should be at least 3W wide from the edge of the upper and lower ground planes, and within the 3W range, there should be no non-grounded vias.
(2) The RF stripline is prohibited from crossing the ground plane gap of the upper and lower layers.
(3) The strip lines in the same layer should be treated with ground copper skin and ground vias should be added on the ground copper skin. The edge of the ground copper foil should be smooth and flat, and sharp burrs are prohibited.
It is recommended that the edge of the ground-clad copper skin be greater than or equal to 1.5W or 3H from the edge of the stripline, and H represents the total thickness of the upper and lower dielectric layers of the stripline.
(4) If the stripline needs to transmit high-power signals, in order to avoid the 50 ohm line width being too thin, the copper sheets of the upper and lower reference planes in the stripline area are usually hollowed out, and the hollowing width is the width of the stripline. More than 5 times the total dielectric thickness, if the line width still does not meet the requirements, then hollow out the upper and lower adjacent reference surfaces of the second layer.
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